 Evolutionary explanations are commonplace when it comes to questions about our physiological nature—why we have opposable thumbs, say, or a bipedal posture. But even a brief look at other animals affords countless examples of adaptive psychological and behavioral traits as well—appetites for food or sex, fear responses, patterns of aggression, parental care and bonding, or patterns of cooperation and retribution; and these traits are likewise often best explained as biological adaptations, i.e., traits that evolved through natural selection due to their adaptive effects. (We will go through a striking example involving cooperative behavior in section 2.2.) This raises the question: to what extent might human psychological and behavioral traits similarly reflect our own evolutionary heritage?