The world according to Trivers
Why do we take care of each other?
Sometimes, Trivers argues, it's because we expect to be repaid later. Most of the time, though, we're just looking out for ourselves, or at least our genes. If evolution is a matter of so-called "selfish genes," then it makes sense to make sacrifices for our kin, who share a lot of the same genetic material. The closer the kin, the greater the sacrifice we're willing to make. In some species, this creates odd arrangements. For example, because of a quirk of their reproductive biology, female ants are more closely related to their sisters than to their own offspring, which may explain why they're willing to forego reproducing to help raise their siblings.
Why do we deceive ourselves?
Whether it's convincing a predator that you're a leaf or fooling another bird into raising your young, deceit is an evolutionary strategy with a long and innovative history. But as evolution selects for better and better cheaters, it should also select for better and better cheating detectors. For example, Trivers argues, humans might have evolved to detect the sort of nervous tics that betray a lie. But there's a counter-strategy: self-deception. If we don't know we're lying, then we won't act like we're lying, and are more likely to get away with it.
Why do babies cry?
Often, of course, it's because they're hungry or tired. Sometimes, though, it's just for attention. Human babies aren't the only ones: Young chimpanzees and pelicans have been observed having temper tantrums. It's part of a strategy that grows out the divergent interests of parents and offspring, Trivers argues. While parents have an equal interest in the well-being of all of their children, each offspring wants the largest possible share of parental resources. But there's a built-in limit: The more that offspring cry wolf, the better parents get at ignoring their fussing.
Why are men bigger than women?
Since in most animal species the male invests less time and energy in the offspring, Trivers argues, his cost per offspring is lower (often much lower) and it's to his evolutionary advantage to mate widely. Of course, many males mating widely makes for competition over females, and in many species the bigger, stronger males will have an advantage over smaller, weaker ones. Females, on the other hand, aren't competing but being competed over, so there's less pressure for size. In fact it can be a disadvantage to waste resources on excess growth. Better to devote them to raising offspring or living longer – which may also explain why women have longer lifespans. 
