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The phenomenon of homosexuality is widespread throughout history and society. Recent studies have estimated the prevalence of predominant homosexuality in adult males of Western societies to be in the range of 2-8%.

As early as the turn of the century, Krafft-Ebing (1922) suggested that sexual orientation was genetically determined, and the recent development of human sociobiology has led to several other theories which link homosexuality exclusively to genetically-controlled processes.

The use of twin studies provides us, in theory, with a method of investigating the relative importance of genetic and environmental factors in the genesis of homosexuality. Three notable twin studies in this area have been conducted to date, by Kallman (1952), Heston and Shields (1968) and Bailey and Pillard (1991). However, the former pair of studies were based on highly biased samples so that their findings cannot be generalized to a larger population. Bailey and Pillard's results suggested that homosexuality has a high component of heritability. Many smaller studies of the sexual orientation of twins have also been reported, which, while not widely generalizable, often give insights of differences in the rearing environment experienced by members of a twin pair which can lead to divergent orientation in adulthood. Eckert et al. (1986) studied sexual orientation in identical twins reared apart, and found concordance in a male pair, but discordance in three female pairs. However, many methodological problems are associated with such investigations.

Several reports have suggested that homosexuality is not a diagnostic entity, but rather that several types of homosexuality, each with a different combination of aetiological factors, may exist. There is some evidence that the determinants of sexual orientation in females may be different to those in males. It is hard to draw any conclusions on this from the existing literature, as very little attention has so far been given to lesbianism. There are also problems associated with the methods of classification used by researchers, and growing dissatisfaction with categorizing an individual as either homosexual or heterosexual.

Theoretical problems of twin studies have been considered, which cloud the distinction between genetic and environmental influences of a trait. Such problems mean that the results of twin studies can, at best, only be considered as suggestive of the relative importance of such influences.

Many theories have been proposed to explain the genesis of homosexuality, including psychoanalytic, sociobiological and gestational neurohormonal theories. Of these, the latter is the most scientifically comprehensive and testable. The gestational neurohormonal theory is consistent with many of the data used as evidence for genetic contributions, because production and control of the critical hormones are under significant genetic control.

From the data reviewed in this report, it seems reasonable to conclude that male homosexuality, or, at least, some 'types' of male homosexuality, are under some degree of genetic control, although various problems with this data prevent more precise conclusions from being drawn. Little can be said of the origins of female homosexuality.

Research, including twin studies of improved design, is continuing at a rapid pace. We can therefore expect clearer answers to many of the questions highlighted in this report within the coming years.

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