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james works at an automobile factory. every day he goes to work and inspects the machines at the factory to make sure they are running smoothly. at the end of the every week, james is paid by his boss, bill, for his good work. james takes his paycheck to the bank every friday and deposits most of it, but keeps a little bit of spending money. he then takes his wife, julie, out for dinner, using the leftover money from his paycheck.

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.

the branch of biology that deals with the normal functions of living organisms and their parts.

the way in which a living organism or bodily part functions.

answer; no longer than a reasonable period of time;

answer; colony;