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A detailed review of the many theories that have been proposed to explain the genesis of homosexuality is beyond the scope of this report. However, a very brief description of three accounts is given in order to demonstrate the different angles from which the question has been tackled.

Within family determinants: Psychoanalysis and beyond

Freud believed that all humans were born bisexual in nature, and from this state, as a result of restriction in one direction or the other, both heterosexuality and homosexuality developed (see his 'Three Essays on the Theory of Sexuality" 1953). He also made the distinction between two types of homosexual (or 'invert', to use his terminology); those who are like women, seeking masculine men, and others who seek feminine qualities in their partners. Some individuals may display predominantly one type of inversion or the other, whereas others might display a certain amount of both types of inversion. Different causal factors were therefore suspected for the two. Freud realised that the aetiology of homosexuality was complex, and suspected that "the choice between 'innate' and 'acquired' is not an exclusive one, or . . . it does not cover all the issues involved".

Early psychoanalytic theory claimed that all homosexual men have unresolved pre-oedipal conflicts, that is, they did not successfully negotiate the separation-individuation phase of early childhood. In this way, early childhood stress leads to obligatory, exclusive homosexuality, whereas stress in the later oedipal phase leads to partial, non-obligatory homosexuality.

As scientific knowledge of homosexuality has expanded, and Western society has become gradually more tolerant towards homosexuals, so the psychoanalytic theory has evolved (e.g. Friedman 1988, Lewes 1989). Indeed, in 1973 the American Psychiatric Association decided to drop homosexuality (per se) from the diagnostic nomenclature. It should be noted that Freud himself had maintained that "it is not scientifically feasible to draw a line of demarcation between what is psychically normal or abnormal; so that the distinction, in spite of its practical importance, possesses only a conventional value."

Freud's earlier work has since been criticised because, for example, he confounded gender-related behaviour and sexual behaviour in his writings. Much recent work in psychoanalytic theory has involved developing more sophisticated classification schemes for the different facets of homosexuality (Lewes, 1989). However, any such theory can be criticised for being generalizable only to those members of a population who attend psychoanalytic clinics.

Many of the case studies described in the previous sections have looked for specific aspects in an individual's environment during development which can lead to adult homosexuality. Such reports have commonly found one or more of the following factors to be unusual in some respect in the childhood of homosexuals: parental hopes before birth for a child of the other sex; difficulties at birth; slight anatomical differences between identical twins leading to a special attachment of one child or the other to their mother; parental attitude toward the role of the individual child, disclosed through the naming of the child; the position of the father in the family; strength of the relationship between father and child; competition for the affections of the mother; and, a 'twinning reaction' or mutual dependence between twins, especially noticeable in identical pairs (Rainer et al. 1960, Joseph & Tabor 1961, Mesnikoff et al. 1963, Bene 1965, Holden 1965, Myers 1982, Ainslie 1985). However, such studies can tell us little of the importance of such factors in the genesis of sexual orientation.

Other factors within a family that have been suggested to correlate with homosexuality include the individual's birth order, parental age at birth, and family size (e.g. Slater 1962, Abe & Moran 1969, Siegelman 1973).

Homosexuality has also been linked to childhood opposite sex-dimorphic behaviour, such as effeminacy in boys (Bell et al. 1981, Green 1987, Buhrich et al, 1991). However, such behaviour is not observed in all pre-homosexual children while it is seen is some pre-heterosexual children. It is quite possible that, rather than being causative of adult sexual orientation, such behaviour is the manifestation of an innate predisposition towards homosexuality.

Genetic theories: Sociobiology and homosexuality

The last decade has seen a revival of interest in genetic theories of homosexuality (e.g. Ruse 1981 & 1988, Kirsch & Rodman 1982), due largely to the development of human sociobiology (e.g. Smith 1983).

Even before these recent theories, there was, of course, a suspicion that genetic factors were involved. Indeed, this is one of the fundamental questions addressed by twin studies. However, many previous reports (e.g. Rosenthal 1970, Fuller & Thompson 1978) have presumed that homosexuality arises through the interaction of particular genetic propensities with specific rearing environments. The sociobiological theories of sexual orientation attempt to show that it is possible that homosexuality can be sustained in a population through purely genetically controlled processes which have been subject to the pressures of natural selection.

The two most popular hypotheses in this field are those of balanced superior heterozygote fitness and of kin selection for altruistic behaviour.

Briefly, the former hypothesis supposes that phenotypic homosexuality is the result of homozygosity for recessive 'homosexual' genes. If a heterozygote, possessing one 'homosexual' genetic allele and one 'heterosexual' allele, is phenotypically heterosexual, and more reproductive than an individual who is homozygotic for the 'heterosexual' alleles, then the heterozygotic combination will be preferentially selected in future generations. In this way, the 'homosexual' genes are preserved. This explanation may seem implausible; for one thing, it hardly seems likely that only a single, major gene is involved in determining sexual preference However, various genetic concepts (incomplete penetrance, epistasis, etc.) may be utilised to expand the hypothesis (e.g. Klintworth 1962, Fuller & Thompson 1978).

The key to the kin selection hypothesis is that it does not matter how one's genes are passed to the next generation, as long as they are. Siblings share, on average, 50% of their genes. Therefore, if an individual shows altruistic behaviour towards its siblings which results in an increased likelihood of the siblings leaving or raising offspring, then that individual is, in effect, favouring its own success. Although it is hard to see how such a process could operate in modern society, sociobiology concerns itself with how behavioural traits have evolved and been selected from primitive societies to the present day. It has been suggested that, in primitive societies, homosexuals may have formed a 'sterile caste' which could devote itself to helping mothers to rear their young.

Sociobiological explanations of homosexuality are recent and very speculative, and are currently intended only to highlight the possibility of genetic transmission of such a trait. Much more research is required before these hypotheses can develop into credible scientific theories.

Neurohormonal theories

Early studies of hormonal influences on sexual orientation concentrated on levels of circulating sex hormones in adults. Different investigations reported conflicting results (see Ruse, 1988), and, in general, little evidence has been found for consistent differences in the levels of these hormones in homosexuals compared to heterosexuals.

Since the 1970s, hormonal theories emerged which concentrated on differences in prenatal hormone levels (see Ellis & Ames, 1987). Many studies have shown that abnormal levels of some prenatal hormones can lead to an increased chance of homosexuality in an individual (e.g. D�rner et al. 1983, Money et al. 1984, Ehrhardt et al. 1985).

Ellis and Ames (1987) have proposed a very comprehensive gestational neurohormonal theory of human sexual orientation, which deals with the genesis of heterosexuality as well as homosexuality. They propose that sexual orientation is primarily determined by the degree to which the nervous system is exposed to testosterone, estradiol, and to certain other sex hormones while neuro-organization is taking place, predominantly between the middle of the second and the end of the fifth month of gestation. According to this theory, "complex combinations of genetic, hormonal, neurological, and environmental factors operating prior to birth largely determine what an individual's (adult) sexual orientation will be."

This theory makes many testable predictions, e.g. that homosexuality should primarily be a male phenomenon, that homosexuals should have higher frequencies of other sexual inversions than heterosexuals, that relationships between parents and homosexual offspring may be strained and/or assume some cross-sex characteristics, and that homosexuality should reflect a significant degree of heritability (as hormone production and action is under significant genetic control). Such predictions seem to agree with previous research and general intuitions regarding homosexuality.

In addition, as prenatal testosterone levels are of great importance according to the theory, and as, during the proposed critical period, intra-uterine testosterone is primarily of foetal, rather than maternal, origin, this theory could explain the observed differences in concordance rates for sexual orientation between monozygotic and dizygotic twins. According to such an explanation, the increased concordance in monozygotic twins is due to their greater similarity in prenatal hormone production (both in quantity and in timing) and hormone control; processes which are under significant genetic control.

Support for the gestational neurohormonal theory includes a recent study (LeVay, 1991) which reported a difference in hypothalamic structure between heterosexual and homosexual men, although Ellis and Ames warn that several decades of intense, further research may be required to adequately test the theory.

Which is correct?

The many different theories of the origins of homosexuality are not all mutually exclusive. For example, psychoanalytic theories describe interfamily and intrapsychic factors leading to inversion, and it is perfectly possible that such environmental factors are largely governed by the genetic makeup of the members of the family. Sociobiological theories, on the other hand, concentrate on genetic mechanisms for the transmission of homosexuality, while accepting that such mechanisms may operate by influencing the environment which an individual experiences during development. The gestational neurohormonal theory allows for significant control from both genetic and environmental factors.

Psychoanalytic theories may be criticised for being largely descriptive rather than explanatory, and for being derived from a sample which is unrepresentative of the wider population. The sociobiology of homosexuality lies more in the realm of the hypothetical than the proven, and neurohormonal theories still require much corroboratory research. However, research in many areas of homosexuality is proceeding at an accelerating pace, so we can expect a much clearer understanding of the genesis of homosexuality within the next few decades.

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