In the discussion of father-daughter and brother-sister incest, we need to appreciate that the preceding observations on mother-son incest are exceptional; that is, a special psychobiological bond exists between mother and child in the juvenilized human species which imbues mother-child relationship with peculiar extra-dimensional features.

Father-daughter incest is by far the most common form of incest in society. We may exclude the disproportionately large incidence of stepfather-stepdaughter incest in the statistics (an estimated 50-75% of reported cases). The relatively high incidence of father-daughter incest is understandable in male dominated society and explains the fact that in most cases the male parent coerces his daughter into the relationship. The Westermarck effect also correctly predicts that in most cases the father and daughter have lived apart from each other for a significant period of their lives. Statistical data shows that fathers who sexually abused their daughters, on the average, spent less time with them than other fathers when the daughter was growing up. Similar statistics suggest that stepfathers who had been with their stepdaughters most of their childhood tend less to abuse them after sexual maturity.

Brother-sister incest is less common than father-daughter incest, but may well tend to 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 relationships; and once again it would appear that the effect is most pronounced with siblings who grow up together. The Israeli kibbutzim of the early twentieth century demonstrated the fact that the key de-sexualizing factor in brother-sister relationship is the fact of an early childhood experience of growing up together. It is reported that in parts of Lebanon, paternal cousins grow up together like siblings but often come under pressure to marry. They are however reported to usually have little sexual interest in each other. It is also reported that in China, parents who adopted a bride for their sons in early childhood and had the children grow up together have trouble convincing them to marry when they are old enough.

Proponents of the Westermarck effect enjoy pointing out that the best known incest offender of all time, Oedipus, had also not grown up under his mother’s care.