The fourth explanation is captured in the title of a book called "The Expanding Circle," by the philosopher Peter Singer, who argues that evolution bequeathed humans with a sense of empathy, an ability to treat other peoples' interests as comparable to one's own. Unfortunately, by default we apply it only to a very narrow circle of friends and family.
第四种解释含在书题中 ”扩张的圈子“，由哲学家彼得辛格所写， 他争辩道进化遗留给人类 同情的感情：一种将他人的利益作为 自己的利益的一种能力。不幸的是，在默认状态下， 我们只将这种能力局限于朋友和家人的狭窄圈子中。