Key Highlights

• Differences in the intensity range of valenced experience across species may affect how we ought to allocate resources to help different types of animals
• Humans and other mammals likely share a roughly similar intensity range
• It is unlikely that any species of animal possesses an intensity range that is exclusively extraordinarily mild
• Some aspects of cognitive sophistication appear to be positively correlated with intensity range; other aspects of cognitive sophistication appear to be negatively correlated with intensity range
• Affective complexity generally appears to be positively correlated with intensity range
• There is as yet no good objective measure of valence intensity, though there is much interesting work ongoing in this area

Executive Summary

This post is the fifth in Rethink Priorities’ moral weight series. In this report, I explain why it would matter if different sorts of animals possessed characteristically different intensity ranges of valenced experience, what factors might affect the intensity range of valenced experience, and how we might begin to measure differences in the intensity of valenced experience.

Animals differ in their perceptual abilities, their physiology and neural architecture, and their cognitive, affective, and social complexity. Given these differences, it would be surprising if the intensity range of valenced experience were uniform across species. To investigate differences in the intensity range of valenced experience across species, I adopted a three-stage methodology:

1. First, I explored what, if anything, theoretical evolutionary biology could tell us about the function of valenced experience.
2. Next, I explored how different aspects of intellectual and emotional complexity might affect the characteristic intensity of valenced experience.
3. Finally, I explored potential neurobiological, behavioral, and physiological markers of the intensity of valenced experience in humans and nonhuman animals.

Broadly speaking, the evolutionary function of valenced experience is to promote fitness-improving behaviors. It’s plausible that natural selection would not produce animals for whom valenced experiences were always extraordinarily weak because subjective experiences that were so faint as to be almost imperceptible would appear to do a poor job motivating behavior. Conversely, it appears unlikely that evolution would select for animals with a non-contiguous range that was exclusively extraordinarily strong because extremely intense experiences are distracting in a way that appears likely to reduce fitness.

‘Cognitive sophistication’ is a nebulous term that may refer to any of a constellation of mostly independent traits. Many of these traits plausibly affect the intensity of valenced experience, but the sign of the effect is often unclear. ‘Affective complexity’ refers to the diversity and depth of emotional sensations an animal can experience. Increased affective complexity may unlock qualitatively unique emotional states—such as fear, depression, or love—that by themselves or in combination with physical states increase the intensity range of experience.

There are currently no good cross-species measures of the intensity of valenced experience, though there is intriguing recent evidence that neural oscillations in the gamma band may track differences in pain intensity in both humans and nonhuman mammals.

Humans and nonhuman mammals possess neurologically and behaviorally similar affective systems, suggesting that most mammals are capable of experiencing roughly the same base set of emotions. It’s unclear how stark the differences in cognitive sophistication are across mammalian species and how these differences might affect the intensity of valenced experience. Given these facts, it would appear that we are more justified in thinking that humans possess an intensity range roughly similar to other mammals than we are in thinking that humans possess a much wider range than other mammals. It is unclear how mammals compare to other groups of animals, in part because there is a much sparser scientific literature on the capabilities of non-mammals and in part because it is unclear how increasing phylogenetic distance ought to influence reasoning-by-analogy about subjective experience.

Moral Weight Series

1. Comparisons of Capacity for Welfare and Moral Status Across Species
2. How to Measure Capacity for Welfare and Moral Status
3. The Subjective Experience of Time: Welfare Implications
4. Does Critical Flicker-Fusion Frequency Track the Subjective Experience of Time?
5. Differences in the Intensity of Valenced Experience across Species

Intensity of Valenced Experience and Its Range

Valenced experiences are those experiences that take on a positive or negative phenomenal affect.^[1] That is, valenced experiences are those experiences that feel good or feel bad to the subject.^[2] The intrinsic value of an experience is the product of its (possibly subject-relative) duration and valenced intensity.^[3] Just as the duration of experiences varies, so too does the valenced intensity of experiences.^[4] Some pains hurt worse than others: a sprained wrist is typically less wrenching than a broken femur. Some pleasures are better than others: an evening watching television typically engenders less joy than the birth of one’s first child.^[5]

Human life contains an assortment of valenced experiences.^[6] Some experiences, like the birth of one’s child, may be profoundly positive. Other experiences, like breaking one’s femur, may be acutely negative. Taking the difference of one’s best experience and one’s worst experience gives us one’s realized intensity range of valenced experience. Of course, the average human probably doesn’t experience the highest possible highs and lowest possible lows. Most of us do not have the misfortune of, say, suffering torture in a prisoner-of-war camp. Neither, though, do most of us taste the pure jubilation of, say, scoring the winning point in a sports championship. Taking the difference of the best experience one might undergo and the worst experience one might undergo gives us one’s possible intensity range of valenced experience.^[7]

Characteristic Range vs. Possible Range

In addition to realized range and possible range, there is a third conception of valenced intensity range that is useful for our purposes. An animal’s characteristic intensity range of valenced experience is defined by those experiences that are common for a member of the animal’s species at a particular point in the history of the species.^[8] Characteristic range is still a modal notion because it is independent of an animal's actual experiences. (It’s possible that an animal’s actual experiences exclusively fall outside the characteristic range.)

One potential reason to be more concerned about characteristic range than possible range is that animals might differ much more in their characteristic than their possible range, which would be especially concerning if inducing intensities outside the characteristic range were much more challenging^[9] than inducing intensities within the characteristic range. If this is the case, then for practical purposes, when deciding how to allocate resources to different species, it might be more informative to look at characteristic rather than possible range.

An example will illustrate the point. Suppose it’s possible (though very difficult) for both pigs and chickens to reach intensities as low as -100 and as high as +100. Suppose as well that the characteristic range of pigs is -90 to +90 and the characteristic range of chickens is -10 to +10. Suppose finally that moving an animal one point outside its characteristic range is ten times as costly as moving an animal one point inside its characteristic range. In this scenario, focusing too much on possible as opposed to characteristic range might lead us to allocate equal resources to pigs and chickens even though, given the assumptions and all else equal, it will usually be more cost-effective to help pigs rather than chickens in this scenario. More generally, differences in possible range that are rarely realized may matter less than differences in characteristic range that are often realized.

The features and considerations I explore below are sometimes more relevant to possible range than characteristic range, sometimes more relevant to characteristic range than possible range, and sometimes equally relevant to both. At the level of generality with which I discuss the topic, the distinction is not usually important. However, a more fine-grained and detailed examination of these issues would need to pull apart the effects on possible range from characteristic range. In so doing, we might discover that one type of range is more important or more variable than the other.

Why Variation in the Intensity Range of Valenced Experience Matters

Theories of welfare agree: experiences matter, morally.^[10] If animals differ in the intensity ranges of their valenced experience, such differences could affect the magnitude of welfare changes we create by helping (or harming) different sorts of animals. Suppose, for instance, the valence intensity of honey bees only vacillates between mildly positive experiences and mildly negative experiences. That is to say, the worst a honey bee might expect to suffer could be the equivalent of a human stubbing her toe; conversely, the best a honey bee might expect to enjoy could be the equivalent of a human eating a decent banana. In that case, improving the experiential states of honey bees would not generate large improvements in welfare. On the other hand, suppose that the valenced experiences of honey bees reach extreme intensities.^[11] A typical honey bee pain might be worse than the worst human migraine; a typical honey bee pleasure might be better than the best human orgasm. In that case, improving the experiential states of honey bees could lead to large gains in welfare.

Humans exploit an incredibly phylogenetically diverse group of animals.^[12] An exponentially greater diversity of animals suffer in the wild. But animal advocates have limited resources. Thus, hard choices must be made. Knowing that we can’t help all animals simultaneously, we must choose where to invest our scarce resources. There are, of course, many considerations that must be examined when deciding how to allocate time and money to different groups of animals. But to fully judge interventions and policies, we need to be able to evaluate (at least roughly) welfare gains and losses across many species.^[13]

Animals differ with respect to their neurobiological makeup,^[14] their cognitive and affective complexity,^[15] and their experiential capabilities.^[16] These differences make plausible the idea that there are characteristic differences in the intensity range of valenced experience across species.^[17][18] Moreover, it appears that pain sensitivity varies among humans. Humans exposed to the same sort of stimuli often report different subjective experiences. One review observes, “Pain perception varies widely among individuals. Even in controlled experimental settings, the same mild nociceptive stimulus can elicit unpredictably intense sensations in one individual, yet be barely perceived by another. In real-world settings, equally serious injuries often result in remarkably different painful percepts. What drives such dramatic variability of pain perception across individuals remains elusive” (Hu & Iannetti 2019: 1782).^[19] Given such differences among humans, who broadly share social, affective, intellectual, behavioral, and neurobiological characteristics, it’s not hard to imagine more profound differences among nonhuman animals. This possibility is sometimes explicitly raised in the literature: “The apparent occurrence of intra-specific variation in individuals’ pain experiences raises the hypothesis that there might be inter-specific variation in individuals’ pain experiences, i.e., individuals of different species may have different capacities for pain experiences” (Yeates 2012: 25).^[20]

How to Discern Differences in the Intensity Range of Valenced Experience

Discerning differences in the intensity range of valenced experience across species is not easy. As I detail below, the limited evidence currently available suggests extreme uncertainty. When researching the topic, I adopted the following three-step methodology, which also serves as a useful conceptual framing for this report.

1. Start with considerations from theoretical evolutionary biology. The evolutionary function of valenced experience may place some general constraints on the scope and location of intensity ranges. Thinking about how selection pressures are likely to operate on valenced experience in various conditions may help us identify some rough principles for broad groups of animals.

2. Next, consider how intensity range is likely to vary with characteristics like cognitive sophistication, affective complexity, and sociality.^[21]

3. Finally, try to identify various neurobiological and behavioral markers of valenced experience and see how these vary across species. The proxies ought to be validated in humans first and in the right conditions extended to some nonhuman animals.

I address these three steps in turn below.

The Evolutionary Role of Valenced Experience

If valenced experiences—pleasures and pains, broadly defined—are relatively widespread throughout the animal kingdom, there is probably an evolutionary story that explains why animals with the capacity for valenced experience accrued a fitness advantage over animals without that capacity.^[22][23][24] The quality and intensity of valenced experience has, like aspects of animal behavior and physiology, been shaped by selection pressures (Browning 2019: 153).^[25] By examining the evolutionary role of valenced experience, we may be able to identify some rough bounds on the likely variation in intensity of valenced experience across species. By taking an evolutionary lens to certain properties, we may even be able to estimate relative differences in the intensity range of valenced experience among some broad groups of animals.^[26]

According to many evolutionary biologists, valenced experience is adaptive because it motivates fitness-improving actions (Phillips 2008: 291)^[27] and discourages fitness-reducing actions (Dawkins 2009: 393).^[28] Orgasm feels good to iteroparous^[29] animals, so those animals are motivated to sexually reproduce.^[30] Tissue damage feels bad, so animals are motivated to avoid physical harm. Juvenile care feels good to altricial^[31] animals, so those animals are motivated to ensure the bearers of their genes survive to adulthood. Rotten fruit looks, smells, and tastes bad (to some animals), so those animals are motivated to avoid what for them would be unhealthy food. And so on.

To motivate fitness-improving actions, valenced experience is thought to play three roles. First, valenced experience represents, in at least a loose sense, fitness-relevant information. Second, valenced experience serves as a common currency for decision-making. Third, valenced experience facilitates learning.

Representation and Information-Gathering

We can think of the felt goodness or badness of an experience as a direct, intrapersonal representation of potentially fitness-relevant information.^[32] For example, an animal might feel a lance of pain when it moves its injured leg in a certain way. This pain can be thought to represent both the disturbance to the leg (including its location, shape, and quality) and the fact that this sort of disturbance is apt to harm the animal to a certain degree (Cutter & Tye 2011: 99).^[33] This information motivates the animal to rest until the injury has healed. Animals that lack access to such intrapersonal representation are often at a severe fitness-disadvantage.^[34] For instance, humans with congenital analgesia (a condition that prevents the experience of pain) almost always die young (Peirs & Seal 2016: 578).^[35]

In many species, valenced experience also induces outward changes that represent fitness-relevant information to observers, especially conspecifics. The outward signs of disgust after ingesting a poisonous berry warn other members of the species to avoid that food. In social species (such as humans), grunts and winces of pain may elicit offers of assistance (Steinkopf 2016: 1).^[36] In fact, some researchers have theorized that highly social animals suffer characteristically worse pains than non-social animals.^[37] For instance, Barbara Finlay and Supriya Syal argue that “over evolutionary time, several stimuli and situations that are painful in neither mammals generally nor in our primate relatives particularly have come to be experienced as pain in humans, accompanied by obvious distress that serves to elicit help from others. In humans, our domesticated species, and perhaps other highly social animals, soliciting help may allow survival in otherwise fatal situations” (Finlay & Syal 2014: 615).^[38]

Reflecting on the representation of valenced experience, especially painful experience, may help us recognize differences in the intensity of valenced experience in certain groups. Predators have evolved to detect evidence of injury in their prey. Predators display a clear preference for injured prey, and this preference can be explained by the fitness benefit of maximizing energy expenditure to consumption efficiency (Butler & Finn 2009). In response, prey species have evolved mechanisms to minimize the outward expressions of pain that would alert a predator to injury (Mogill 2009: 290; McLennan 2018: 1).^[39] This feat might have been accomplished in two (non-exclusive) ways: (1) individuals better at hiding their pain may have outcompeted conspecifics worse at hiding their pain or (2) individuals with a higher pain tolerance may have outcompeted conspecifics with lower pain tolerance. If the latter strategy predominated in particular species, we should expect that members of that species to experience characteristically less intense pains than members of a comparable species that did not face predation pressures of this kind.^[40][41]

Decision-Making

Another proposed purpose of valenced experience is to serve as a common currency for complex decision-making (Ginsburg & Jablonka 2019: 374).^[42] Animals face many competing demands. They must forage for food, avoid predators, care for juveniles, seek reproductive mates, and shelter from environmental hazards. Animals need a process to weigh, say, the fear of predatory attack with the pang of an empty stomach and make the appropriate tradeoff. Valenced experiences, as representations of fitness-relevant information, may allow animals to compare and evaluate disparate risks and rewards.^[43]

Reflecting on the role that valenced experience plays in decision-making may help us put rough bounds on the likely variation in intensity ranges across species. In particular, evolutionary considerations may give us reason to rule out both intensity ranges that are exclusively extraordinarily weak and intensity ranges that are exclusively extraordinarily strong.^[44]

Some activists have suggested that animals with relatively simple nervous systems are only capable of extraordinarily weak valenced experiences. For instance, Matt Ball writes, “So even if insects can have any subjective experience, their most intense sensation would be the palest hint of a feeling—a tiny fraction of the worst suffering we can experience.” The motivation for this view seems to be the idea that the intensity of an animal’s valenced experience should scale with the complexity of the animal’s nervous system.^[45] However, it’s hard to square this idea with the evolutionary function of valenced experience. It's unclear what fitness advantage the palest hint of a feeling could convey. Pain motivates animals to do things like avoid bodily damage; pleasure motivates animals to do things like reproduce. Subjective experiences so faint as to barely register would do a poor job motivating anything.

One possible reply to this line of argument is to suggest that motivational requirements vary by taxa. Perhaps mammals require intense experiences in order to motivate actions, but insects only require weak experiences to motivate similar actions. The thought is plausible if you think that motivation depends on the relative strengths of one’s experiences. If all an insect’s experiences are faint, then competition for the insect's motivational resources will not be fierce. So long as the experiences are still ordered by intensity, the insect will be motivated to pursue pleasures at the top end of the scale more than pleasures at the bottom end of the scale and motivated to avoid pains at the top end of the scale more than pains at the bottom end of the scale.

Here, though, we tread on conceptually precarious ground. If one adopts a subject-relative account of intensity, then it follows that insects do not have characteristically less intense experiences than other animals. So the position must be that insects have objectively faint experiences but subject-relative motivational requirements. But it’s unclear if that is a coherent position. There appears to be some sort of connection between motivation and intensity. It would be strange if there were an individual in extreme pain that, all else equal, were not motivated to escape the pain. Conversely, if an individual were extremely motivated to avoid mild pains, such an aversion would, all else equal, be regarded as pathological. The connection between intensity and motivation may not be conceptual, but if the evolutionary function of valenced experience is to motivate actions, it’s plausible that intensity and motivation generally co-vary under normal circumstances. Anyone who claims that insects have extraordinarily faint experiences faces a dilemma: either deny that insects are motivated to act (which seems empirically false) or deny the connection between intensity and motivation. One who adopts the latter horn of the dilemma owes us an evolutionary story about how the connection between intensity and motivation was severed.^[46]

On the other hand, others have speculated that animals with simpler nervous systems have characteristically much more intense experiences than humans. For example in his blog post “Is Brain Size Morally Relevant?” Brian Tomasik explores the idea that “to a tiny brain, an experience activating just a few pain neurons could feel like the worst thing in the world from its point of view.” Insofar as this notion is plausible, it depends on two ideas: (1) that pain intensity depends, at least roughly, on the proportion of total “pain neurons” that are firing at a given time and (2) that small brains are more likely to have a total proportion of their pain neurons firing at once. As far as I can tell, there is little empirical evidence to support either (1) or (2).

Moreover, there may be theoretical reasons to regard this notion with suspicion. There appears to be a limit to how intense experiences can be before they lose the fitness advantage for which they were selected in the first place. In humans the conscious sensation of pain is distracting (Chan et al. 2012). Mild pain can be an annoyance, and intense pain can be so overwhelming that it effectively incapacitates the sufferer. (A similar story is probably true of pleasures.) If an animal’s experiences were exclusively extremely strong, it seems the animal’s decision-making would be impaired.

Learning

It seems that one of the most important evolutionary functions of valenced experience is to facilitate learning (Damasio & Carvalho 2013: 145).^[47] Behaviors which improve evolutionary fitness, such as mating, tend to have a positive valence, while behaviors which reduce evolutionary fitness, such as eating spoiled meat, tend to have a negative valence. When these valences are paired with the appropriate behaviors often enough, an animal learns to pursue fitness-boosting behaviors and avoid fitness-reducing behaviors. Lynne Sneddon and her collaborators argue that the “key function [of pain] appears to be that the aversive experience of pain creates a strong and lasting motivation that enables the animal to avoid getting into a similar situation in the future” (Sneddon et al. 2014: 202).^[48]

Animals less likely to benefit from long-term learning may be less likely to benefit from valenced experiences. This idea has led some researchers to speculate that animals with a shorter lifespan are less sensitive to pain than animals with a longer lifespan (Walters 2018: 12).^[49] The basic idea is that the reproduction-survival tradeoff will push some animals to occupy an evolutionary niche that favors traits that increase early reproductive success, even at the expense of traits that promote long-term survival (Stearns 2012: 4309).^[50] One important function of painful experiences is to teach an animal which stimuli to avoid in order to promote long-term survival. Short-lived animals that won’t be around long enough to benefit from such learning may be unlikely to invest in costly fine-grained nociceptive systems (Finlay 2019: 6).^[51] These animals would still benefit from a nociceptive system that alerted the animals to a threat of imminent death, but they would be less likely to benefit from a system that, say, induced an unpleasant sensation when the animals ate foods that were only slightly bad for them.

Similar reasoning may be applied to animals (again, like many insects) that produce large numbers of offspring that don’t survive to adulthood and that change drastically between life stages. If most individuals in the larval or pupal stage are likely to benefit less from valenced experience than adult members of the species,^[52] then the intensity range of adults might be characteristically larger than the range for juveniles.^[53] This consideration may be especially important for invertebrate species welfare prioritization. Some invertebrate species, such as mealworms and black soldier flies, are slaughtered before they reach adulthood; other species, such as crickets, are farmed to adulthood. However, recent research suggests (or perhaps merely assumes) that fruit fly larvae process noxious stimuli in the same way as adult flies (Lopez-Bellido et al. 2019).^[54]

Finally, the didactic role of valenced experience may make differences in the subjective experience of time relevant to differences in the intensity of valenced experience. Suppose it’s the case that for some range of values, x seconds of valenced experience at intensity n is as effective a teaching tool as 2x seconds of valenced experience at intensity 0.5n. If animal A has a rate of subjective experience twice that as animal B, A could learn the same lesson as B in either half the time or at half the intensity.

Cognitive Sophistication and Intensity of Valenced Experience

There are two ways in which an animal’s degree of cognitive sophistication could plausibly affect the intensity range of its valenced experiences. Certain types of cognitive sophistication may directly alter valenced states, amplifying or diminishing the intensity of certain pains and pleasures. Other types of cognitive sophistication may alter intensity range by unlocking new states that characteristically accompany certain pains and pleasures, the combination of which serves to reduce or magnify the overall intensity of the combined experiences. As we’ll see below, the means by which an animal’s level of cognitive sophistication affects its experiences do not all cut in the same direction. Some types of cognitive sophistication appear to increase intensity range, while other types of cognitive sophistication appear to decrease intensity range.

The primary reason for this tension is that—as I hope is independently plausible—cognitive sophistication is not a single, unidimensional characteristic. Instead, the thing we somewhat nebulously refer to as ‘cognitive sophistication’ is actually a loose, heterogeneous bundle of independent traits. These various traits affect the intensity of valenced experience in various ways. Thus, the claim that cognitive sophistication affects the intensity of valenced experience, while true, is conceptually too simple for our purposes. We need a more fine-grained analysis that recognizes the different ways that different elements of cognitive sophistication are apt to influence the intensity of valenced experience.^[55]

A further complication is that different elements of cognitive sophistication may affect different types of valenced experience in different ways. This multitude of relationships between different elements of cognitive sophistication and different types of valenced experience makes for a complicated web of effects (Yeates 2012: 33).^[56] Great care must be taken to untangle this web. We should thus be wary of claims that report a clear and general relationship between the intensity of valenced experience and cognitive sophistication.^[57]

Mental Time Travel

The capacity to remember past events and imagine future events may influence the intensity of valenced experiences. Psychologists call this ability mental time travel. Evidence of mental time travel is currently limited to birds and mammals (Cheke & Clayton 2010), though any attribution of the ability to nonhuman animals is at least somewhat controversial.^[58] Like many of the other cognitive traits discussed in this section, mental time travel seems to reduce the intensity of experiences in some circumstances and amplify the intensity of experiences in other circumstances. It is thus unclear whether animals that possess this ability have characteristically more or less intense valenced experiences overall.

For animals experiencing acute pain with a fixed endpoint, mental time travel may reduce the intensity of pain. Sahar Akhtar writes, “Greater time horizons in either direction might lessen the intensity of one's pain: being able to think about the past can allow us to recollect and take comfort in painless past experiences. Similarly, by thinking about future periods we may be able to take relief in the thought of future periods without pain and we are capable of recognizing that the pain will end. An animal may not be able to remember times without pain or imagine future periods without the pain” (Akhtar 2011: 507). In addition to remembering pain-free moments in the past, if the current painful experience is of a type that an animal has undergone in the past, reflecting on that past painful experience may diminish the intensity of the current experience. In humans, familiarity with a pain reduces the intensity of the pain (Reicherts et al. 2017: 544),^[59] and it’s plausible that nonhuman animals with sufficient memorial skills experience a similar reduction in intensity. Animals unable to become familiar with a repeated painful stimulus may experience the pain at its peak intensity every time. Akhtar concludes that animals “that have a limited temporal sense would seem to be unable to escape, or get outside of, their present mental state, including a present painful state” (Akhtar 2011: 507).^[60]

On the other hand, the ability to recall past suffering and anticipate future suffering adds an additional dimension to experience. Some authors have thus argued that mental time travel increases rather than decreases the intensity range of valenced experience. Huub Brouwer and Willem van der Deijl write, “Also, while animals, especially lower animals [sic] such as mice and rattlesnakes, may not be capable of experiencing as high levels of wellbeing as humans, they might also not have the capacity to suffer as much as humans can. One reason to think that this is so, is that humans can remember past suffering and anticipate future suffering to a much greater extent than, for instance, mice and rattlesnakes can” (Brouwer & van der Deijl 2020: 353). In humans the anticipation of certain (especially psychological) stressors can induce anxiety-driven hyperalgesia (enhanced pain sensitivity) (Reicherts et al. 2017: 551).^[61] James Yeates summarizes the dual nature of mental time travel thusly: “The ability to temporally locate pain allows animals to know when pain will begin or end and that pain will begin, end or continue. The latter knowledge may help animals to cope, and higher animals that are able to anticipate pain’s end may therefore find pain experiences less unpleasant. Conversely, higher animals’ knowledge that pain is likely to continue may exacerbate their negative feelings” (Yeates 2012: 28).

Whether mental time travel increases or decreases the intensity of experience may depend on how fully developed the ability is in a given animal. Akhtar speculates that “beings with only a basic or rudimentary sense of time and of self may be at the greatest disadvantage from pain. They may possess enough self- and time-awareness to suffer from the anticipation and memory of pain, but not enough to be able to discount pain, choose to refrain from focusing on pain, form expectations about the cessation of pain, or to consider other interests or times without pain” (Akhtar 2011: 510). It’s unclear which animals fall into this category, but it plausibly encompasses many bird and nonhuman mammal species.

Rationalization and Normative Reasoning

The ability to rationalize valenced experiences may reduce their intensity. When one understands that the pain one endures is suffered for good cause (the fatigue of exercise or the prick of a flu shot, say), it’s plausible that the overall experience is less intense than comparable stimuli endured for no good cause. Rationalization may reduce the overall intensity of negative experience by removing other negative states. A dog receiving a parvovirus vaccination might feel alarm and betrayal that their best friend would subject them to such discomfort, whereas a human undergoing a similar procedure might only feel the physical pain of the shot. Alternatively (and non-exclusively), rationalization might reduce the overall intensity of negative experiences by introducing positively valenced states that counteract the negative state. Although a human might dislike the nature of a painful medical procedure, she might find solace in the fact that the procedure improves her health. Donald Broom argues that “more sophisticated brain processing will also provide better opportunities for coping with some problems. For example, humans may have means of dealing with pain that fish do not, and may suffer less from pain because they are able to rationalise that it will not last for long” (Broom 2007: 103).^[62] Sahar Akhtar adds, “Without the belief that there are good reasons to endure their pain—even if there are good reasons, such as a painful but necessary veterinary procedure—animal pain would likely be more intense” (Akhtar 2011: 506).

On the other hand, the ability to recognize good reasons for an experience is probably a manifestation of a more general capacity to reason normatively about the world. In some circumstances, this capacity appears to increase rather than diminish the intensity of experience. For instance, witnessing or undergoing an event that one recognizes as unjust adds a moral dimension to the experience, and adding a moral dimension to an experience plausibly amplifies the intensity of the experience. Recent research suggests that pains perceived to be unjust or unfair are judged to be more severe (McParland & Eccleston 2013: 485),^[63] though a causal explanation of the correlation has yet to be established. Still, it’s plausible that pains and pleasures perceived to be undeserved might feel worse, all things considered, than comparably evoked pains and pleasures without a normative component. Pains and pleasures perceived to be deserved might feel better, all things considered, than comparably evoked pains and pleasures without a normative component. If that is true, then, all else equal, creatures capable of forming normative judgments will possess a wider characteristic^[64] intensity range than creatures incapable of forming such judgments.

It’s unclear whether nonhuman animals are capable of normative reasoning. For recent discussion of the question see Clement 2013, Johannsen 2019, and Andrews 2020.

Attention and Focus

The ability to consciously direct one’s attention and focus may affect the intensity of valenced experience. Because valenced experiences evolved to make fitness-relevant information salient, they tend to grab our attention (Bjekić et al. 2018: 587).^[65] Many animals seem to lack the ability to deliberately redirect their attention and focus away from valenced experiences, and this inability may increase the intensity of the experiences. James Yeates speculates that “animals more able to divide attention would experience pain of lower intensity (because the attention is divided)” (Yeates 2012: 29). Sahar Akhtar concurs, writing, “The facts, if they are facts, that many animals do not have many substitutes for focusing on their pain and cannot will themselves to focus on other things, cannot form expectations about the ending of pain, think about other times without pain, or consider more complex interests for which pain may be a necessary means, provide us with reasons for thinking that the overall pain experience caused by a given measure of pain might sometimes be worse for animals than it is for us” (Akhtar 2011: 509).

Although Yeates and Akhtar appear to base their reasoning on a priori considerations, there is at least modest empirical evidence to support their position. Cognitive inhibition is the ability to disregard stimuli that are irrelevant to the task at hand. Recent research suggests that, in humans, “a higher level of cognitive inhibition is associated with lower experimental pain sensitivity” (Bjekić et al. 2018: 580). Hence, it appears that humans that are better able to consciously redirect their focus also suffer characteristically less intense pains.^[66]

The extent to which nonhuman animals can consciously control their attention is unclear. Nonhuman primates are generally thought to be able to redirect their focus non-reflexively (Scerif, Gomez, & Byrne 2004), and recent research suggests rodents may also possess this ability (Matzel, Wass, & Kolata 2011; Lustig et al. 2013). It appears this ability has not been widely studied in other nonhuman animals.

Learning Ability

As noted above, one of the main evolutionary functions of valenced experience is to facilitate the learning of fitness-improving behaviors. Pain teaches animals which stimuli are noxious, how to avoid those stimuli, and what to do to recover from injury. Pleasure teaches animals which foods are healthy and which activities increase the likelihood of gene propagation. Because extremely intense experiences are often distracting, animals that undergo characteristically extremely intense experiences appear to be at a selective disadvantage compared to conspecifics with less intense experiences. Thus, we might expect evolution to select for animals with experiences just intense enough (in general) to play the primary instructive role of pleasure and pain.

Animals differ with respect to the ease with which they are able to learn new information. In controlled settings, some species can be taught a new behavior in fewer trials than other species.^[67] (The potential complexity of learned behavior also varies by species.) Some types of animals appear better equipped to pick up patterns in signals only weakly conveyed. In general, less cognitively sophisticated animals probably require stronger signals for pattern-learning. If valenced experience is the signal, then we might reasonably expect the intensity of valenced experience to correlate inversely with learning ability. That is, all else equal, the better an animal is at learning, the less intense its valenced experiences might normally be.

This line of reasoning is, however, entirely speculative. It’s possible learning depends more on the statistical regularity of the pattern than the strength of the stimulus during trials, in which case we should not expect a relationship between learning ability and the intensity of valenced experience.

Self-Awareness

Self-awareness may influence the intensity of valenced experience. However, like many of the features described above, the direction of this influence is unclear. Some authors emphasize the ways self-awareness may unlock new states that increase intensity range. For instance, Gregory Berns writes, “An animal who is aware of his or her own pain and suffering may well experience the existential fear associated with imminent death” (Berns 2017: 245). If self-awareness is a necessary condition on existential fear, and existential fear is one of the worst possible experiences, then, all else equal, it seems self-awareness extends both an animal’s possible intensity range of experience and, if existential fear is common enough, an animal’s characteristic intensity range. However, other authors have suggested that the lack of self-awareness increases the intensity of valenced experience. Sahar Akhtar argues that “the absence of self- and time-awareness may mean that the feeling of pain for animals is much worse in certain respects than has previously been thought. The absence of awareness may mean that in some cases pain is worse for animals than a comparable measure of pain for us” (Akhtar 2011: 499). Akhtar’s argument for this claim proceeds by way of mental time travel, for which she takes self-awareness to be a necessary condition. She writes, “Animals without an awareness of self and time would not be able to anticipate the cessation of the pain or take relief in the thought that it may or will end” (Akhtar 2011: 507).

Of course, what exactly constitutes self-awareness is also contentious. There is a philosophical tradition dating back at least to Kant that claims sophisticated language ability is the only hallmark of true self-awareness. More recently, empirical evidence has suggested that many nonhuman mammals, such as mice, appear to be self-aware (Wada et al. 2016). There is even controversial evidence for self-awareness in cleaner wrasse, a species of bony fish (Khoda et al. 2019). If metacognition is a type of self-awareness, then self-awareness is probably even more widespread. There is evidence of uncertainty monitoring, a type of metacognition, in bees (Perry & Barron 2013) and ants (Czaczkes & Heinze 2015).^[68]

Doxastic Capacity

A creature’s doxastic capacity refers to its ability to adopt doxastic attitudes (canonically: belief, disbelief, and suspension of judgment^[69]) toward various propositions. Some animals have a wide range of beliefs concerning a large variety of topics. Other animals have a limited set of beliefs concerning a narrow range of topics.

An ability to form beliefs about one’s valenced experiences may influence the intensity of those experiences. James Yeates writes, “Insofar as pain is ‘information,’ an animal’s doxastic reasoning abilities can affect how it processes painful stimuli. Higher animals may entertain beliefs about a pain’s cause, duration, permanence and the animal’s ability to control it” (Yeates 2012: 32).

This ability, however, may cut both ways. Knowing that a pain to come will be fleeting plausibly reduces the intensity of the pain. Animals that are unable to form such beliefs may suffer more from comparable stimuli. (Compare a routine medical procedure for humans with a routine veterinary procedure for dogs.) On the other hand, knowing that, say, a migraine could strike at any moment induces a dread that would be absent if one were unable to entertain beliefs about future pain states.^[70] Yeates summarizes the relationship thusly: “Higher doxastic capacities may be beneficial for animals who experience predictable, controllable and impermanent pain, whereas animals experiencing pain that appears unpredictable, uncontrollable and permanent may suffer more from knowing this” (Yeates 2012: 33).

Affective Complexity and Intensity of Valenced Experience

Affective complexity is a measure of the potential diversity and depth of emotional states that an animal can experience. In humans, emotional states are a pervasive and universal component of experience. In experience-sampling studies,^[71] humans report the presence of some emotion virtually every waking moment, and these emotions almost always have a valence (Lucas & Diener 2008: 471-472). It’s plausible that affective complexity is, in general, positively correlated with intensity range of valenced experience. That is, species that experience a greater variety and/or greater complexity of emotional states are, all else equal, capable of more intense positive and negative experiences.

There are two ways in which affective complexity might increase the possible intensity range of valenced experience. Different degrees of affective complexity might unlock qualitatively different kinds of pleasures and pains that are emotional rather than physical in nature. If these emotional states can be more intense than mere physical states,^[72] then adding them to an animal’s valence repertoire increases the possible intensity range. Even if the emotional states by themselves are not more intense than mere physical states, they might combine with physical states to produce valences that are overall more intense than emotional or physical states alone.^[73]

Combination may take different forms. Distinct valence types may be roughly additive, such that the combination of a physical pain and an emotional pain generates greater overall suffering than the physical or emotional pain alone. Alternatively (but not exclusively), the different valence types may interact in some way, with emotional states amplifying or reducing the intensity of physical states. There is a rich literature documenting the effect of emotional states on physical pain in humans. Summarizing this literature, Philipp Reicherts and his collaborators write, “Emotions have tremendous influence on the perception of pain. In general, pain is diminished by positive while it is increased by negative emotions” (Reicherts et al. 2017: 544). Unfortunately, the literature on emotional states in nonhuman animals is much sparser. Elodie Briefer explains, “Because the emotions of non-human animals have long been considered as unobservable processes that could not be objectively studied, scientific interest in this topic is relatively recent” (Briefer 2018: 1).

Even if affective complexity doesn’t affect an animal’s possible intensity range, it still may affect an animal’s characteristic intensity range in important ways. There may be some physical pains and pleasures that are so intense they effectively crowd out all other feelings, such that layering an emotional state on top of the physical state doesn’t alter the overall valence.^[74] Nonetheless, if such physical states are rare, and if emotional states often exacerbate the intensity of common experiences, then animals with greater affective complexity will tend to experience a greater intensity range in their everyday lives.

Notably, some of the differences in affective complexity are plausibly the result of differences in social complexity. As such, the question of how affective complexity affects the intensity of valenced experience is inextricably connected to the question of how differences in sociality affect the intensity of valenced experience.

Fear and Anxiety

Although basically all animals react to perceived danger, it appears only some animals are capable of conscious fear and anxiety. Genuine^[75] fear and anxiety may be among the worst emotional states an animal can experience. The negative effects of fear and anxiety on human welfare are well-documented.^[76] The experience of fear is associated with certain physiological and behavioral responses. Behavioral markings of fear include fleeing, hiding, freezing, and suspending unnecessary bodily functions. Physiological reactions to fear can include elevated heart rate, hyperventilation, increased muscle tension, constriction of blood vessels, nausea, and dizziness. Insofar as other animals display relevantly similar physiological and/or behavioral responses, that is evidence that they too experience conscious fear.^[77]

Anxiety is related to but distinct from fear. Anxiety is sometimes said to be the result of danger that is perceived to be unavoidable (Öhman 2008) or situations in which the threat is ambiguous or unknown (Belzung & Philippot 2007). Anxiety is often considered a secondary emotion, that is, an emotion in response to another emotion. For example, in humans, generalized fear often leads to anxiety. Physiological and behavioral signs of anxiety include elevated heart rate, rapid breathing, increased perspiration, increased motor tension, changes in sleeping patterns, and/or changes in food intake.^[78] In addition to physiological and behavioral reactions, anxiety is also associated with certain cognitive changes. For example, anxiety increases general apprehension and is correlated with pessimistic cognitive biases (Anderson and Adolphs 2014). Anxiety tends to amplify physical pain in both humans and (at least some) nonhuman animals (Rhudy & Meagher 2000).

It’s plausible that many nonhuman animals also experience these effects. There is evidence that exposure to fear- or anxiety-producing stimuli reduces the lifespan of domestic dogs (Dreschel 2010). Other researchers have argued, on empirical grounds, that predator-induced fear commonly causes post-traumatic stress disorder-like symptoms in wild mammals and birds and perhaps other animals (Zanette et al. 2019).

Two worries are worth noting with respect to the detection of fear and anxiety in nonhuman animals. The first is that applying emotional terms to nonhuman animals, especially phylogenetically distant animals, may result in problematic anthropomorphizing. Both crayfish (Fossat et al. 2014) and honeybees (Bateson et al. 2011) exhibit many of the behaviors associated with human fear and anxiety, but, because those terms come loaded with anthropocentric connotations, applying those terms to those animals may inevitably be misleading.

The other worry runs in the opposite direction. Because the lifestyle and environment of some animals is so alien to us, we should not expect negative emotional states to be expressed in precisely the same manner. William Gibson and his collaborators remind us that “distantly related species may express emotion states through behaviors that have no obvious homology to human behaviors” (Gibson et al. 2015: 1401). They advocate for an “alternative approach to identifying instances of emotional expression, which does not depend on anthropocentric homologies” by establishing “general features of emotion states, or ‘emotion primitives,’ which apply both to different emotions in a species and to emotions across phylogeny. One can then search for behaviors that exhibit evidence of such emotion primitives in model organisms” (ibid.).^[79]

Depression

All else equal, animals that are capable of becoming depressed may generally possess wider intensity ranges than animals that are not capable of becoming depressed. In humans, severe depression appears to be one of the very worst valenced states. One 2011 study reported that, on average, subjects with severe depression lost about 80% of their wellbeing, nearly twice the wellbeing loss associated with extreme pain/discomfort (Graham, Higuera, & Lora 2011).^[80] A 2012 study found similar results: severe depression had a bigger effect on life satisfaction and day affect than extreme pain/discomfort (Dolan & Metcalfe 2012).^[81] A 2013 study found that depression had a bigger impact on the self-reported frequency of happy states than physical pain did (Mukuria & Brazier 2013). And in the United States, among individuals who die by suicide, mental health problems are about twice as common as physical health problems (Karch, Logan, & Patel 2011).^[82] If nonhuman animals experience depression in a similar way, it appears plausible that the ability to experience severe depression extends a creature’s intensity range in many circumstances.

Components of depression include sadness, anhedonia, and learned helplessness. Each of these components has been detected in nonhuman animals.

Monkeys, elephants, whales, bears, buffalo, cats, dogs, rabbits, goats, and horses display behavior reminiscent of sadness (King 2013). In fact, the capacity for sadness appears to be adaptive. In humans, sadness increases the accuracy of some types of memory and generally improves decision-making and problem-solving (Bonanno, Goorin, & Coifman 2008: 799).^[83] In social mammals, including humans, displays of sadness also tend to invoke offers of assistance from others (ibid.).^[84] When sadness slides into depression, however, these benefits dissipate. The decrease in motivation wipes out the cognitive gains, and prolonged depression tends to hurt relationships, reducing assistance from others (Bonanno, Goorin, & Coifman 2008: 800-801). Thus, whereas sadness appears to play an evolutionarily useful role, depression looks to be maladaptive.

Anhedonia is a loss of interest in activities previously found to be rewarding (Willner, Muscat, & Pap 1992). In nonhuman animals, external symptoms of anhedonia may include “behavioral deficits consistent with a loss of responsiveness to reward, such as decreased sucrose consumption, decreased ability to associate rewards with a distinctive environment, and decreased sensitivity to rewarding electrical brain stimulation” (Moreau 2002: 351). Anhedonia-like behavior can be induced in fruit flies by exposing them to aversive, uncontrollable vibrations over several days. The shaken flies show reductions in various voluntary behaviors, although reflexive behavior remains unchanged. In particular, shaken flies consume much less glycerol (commonly used as a reward in fruit fly studies) than non-shaken controls, suggesting that the shaken flies have lost their taste for sweets (Ries et al. 2017).

Learned helplessness is a condition in which a human or nonhuman animal adopts an overly passive reaction profile that disrupts important voluntary behavior.^[85] A creature in the throes of learned helplessness will often neglect basic survival needs, ignoring food, water, predators, and potential mates. Learned helplessness is triggered by repeated, uncontrollable exposure to traumatic stimuli, and it can be reliably reproduced in a number of nonhuman animals. For example, if dogs are subjected to repetitive and unavoidable electric shocks, the dogs will eventually stop trying to evade the shocks, sitting passive and still even if later explicitly given the opportunity to escape (Seligman 1968).^[86]

Given the prevalence of these symptoms, many nonhuman animals, including fish, are used as models for depression research.

Friendship, Love, Social Buffering, and Social Isolation

Different sorts of animals exhibit different types and different degrees of sociality.^[87] These differences may unlock qualitatively different emotional states that extend an animal’s intensity range. Many of the putatively most positive types of human experiences involve a loving relationship (with love liberally defined to include Platonic (friendship) love, familial love, and romantic love). In humans, the number and quality of interpersonal relationships are among the greatest determinants of self-reported happiness (Bruni 2010). Thus, the ability to engage in loving relationships plausibly extends a creature’s intensity range in many circumstances.

Many mammalian species appear to have the capacity for something like maternal love and non-familial friendship. Analogues of romantic love, however, appear to be much rarer in mammals (though perhaps common in birds).^[88]

According to one review, “[m]other-infant bonding is a characteristic of virtually all mammals” (Numan & Young 2016: 98). Numerous behavioral, hormonal, and neural similarities have been observed between human and non-human mammalian mothers. Lactation and its associated behavioral suite is common to most mammals, including humans. Mammalian mothers protect their young from adverse weather, predators, and threatening conspecifics (Lonstein, Lévy, & Fleming 2015: 157).^[89] The facilitation of this behavior is accomplished by similar hormonal agents and a similar neural reward mechanism (Lonstein, Lévy, & Fleming 2015: 177).^[90]

While less common than maternal bonding, relationships akin to friendship may exist among some nonhuman animals. Group-living animals display their preference for social contact in a number of ways. Bull calves actively seek out companion animals (Mounier et al. 2006). Young chicks increase their peeping calls when isolated (Sufka & Hughes 1991). In many group-living species (especially but not exclusively primates^[91]), individuals affiliate with some conspecifics more than others, and these affiliation preferences cannot be explained by kin relations or social hierarchies alone (Massen, Sterck, & de Vos 2010). Affiliation preferences appear to promote cooperation, health, and reproductive success (Massen, Sterck, & de Vos 2010: 1386-1390).

Social buffering is the phenomenon by which animals recover better from aversive events in the presence of conspecifics. Social buffering has been documented in many mammals, as well as chickens (Edgar et al. 2015) and zebrafish (Faustino, Tacão-Monteiro, & Oliveira 2017). One review describes social buffering in rats and mice [citations omitted]: “Rodents have demonstrated surprising responsiveness to conspecifics' needs. Mice orient to a littermate's pain and synchronize pain behaviour [...] Female mice provide social analgesia to another in pain, choosing to spend time close to a ‘jailed’ mouse in pain at one end of the apparatus rather than to another ‘jailed’ mouse without pain at the other end, with the effect of reducing pain behaviour in the former. [...] Rats show emotional contagion and will work to terminate another's distress [...] These findings are intriguing, because they occur between adults, in rodent species with different social structures and behaviours, and imply social analgesia from the unaffected individual's voluntarily staying close to the individual in pain” (Williams 2019: 4-5). Social buffering thus tends to reduce the intensity of negatively valenced experiences (perhaps by ‘diluting’ the intensity across individuals). But being a social animal raises the possibility of intense suffering due to social isolation.

In addition to the positive emotions associated with maternal love and friendship, the capacity for these relationships also raises the specter of extremely negative states when access to such relationships is denied. The adverse effects of social deprivation in group-living vertebrates is well documented. When raised in isolation, such animals are less able to appropriately process social and environmental stimuli. For example, rat pups reared in isolation exhibit a wide range of long-term behavioral and physiological abnormalities, including “neophobia [fear of new things], impaired sensorimotor gating, aggression, cognitive rigidity, reduced prefrontal cortical volume and decreased cortical and hippocampal synaptic plasticity” (Fone & Porkess 2008: 1087). In humans, social isolation is about as deadly as smoking (Pantell et al. 2013). According to some researchers, social exclusion is literally painful (MacDonald & Leary 2005).^[92]

Self-Conscious Emotions

Self-conscious emotions paradigmatically include shame, pride, guilt, embarrassment, jealousy, and envy (Lewis 2008). The capacity to experience self-conscious emotions may increase an animal’s characteristic intensity range. Negative self-conscious emotions like shame and guilt increase one’s feelings of separateness, which according to some (notably Buddhist) traditions is the root cause of suffering. Anecdotal evidence and received wisdom suggest that self-conscious affect tends to make many negatively valenced experiences more negative. Although the empirical literature on this topic appears relatively sparse, it has long been known that neurotic individuals are more likely to be self-conscious (Widiger 2009).

Self-conscious emotions are thought to require sophisticated cognitive and social skills. Nonetheless, Frans de Waal has suggested that nonhuman primates share with humans the basic building blocks for such emotions. Regarding shame, he writes, “Human shame, for example, typically stems from the violation of social norms and is characterized by a desire for invisibility. It is expressed in a shrinking body posture and downcast gaze, which brings it morphologically close to the submission displays of primates and other animals. Due to its self-conscious nature, human shame appears cognitively more complex than submission, but the associated emotions may not be so different” (de Waal 2011: 202). Regarding guilt, de Waal writes, “Similarly, guilt reflects regret at an action that may have hurt another, but that at the same time also hurt the relationship between actor and recipient. Guilt may thus help regulate reciprocity relationships. We know that nonhuman primates exhibit external signs of anxiety after aggressive acts that undermine their social relationships, and do so more often the more valuable their partner. Bonobo aggressors, for example, often approach their victim immediately after having attacked them to inspect and lick the injuries they themselves inflicted, which appears close to regret of previous behavior” (de Waal 2011: 202).^[93] Although attributing self-conscious emotions to nonhuman animals runs the risk of anthropomorphism, de Waal has elsewhere (de Waal 1999) urged researchers to give equal weight to avoiding the converse: anthropodenialism, the mistaken refusal to attribute human mental characteristics to nonhuman animals.^[94]

Measuring the Intensity of Valenced Experience

Thus far I have discussed, in broad terms, what sorts of characteristics might correlate with differences in the intensity range of valenced experience. But if these considerations are ever to amount to more than theoretical speculation, we will need to find a way to actually measure the intensity of valenced experience (at least in humans, and preferably in a variety of animals). And if we could quantify the intensity of valenced experience, we could perhaps dispense with correlative investigations altogether and instead directly assess differences in the intensity of valenced experience across species.

Regrettably, measuring the intensity of valenced experience across species is going to be difficult, for at least three reasons.

First, the neural correlates of valenced experience are poorly understood, even in humans (Walters & Williams 2019: 6).^[95] In their search for the mechanisms that govern valenced experience (especially pain experience), neuroscientists have advanced a number of candidate brain regions and neural processes. Despite a huge volume of research in recent years, little consensus has emerged.^[96] Significant progress appears to be perennially just on the horizon. With so much still unknown about the causal mechanisms governing valenced experience in humans, it is difficult to search for neural evidence of differences in the intensity of valenced experience across species. As an animal’s phylogenetic distance from humans increases, the animal’s neural architecture tends to become increasingly alien,^[97] making neural evidence all the more difficult to interpret.

With neural evidence not yet helpful, we must turn to behavioral evidence. Unfortunately, behavioral evidence of valenced experience varies considerably by species.^[98] This is true even for some closely-related species.^[99] For example, “urination and defecation in a new environment is a scent-marking behaviour in mice but a sign of fear in rats, and bulls show decreased corticosteroid response after tethering, while pigs show increased response” (Browning 2019: 156). Pain behavior even differs by breed in dogs (Dobromylskyj et al. 2000), mice (Mogil 2019),^[100] and presumably other less well-studied species. As one review puts it, “The challenge in developing pain scales for use in animals is that it requires detailed knowledge of many different species-specific behaviors” (Stasiak et al. 2003: 14).^[101] In phylogenetically distant taxa, behavioral responses to pain may be radically different.^[102]

Finally, we must carefully distinguish genuine differences in the intensity of experience from mere differences in what we take to be markers of the intensity of experience.^[103] We already encountered an instance of this problem in the section on the evolutionary function of valenced experience: prey animals display different behavior than predator animals in response to similar putatively painful experiences. It could be the case that evolution selected for prey animals with high pain thresholds because prey animals with lower pain thresholds were more likely to display the sort of weaknesses that got them attacked by predators. Alternatively, it might be that evolution merely selected for prey animals that were better at hiding their pain. Thus, in some contexts, differences in pain behavior may not be evidence for differences in pain experience.

Nonetheless, it’s a good idea to briefly survey the methods we might use to measure the intensity of valenced experience. If any of these methods proved even partially successful, that success could deliver action-relevant information. Given our large uncertainty regarding variation in the intensity of valenced experience across species and the potential importance of such variation, we ought to be willing to simultaneously pursue many different strategies for making progress in this area. In that vein, it might be worth following (and perhaps selectively supporting) the work of Morten Kringelbach’s lab (jointly housed at Oxford and Aarhus), Kent Berridge’s lab (Michigan), Gregory Berns’ lab (Emory), and Giandomenico Iannetti’s lab (jointly housed at the Italian Institute of Technology and University College London).

Below I overview three broad categories of measures: neurobiological markers of valenced experience, behavioral markers of valenced experience, and physiological markers of valenced experience.

Neurobiological Markers

Neurobiology appears to have the potential to shed light on valenced experience. If we can map the structure and function of the human brain and correlate differences in neural activity with self-reported differences in subjective experience, we may be able to identify the causal mechanisms that govern the intensity of pleasurable and painful experience. Some philosophers and neuroscientists believe that, once validated in humans, we can extend this approach to (some) nonhuman animals.^[104]

The neurobiological evidence suggests we should not expect big differences in the intensity of valenced experience between humans and other mammals.^[105] In humans, a dizzying array of brain regions have been associated to some degree with valenced experience: the orbitofrontal cortex, the dorsal frontal cortex, the anterior cingulate cortex, the primary somatosensory cortex, the nucleus accumbens, the ventral pallidum, the basolateral amygdala, the mesolimbic tegmentum, and probably others.^[106] Most of these same (or homologous) regions have been implicated in other mammals. Jaak Panksepp writes, “there is now abundant experimental evidence indicating that all mammals have negatively and positively-valenced emotional networks concentrated in homologous brain regions that mediate affective experiences when animals are emotionally aroused. That is what the neuroscientific evidence indicates” (Panksepp 2011a: 1). In a separate review, Panskepp writes, “The basic neurochemistries for emotional feelings, as far as we presently understand them, are essentially the same in all mammals” (Panksepp 2011b: 1797).^[107] New research has demonstrated similar brain connectivity and wiring across 123 mammalian species (Assaf et al. 2020). Summarizing what we know about the neuroscience of affect, Kent Berridge and Morten Kringelbach write, “The brain’s circuitry for affective reactions spans from front to back of nearly the entire brain. Much of this circuitry is remarkably similar between humans and other mammals” (Berridge & Kringelbach 2013).^[108]

An alternative to looking at differences in the activity of different brain regions is instead to look at differences in brain-wide neural oscillations. Neural oscillations (often called ‘brainwaves’ in the popular press) are rhythmic patterns of neural activity caused by interactions between neurons. Different types of neural oscillations produce different electrophysiological signatures, and the frequency of these oscillations can be measured noninvasively via electroencephalogram (EEG). Although EEG readings are a relatively superficial measurement of brain activity, they are nonetheless correlated with many interesting differences in cognitive state. (For example, a sleeping brain produces a characteristically different pattern of neural oscillations than an awake brain.) Traditionally, neural oscillations are divided into five categories, based on their frequency: delta band (1-4 Hz), theta band (4-8 Hz), alpha band (8-12 Hz), beta band (12-30 Hz), and gamma band (30-150 Hz).

There are reasons to think that valenced experiences might produce measurable differences in neural oscillations. One review summarizes the case for thinking pain experience will manifest in neural oscillations: “Pain is a complex phenomenon that serves to protect the body. To this end, the brain dynamically integrates sensory and contextual information to guide behavior that aims to limit and avoid harm. Thus, pain is essentially an integrative phenomenon. In the brain, integrative functions are served by neuronal oscillations and synchrony [1]. Therefore, understanding the role of oscillations in the processing of pain can shed new light on how functionally diverse processes merge into the experience of pain” (Ploner & Gross 2019: 450).

It has recently been discovered that variation in gamma band oscillations is correlated with differences in self-reported pain intensity in human subjects (Hu & Iannetti 2019).^[109] Importantly, gamma band activity also appears to be correlated with pain experience in mice (Tan et al. 2019) and rats (Peng et al. 2018). Gamma band oscillations thus appear to be one of the most promising biomarkers of pain across species, at least within mammals (Yue, Iannetti, & Hu 2020: 3478).^[110] Even supposing these correlations reflect a fundamental causal connection, there is as yet much work to be done before such data would allow us to quantify and compare the intensity of pain experiences across species. Still, this may be an area that merits more attention and perhaps carefully targeted funding in the future.

Behavioral Markers

The most obvious behavioral marker of valenced experience is a human’s self-report of the experience. Psychologists and clinicians have developed and utilized a number of pain scales and pleasure scales to measure the intensity of various experiences in humans.^[111] Although there are a variety of challenges concerning the reliable elicitations of subjective experience,^[112] human self-reports nonetheless constitute crucial and invaluable data. Carefully calibrated self-reports can help us appreciate which circumstances typically engender the most intense valenced experiences in humans. Understanding the nature of the human intensity range will help us compare that range to those of nonhuman animals.^[113]

Since self-reports are unavailable for the vast majority of animals,^[114] we must look to less obvious behaviors to infer differences in the intensity of various experiences. Unfortunately, there appear to be few, if any, good options. Behavioral pain scales have been developed for a handful of species. For instances, Lynne Sneddon reports that “grimace scales based on changes in facial expression using Facial Action Coding Schemes have been developed for a variety of mammals including rats [7], mice [8,9], rabbits [10], horses [11] and piglets [12]” (Sneddon 2019: 2). However, these scales are at present rather coarse-grained, and it’s unclear if they are able to adequately capture relevant differences in the intensity of different types of experiences. (Even with the aid of self-reports, measuring the intensity of human experiences is fraught with both theoretical and practical challenges.) More importantly, there is as yet no way to compare these scales across species.

Alternatively, we could look at willingness-to-pay studies, in which a nonhuman animal makes tradeoffs that reveal the strength of its preferences (Kahnau et al. 2020: 5).^[115] ‘Payment’ can be quantified in a number of ways: how long and at what magnitude an animal will endure an electric shock to access some reward,^[116] how much food an animal is willing to forgo to access some reward, or how much work (operationalized in terms of lever depressions, say) an animal will perform to access some reward. These sorts of tests have been conducted on a wide range of animals including mammals, birds, fish, insects, and crustaceans. The results of these experiments may give us insight into the relative intensity of different sorts of experiences.^[117] However, like other behavioral measures, the problem of interspecies comparisons remains (Browning 2019: 138).^[118]

Physiological Markers

Physiological markers of valenced experience can be conceptualized as lying midway between the neural mechanisms that govern pains and pleasures and the voluntary behaviors that animals undertake in response to those experiences. Involuntary changes in an animal’s heart rate, blood pressure, pupil dilation, perspiration, hormonal levels, or metabolic activity may indicate the presence of valenced states. Differences in the magnitude of such changes may indicate differences in the intensity of the associated experiences. This sort of physiological activity may be more easily studied in nonhuman animals than neural activity, and if these physiological changes are truly involuntary, they may be more reliably observed than voluntary behaviors.

Recent research has attempted to identify physiological markers of pain intensity in humans. Physiological activity, however, is affected by many factors. No single physiological variable is likely to track the intensity of pains with much precision. In response to this problem, researchers have created composite algorithms that combine and weight information from a variety of physiological sources. Using large datasets and sophisticated statistical modelling, researchers hope to identify subtle shifts in physiological activity that mirror changes in pain intensity. According to one review, “These multi‐variable approaches appear to be superior predictors of pain intensity and intra‐operative nociception to any individual parameter alone” (Cowen et al. 2015: 838). If this approach is validated in humans, it could plausibly be extended to closely related nonhuman animals.

Ultimately, however, even these composite algorithms have failed to deliver a measure that is reliable enough to be used in clinical settings.^[119] In the same review, Ruth Cowen and her collaborators conclude, “Although clinically needed and theoretically promising, currently there is not enough evidence to support the widespread use of any physiological markers as ‘objective’ measures of pain and nociception. This is despite recently increased efforts, raising the question whether this is possible in the foreseeable future” (Cowen et al. 2015: 840). Nonetheless, Cowen and her collaborators end the review on a hopeful note, observing that “there are some promising avenues on the horizon. Biomarker research as part of clinical phenotyping, and the development of composite algorithms, should be closely watched” (ibid.).

Conclusion

The moral significance of a pain or pleasure varies, in part, with its intensity. A cluster headache typically demands more of our moral attention than a stubbed toe. The joy of holding one’s child for the first time is typically worth more, morally, than the pleasure of a good meal. We intuitively and naturally recognize such differences in our own lives and in the lives of those close to us. Our everyday decision-making would be much worse if we did not account for differences in the intensity of experiences.

Like all our subjective experiences, valenced experiences are the product of intricate neural mechanisms that have been shaped by countless generations of natural selection. At a higher level of abstraction, the structure of our social, emotional, and intellectual nature contributes in complex ways to the intensity of our feelings. Given the enormous diversity of the (plausibly sentient portion of the) animal kingdom, it would be surprising if (sentient) animals were uniformly capable of the exact same magnitude of pleasures and pains.

Differences in intensity range could entail big and unrecognized differences in the cost-effectiveness of interventions targeting different species. We want our resources to be allocated in the way that maximizes improvements to welfare. To properly compare interventions that aid phylogenetically distant species—pigs versus carp, say, or chickens versus honey bees—we need to know, at least roughly, how the experiences of those animals are likely to differ.

Finally, thinking about differences in the intensity range of valenced experience might help us better judge the ideal split between (neartermist^[120]) human-focused spending and spending on nonhuman animals. In many circles, it is taken for granted that humans have a larger capacity for welfare than nonhuman animals. This is a plausible claim, but plausibility does not entail truth: we must weigh the evidence as impartially as we can. There is at least tentative reason to believe that the intensity range of valenced experience doesn’t vary dramatically among mammals, including humans (though of course capacity for welfare encompasses more than the intensity of valenced experience). In any event, that humans have a larger capacity for welfare than nonhuman animals is a less informative claim than the claim that humans have x times as large a capacity for welfare as nonhuman animals (of type y). There is widespread disagreement about what the plausible range of x is (for a given y). Further investigation into differences in the intensity of valenced experience across species may help us narrow that range, which in turn will help us evaluate the degree to which we want to prioritize human welfare over nonhuman animal welfare.^[121]

There is as yet much we do not know about differences in the intensity of valenced experience across species. In Appendix 1 I have compiled a partial list of open research questions. There is little reason to think that our current distribution of resources is perfect, and many reasons to think that our understanding of the nature and moral worth of nonhuman animal experiences may change dramatically over the next century.

Credits

This essay is a project of Rethink Priorities. It was written by Jason Schukraft. Thanks to Marcus A. Davis, Michael Dickens, Holly Elmore, Persis Eskander, Derek Foster, Peter Hurford, David Mathers, and Gavin Taylor for helpful feedback on earlier drafts. If you like our work, please consider subscribing to our newsletter. You can see all our work to date here.

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Appendix 1: Open Research Questions

1. Which animals have the capacity for valenced experience?

2. Is valence incomparably multidimensional or is a unidimensional analysis of valence appropriate?

3. Did valenced experience evolve once or multiple times?

4. What is the evolutionary function of valenced experience? Does valenced experience serve the same role in all animals capable of valenced experience?

5. Do animals with shorter lifespans have reduced pain sensitivity or increased pleasure sensitivity (compared to similar animals with longer lifespans)? Neither? Both?

6. Which nonhuman animals are capable of mental time travel?

7. Which nonhuman animals can reason normatively?

8. Which influences learning more: the strength of the stimulus/reward or the statistical regularity of the stimulus-reward pattern?

9. Are intensely pleasurable experiences as distracting as intensely painful experiences? If not, why not? Can we quantify level of distraction and use it to estimate the intensity of a valenced experience?

10. Is it a contingent empirical fact that intense experiences are distracting, or is intensity partly constituted by its grip on the subject’s attention?

11. What are the neurobiological constraints (if any) that impose a limit on the maximum intensity of a valenced experience?

12. How large is the typical gap between an animal’s characteristic intensity range of experience and an animal’s possible intensity range of experience?

13. How do nociceptive thresholds differ across species, and what drives this variation?

14. Are pleasures judged to be undeserved perceived to be less intense than comparable pleasures either judged to be deserved or lacking a normative evaluation altogether?

15. Are pains judged to be endured for good reason perceived as less intense than pains that are not so judged?

16. Under what conditions (if any) does stress or anxiety increase pain sensitivity?

17. Does focusing on a pleasure increase the intensity of the pleasure?

18. How does the anticipation of a pain/pleasure influence the intensity of the pain/pleasure?

19. How does the pain behavior of prey animals differ in the presence vs. the absence of their predators? Is there a corresponding neurobiological difference?

20. How does the pain behavior of prey species compare to similar non-prey species? Is there a corresponding neurobiological difference?

21. Which nonhuman animals are capable of deliberately redirecting their focus and attention?

22. When someone ‘learns to live’ with chronic pain that is initially debilitating, does the person experience a reduction in pain or is the pain experience the same, the person just learns to cope with it?^[122]

23. How asymmetrical are intensity ranges? How does this vary across species? What drives this variation?

24. Are there any animals whose possible valence range is exclusively negative? Exclusively positive?

25. How often do combination effects affect the intensity of valenced experience? What are the most common combination effects?

26. What is the difference in suicide rates for people experiencing purely emotional distress (perfect health but severe depression, say) versus people experiencing purely physical distress (chronic intense pain but no depression, say)?^[123]

27. Is sadness more likely in social animals?

28. Does intensity of valenced experience change with life stage (especially in animals like insects with radically different life stages)?

29. What circumstances typically induce the most intense valenced experiences in humans?

30. Which nonhuman animals, if any, are capable of directly communicating the nature of their inner experiences to us?

Notes