Duncan, an inquisitive 8-year-old, saw his grandfather’s dentures soaking overnight in a glass of water. he asked his grandfather how his real teeth had fallen out. assuming the grandfather remembered the events correctly but was not a medical expert and used layperson’s terms, reconstruct the kind of story the man is likely to have told.

The grandfather would have told him that his teeth fell out because they could not be held in place due to his old age.

Explanation:

The grandfather would have told the kid that he was getting old and as a consequence his body was getting weaker. He would say that the teeth were especially prone to be affected because his mouth had lost the gums and supporting structures of teeth. This most likely causes his teeth to fall out.

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.