Another Boring Just-so Story

 
From ScientificAmercian.com.

Child molestation and rape top the social taboo list, according to a survey of 186 people between the ages of 18 and 47, and smoking marijuana ranks lowest among the 19 choices of forbidden behavior. In the middle—worse than robbing a bank but better than spousal murder—lies incest between brothers and sisters. Given the deleterious genetic impacts of offspring from such mating, some researchers have suggested that there may be an evolved mechanism designed to prevent that from occurring. And now evolutionary psychologist Debra Lieberman of the University of Hawaii–Honolulu believes she may have elicited some of its functions from this simple questionnaire....

The evolutionary psychologists hypothesize that some form of mental mechanism assesses various cues to come up with an estimate of how related two people are. "The real question is: What are these cues?" Lieberman says. "A potent cue is seeing your mom caring for a newborn. That would have served as a great cue that the infant is a sibling, at least a half sibling." But for younger siblings, who would have no opportunity to make this observation, another cue might be the amount of time spent living with another child/potential sibling. Dubbed the "Westermarck hypothesis"—after the Finnish sociologist who first noted it in a book published in 1889—it posits that children reared together do not often end up being sexually attracted to each other.