"[Evolutionary biologist Robert] Trivers’ first paper, on the evolution of reciprocal altruism, described a theoretical model showing how altruism among strangers could naturally develop—people cooperate with the expectation of similar treatment from others. This model explained a wide variety of feelings and behaviors, from friendship to moralistic aggression. The emotion of gratitude, for instance, evolved to motivate us to return favors, encouraging cooperation. Guilt motivates us to repair relationships we’ve harmed. Anger makes us avoid or punish those who have harmed us. And gossip makes us mindful of our reputations. Trivers suggested that complex strategies of cheating, detecting cheating, and the false accusation of cheating (itself a form of cheating) pushed the development of intelligence and helped increase the size of the human brain.
Next, in Trivers’ second paper, he hypothesized that a single factor drives sex differences across all species. He argued that differences in parental investment—the energy and resources invested in an offspring—lead the sex that invests more (females, in most species) to focus on mate quality and the sex that invests less (males) to seek quantity. So in humans we expect choosiness in females and aggression between males as they vie for females. The theory has tremendous explanatory power, from justifying the brightly colored feathers of male birds to illuminating why sexual jealousy is a leading (and, until recently, legally defensible) cause of homicide—men prize their mate’s fidelity above all.
In another paper, Trivers conceptualized offspring not as passive recipients of parental investment, but as independent actors, generating the theory of parent-offspring conflict. A child wants disproportionate attention and resources for him- or herself, but a parent wants to spread the goods equally between all offspring. And so we have kids who bawl until they get what they want, siblings who maintain lifelong rivalries, and parents who try to instill equality no matter how selfish the kids’ tendencies. It was for these three papers, plus another two, on insect colonies and on parents’ ability to vary the sex ratio of their offspring, that he won the Crafoord.
After writing papers addressing how we treat strangers, friends, lovers, parents, and children, Trivers offered a no-less-powerful theory on how we deal with ourselves. In a sentence in the foreword to Dawkins’ book, he proposed that self-deception evolved to facilitate the deception of others. Trivers says he’d planned to flesh out the theory but didn’t get around to it because he was “smoking too much strong herb.”"