Humans first: Why people value animals less than humans

People routinely give humans moral priority over other animals. Is such moral anthropocentrism based in perceived differences in mental capacity between humans and non-humans or merely because humans favor other members of their own species? We investigated this question in six studies (N = 2217). We found that most participants prioritized humans over animals even when the animals were described as having equal or more advanced mental capacities than the humans. This applied to both mental capacity at the level of specific individuals (Studies 1a-b) and at the level typical for the respective species (Study 2). The key driver behind moral anthropocentrism was thus mere species-membership (speciesism). However, all else equal, participants still gave more moral weight to individuals with higher mental capacities (individual mental capacity principle), suggesting that the belief that humans have higher mental capacities than animals is part of the reason that they give humans moral priority. Notably, participants found mental capacity more important for animals than for humans-a tendency which can itself be regarded as speciesist. We also explored possible sub-factors driving speciesism. We found that many participants judged that all individuals (not only humans) should prioritize members of their own species over members of other species (species-relativism; Studies 3a-b). However, some participants also exhibited a tendency to see humans as having superior value in an absolute sense (pro-human species-absolutism, Studies 3-4). Overall, our work demonstrates that speciesism plays a central role in explaining moral anthropocentrism and may be itself divided into multiple sub-factors.