I would guess most arguments for global health and poverty over animal welfare fall under the following:
- animals are not conscious or less conscious than humans
- animals suffer less than humans
See A Debate on Animal Consciousness, Why might one value animals far less than humans? and this thread here.
The arguments for the former mostly depend on nonhuman animals not being sufficiently self-reflective in the right way, and the arguments for the latter depend on brain size/complexity or particular welfare-relevant capacities that humans have that nonhuman animals lack or have less of, e.g. preferences for the future.
But also see Rethink Priorities' research:
https://www.rethinkpriorities.org/publications#moralweight
https://www.rethinkpriorities.org/invertebrate-sentience-table
ASENT's research:
https://www.lse.ac.uk/cpnss/research/ASENT
And Luke Muehlhauser's older work:
https://www.openphilanthropy.org/2017-report-consciousness-and-moral-patienthood
https://www.lesswrong.com/posts/2jTQTxYNwo6zb3Kyp/preliminary-thoughts-on-moral-weight
we can't know how much we are improving animal suffering
There's also skepticism about the specific interventions supported to help nonhuman animals, because their welfare effects actually seem plausibly bad, or because the evidence is just much less rigorous, or because of interactions with wild animals (as discussed in this thread).
My own view is that farmed vertebrates are very likely to be conscious and brain size/complexity is relevant and probably the most important factor, and I would tentatively use something between square root and linear weighting for brain regions involved in hedonistic experience. I also think that nonhuman animals (farmed and wild vertebrates) suffer about as much in practice as some of the worst off humans in the world, because of the horrible conditions they face. I also worry about the meat-eater problem, so that helping humans may harm nonhuman animals. Overall, I currently prioritize helping farmed chickens, and doing research on how to best help them and other nonhuman animals (including prioritizing between species).
On estimates, from Lewis Bollard's recent AMA:
Rethink Priorities' estimate "Corporate campaigns affect 9 to 120 years of chicken life per dollar spent" (on average, not on the margin).
Founders Pledge also did a direct comparison between The Humane League and the Against Malaria Foundation here. They've done their own research, and summarizing others' and theirs together: "Cost-effectiveness analyses have found that for every dollar donated, the lives of between 10 and 160 birds are affected."
Charity Entrepreneurship has some research on this, and Animal Charity Evaluators has estimates of number of animals affected by corporate campaigns for specific charities, too; you can see their reviews of recommended charities working on corporate campaigns, and this model and this spreadsheet. (I'm an intern at ACE, but only speaking for myself.)
But also see Global poverty could be more cost-effective than animal advocacy (even for non-speciesists) by Peter Hurford.
Thank you so much!! This is really helpful and I'm taking a look at it now, and that last article looks like it gets to the center of my concern.