There’s an interesting evolutionary psychology paper in the new Nature. It’s by Tooby and Cosmides, and it investigates the roots of the incest taboo. The researchers found that, on average, our repulsion at the idea of having sex with a sibling correlates with two variables: how long we lived with that sibling and how long we watched our mother care for that sibling (their “perinatal association”). We use these two variables to compute a “kinship index” that “corresponds to an estimate of genetic relatedness between self and other.” Siblings with a high “kinship index” not only triggered the incest taboo – the idea of having sex with them is disgusting – but exhibited increased altruistic behavior. That’s a nice proof of kin selection.