Agroup of chemists, who have been working together on a new diet pill for the last four years, meet to discuss their new drug. over the years, they have all come to respect each other and have formed a close bond. upon discussing the effectiveness and safety of the diet pill, many members of the group found themselves agreeing with most of the comments that were made. somewhere along the way, no one seemed to pay much attention to the fact that some of the subjects in the clinical trials of that drug became ill. it seemed that because nobody wanted to disrespect anyone else, the topic of ill subjects was dropped. what psychological phenomenon occurred here?

as university of miami psychologists debra lieberman and adam smith pointed out in a recent article in the journal current directions in psychological science, humans have social and psychological mechanisms to deter incest. with very few exceptions, marriages between brothers and sisters and between parents and their children are verboten in every human culture. the primary psychological anti-incest mechanism is the yuck response. even the idea of sex with their mom or dad, bro or sis is upsetting to most people. the psychologist jonathan haidt has found that nearly everyone is repelled by the prospect of brother-sister sex, even in hypothetical situations in which there is no chance of pregnancy).

the biological cost of incest

this raises an interesting question: just what’s so bad about incest? sure, having sex with your dad or your sister seems gross — but why? some anthropologists have argued that incest taboos are learned social conventions. this explanation, however, doesn’t make sense to me as it does not explain the widespread existence of anti-incest mechanisms in creatures ranging from cockroaches to chimpanzees. second, the incest taboo is about as close to a universal law as human moral rules get.

why should mechanisms to avoid incest be so widespread both in nature and across human societies? the answer is simple. the problem with having sex close with relatives is that there is an astonishingly high chance that your offspring will be born with a serious birth defect.