Empathy is a complicated emotion, even for mice. On seeing another in pain, a mouse will act as if it itself is also hurting—much more, though, if it knows the first mouse. Capuchin monkeys will help out another monkey, without any reward, but only if they’re on friendly terms.
People also feel less empathy for those they dislike. But our species adds another layer of complication: We empathize more with people who are “like us” than with “them.”