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. Take the results of a study of Czechoslovakian children whose fathers were first degree relatives. Fewer than half of the children who were the product of incestuous unions were completely healthy. Forty-two percent of them were born with severe birth defects or suffered early death and another 11 percent were mildly mentally impaired. This study is particularly instructive as it included a unique control group — the offspring of the same mothers but whose fathers were not the mothers’ relatives. When the same women were impregnated by a non-relative, only 7 percent of their children were born with a birth defect (Figure 1).
A group of genetic counselors reviewed the research on the biological consequences of sex between relatives (consanguineous relationships) (here). They found a surprisingly small increase (about 4 percent) in birth defects among the children of married cousins. Incest between first degree relatives, however, was a different story. The researchers examined four studies (including the Czech research) on the effects of first degree incest on the health of the offspring. Forty percent of the children were born with either autosomal recessive disorders, congenital physical malformations, or severe intellectual deficits. And another 14 percent of them had mild mental disabilities. In short, the odds that a newborn child who is the product of brother-sister or father-daughter incest will suffer an early death, a severe birth defect or some mental deficiently approaches 50 percent.