Figures on incidence are difficult to secure. According to the early reports of Kinsey and colleagues, approximately 4 percent of American males are exclusively homosexual and 8 percent have been “more or less exclusively homosexual for at least 3 years, sometime between the ages of 16 and 65.” For females, the incidence is lower, perhaps half that for males. It was estimated, on the basis of the examination of large numbers of military personnel during World War II, that 1 to 2 percent of servicemen were exclusively or predominantly homosexual. More recent estimates, both in men and women, range from 1 to 5 percent (see LeVay and Hamer). These widely variable figures share a problem with all estimates derived from surveys and questionnaires: they cannot count people who do not wish to be counted.
The origins of homosexuality are obscure. The authors favor the hypothesis that differences or variations in genetic patterning of the nervous system (possibly of the hypothalamus) set the sexual predilection during early life. Several morphologic studies of the hypothalamus are significant in this regard. Swaab and Hofman have reported that the preoptic zone is three times larger in heterosexual males than it is in females, but it is about the same size in homosexual males as it is in females. As mentioned in Chap. 26, LeVay found that an aggregate of neurons in the suprachiasmatic nucleus of the hypothalamus is two to three times larger in heterosexual men than it is in women, and also two to three times larger in heterosexual than in homosexual men. If confirmed, these findings, which have been disputed by Byne and others, would support the view that homosexuality has a biologic basis. Genetic studies point in the same direction. Pooled data from 5 studies in men show that approximately 57 percent of identical twins (and 13 percent of brothers) of homosexual men are also homosexual. The figures for lesbians are much the same. In most studies, the inheritance pattern of male homosexuality comes from the maternal side, implicating a gene on the X chromosome (LeVay and Hamer) but to suggest there is a simple genetic connection is oversimplified.
Psychoanalytic explanations of homosexuality have never been substantiated. Attempts to demonstrate an endocrine basis for homosexuality have also failed. The most widely held current view is that homosexuality is not a mental or a personality disorder, though it may at times lead to secondary reactive disturbances. The studies of Kinsey and colleagues indicate that a homosexual orientation cannot be traced to a single social or psychologic root. Instead, as indicated earlier, homosexuality seems to arise from a deep-seated predisposition, biologic in origin and as ingrained as heterosexuality. The status of bisexuality is undetermined.
The Development of Personality and Social Adaptation (See Also Chap. 50)
Personality, the most inclusive of all psychologic terms, encompasses the entirety of psychologic traits that distinguish one individual from every other. The notion that one’s physical characteristics are determined by inheritance is a fundamental tenet of biology. One has but to observe the resemblances between parent and child to confirm this view. Just as no two persons are physically identical, not even monozygotic twins, so, too, do they differ in any other refined quality one chooses to measure, particularly those that determine behavior and modes of thinking. Strictly speaking, the normal person is an abstraction, just as is a typical example of any disease.