In discussing father-daughter and brother-sister incest, we must appreciate that the above remarks on mother-son incest are exceptional; that is, there is a special psychobiological bond between mother and child in the juvenile human species that imbues the mother-child relationship with peculiar extradimensional characteristics.
Father-daughter incest is by far the most common form of incest in society. We can exclude the disproportionately large incidence of stepfather-stepdaughter incest in the statistics (an estimate of 50-75% of reported cases). The relatively high incidence of father-daughter incest is understandable in a male-dominated society and explains the fact that in most cases the father coerces his daughter into the relationship. The Westermarck effect also correctly predicts that, in most cases, father and daughter have lived apart for a significant period of their lives. Statistical data shows that fathers who sexually abused their daughters, on average, spent less time with their daughters than other fathers when the daughter was growing up. Similar statistics suggest that stepfathers who had been with their stepdaughters for most of their childhood tend to abuse them less after sexual maturity.
Brother-sister incest is less common than father-daughter incest, but may be less frequently reported than father-daughter incest. The Westermarck effect is most clearly demonstrated in the tendency of brothers and sisters to avoid sexual intercourse; and once again it would appear that the effect is more pronounced with siblings growing up together. Israeli kibbutzim of the early 20th century demonstrated the fact that the key desexualizing factor in the brother-sister relationship is the fact of an early childhood experience of growing up together. In some parts of Lebanon, paternal cousins are reported to grow up together as siblings, but are often pressured to marry. However, they are reported to generally have little sexual interest in each other. It is also reported that in China, parents who adopted a girlfriend for their children in early childhood and raised them together have trouble convincing them to marry when they are old enough.
Proponents of the Westermarck effect take delight in pointing out that the best-known incest offender of all time, Oedipus, did not grow up in his mother’s care either.