Both reactions impart a powerful sensation of well-being. Oxytocin creates a feedback loop of good-feeling and group loyalty that ultimately leads members to “self-sacrifice to promote group welfare,” in the words of one study. Hominids that cooperated with one another—and punished those who didn’t—must have outfought, outhunted, and outbred everyone else. These are the hominids that modern humans are descended from.
