Sexual activity between persons of closer blood relationship than the culture allows.The degree of relationship varies considerably from society to society. In some societies marriage between cousins or between uncles and nieces, aunts and nephews is prohibited, in others it is permitted. Privileged groups have been exempted from incest taboos in some cultures—for example, during the Ptolemaic period in Egypt marriage between brother and sister in the royal family was not only permitted but required.Incest taboos of one kind or another have been found in practically every society. At one time these prohibitions were attributed to the dangers of in- breeding, but today this theory is largely discredited because it applies only when there are latent hereditary defects in the family line. A more widely accepted explanation is that incest creates rivalries that disrupt family life and prevent the society from enlarging and strengthening itself through outside relationships.The prevalence of incest taboos indicates that the urge to form these relationships must be widespread. In his theory of psychosexual development, Freud held that every child experiences incestuous impulses directed toward the parent of the opposite sex, although during the latency period an “incest barrier” is ordinarily erected, which deflects sexual interest from the parents to other people and activities. Recent investigations, however, tend to favor environmental explanations. It has been found that incestuous relationships occur most frequently where living quarters are crowded, family moral standards lowest, and opportunities for sexual experimentation constantly present themselves. This view is supported by the fact that the practice appears to be most common in the lower socioeconomic groups where brother and sister frequently share the same room and the father lives in close contact with adolescent daughters. Other precipitating factors in father-daughter incest are disturbed marital relationships, death or absence of the wife, and alcoholism. In some cases this form of incest is the result of a lowering of ethical and emotional controls due to psychosis, senile deterioration, manic reaction, or paresis. See OEDIPUS COMPLEX.It is difficult to obtain accurate figures on incest, but the incidence is believed to be relatively low in our society. Mother-son incest is rare, usually occurring only where one or both are psychotic. Brother-sister sex play is not uncommon, but a full incestuous relationship seldom occurs. Father-daughter incest appears to be considerably more common than any other type, and is the most frequent form among men who have been convicted of sexual offenses. Incest frequently gives rise to emotional disturbance, particularly when it occurs during adolescence rather than earlier. Some young people develop severe conflicts as a result of fantasies centering around incestuous wishes. In cases where girls have been forced into sexual relations with their father or an older brother, the experience is often so traumatic that it leads to intense guilt feelings and a lasting revulsion toward sex. Some girls who have tolerated or accepted the relationship develop feelings of degradation and sin only after they have become more mature. On the other hand, many girls who have had incestuous experiences at home adopt a permanent pattern of promiscuous behavior.