Tl;dr: You need to make specific moral weight assumptions to decide who to prioritize between animals with different degrees of sentience. And the moral weight of a given being is not its p(sentience). One could very well be certain that shrimp and insects feel pain (and are as badly off as they could possibly be), yet believe that they do so to a degree too small for them to be prioritized. (EDIT: Note that I do not mean to defend this view.) Debates about interspecies tradeoffs should outline welfare ranges, over p(sentience), as the relevant crux.

When people defend prioritizing cognitively simpler animals over more complex ones, they tend to appeal to i) their numbers, ii) evidence of their sentience, and iii) some form of expected value maximization, even if slightly risk-averse.

While I am pretty sympathetic to prioritizing cognitively simpler beings, I think the above appeal misses the key crux, by mistakenly making (ii) about p(sentience) rather than about degree of sentience, or welfare range. For the conclusion to follow, (ii) needs to be: evidence of a welfare range that is not too insignificant to crucially undermine the importance of their large numbers.

It is quite easy to believe that shrimp and many insects are likely sentient.^[1] In fact, let us, from here on, assume that they are, without doubt. What may be a harder bullet to bite is that their degree of sentience is high enough for their numbers to “carry you the rest of the way”. Maybe you think it’s obvious that what a pig experiences when being pushed into a slaughterhouse is nothing compared to the suffering of millions of mealworms (or whatever the fairest to-scale comparison is).^[2] But you need an argument.

And finding a decisive one is non-trivial. It is already astonishingly hard to estimate how many pinpricks could be worse than torture at the individual level.^[3] Here, figuring out who to prioritize between, say, mealworms and pigs, not only requires overcoming the high uncertainty this sort of question brings, but also the extra uncertainty of how to compare welfare across species (i.e., how many pig pinpricks is equivalent to a malnourished mealworm).^[4] So while I largely share Browning and Veit’s (2023) optimism about our scientific ability to determine, with non-trivial confidence, which beings are sentient, I think r...

On moral weights and cross-species tradeoffs