Rethink Priorities’ Moral Weight Project ran from May 2021 to February 2023. During that time, we developed a from-first-principles method for estimating differences in animals’ capacity for welfare. In the year and a half since we finished the project, we turned it into a book for Oxford University Press: Weighing Animal Welfare: Comparing Well-being Across Species. We’re pleased to announce that it’s available now and free to access.

Why did a project about “moral weight” focus on differences in capacity for welfare? In this context and very roughly, a moral weight is the adjustment that ought to be applied to the estimated impact of an animal-focused intervention to make it comparable to the estimated impact of some human-focused intervention. Given certain (controversial) assumptions, differences in capacity for welfare just are moral weights. But in themselves, they’re something more modest: they’re estimates of how well and badly an animal’s life can go relative to a human’s. And if we assume hedonism—as we did—then they’re something more modest still: they’re estimates of how intense an animal’s valenced states can be relative to a human’s. The headline result of the Moral Weight Project was something like:

While humans and animals differ in lots of interesting ways, many of the animals we farm can probably have pains that aren’t that much less intense than the ones humans can have.

I, for one, stand by that claim.

Much of the book is adapted from material we first shared here. A great deal is new. The basic methodology remains unchanged, though we’ve worked hard to clarify it, provide better motivations for it, and highlight its limitations.

As we say in the conclusion:

Our estimates will change as we learn more about all animals, human and nonhuman. They will change as we learn more about the various traits we share with nonhuman animals and the various traits we don’t share with them. They will change with advances in comparative cognition, neuroscience, philosophy, and various other fields. In sum, then, we’re under no illusions that we’re providing the last word on this topic. Instead, we’re providing a starting point for more rigorous, empirically-driven research into animals’ [capacity for welfare]. At the same time, we hope we’re offering guidance for decisions that have to be made long before that research is finished.

To make good on that, we’ve been figuring out how to integrate our work into existing decision-making, ranging from intra-animal cause prioritization to benefit-cost analysis. It would be better, of course, if we didn’t face tradeoffs involving animals. But since we do—and will for the foreseeable future—we ought to make them explicitly and thoughtfully. We hope that Weighing Animal Welfare is a valuable step in that direction.

Acknowledgments

Rethink Priorities is a think-and-do tank dedicated to informing decisions made by high-impact organizations and funders across various cause areas. Bob Fischer wrote this post. Thanks to the many contributors to the Moral Weight Project for their work.