"3. Genetic Studies.
Bailey and Pillard (1991) interviewed 115 gay males and their twin brothers (MZ and DZ), and a further 46 gay males with their adopted brothers. They found that 52% of the MZ co-twins, 22% of the DZ co-twins, and 11% of adopted brothers were either homosexual or bisexual. In a later study Bailey et al., (1993) carried out a similar study in female homosexuals and found similar results, i.e. 48% of MZ co-twins, 16% of DZ co-twins, and 6% of adopted sisters were either homosexual or bisexual.
4. Childhood Behaviours.
The connection between early effeminate behaviour in boys and subsequent incidence of homosexuality has been of interest for some time. Studies have typically failed to find consistent differences between homosexual and heterosexual adults in sociocultural factors such as parenting styles, indicating that homosexuality may be inborn and not created. However, most of the studies have been retrospective, and the few prospective studies have used small populations and have been limited to short observation periods. Zuger (1984) carried out a long-term follow-up of 55 boys showing early effeminate behaviour. They were first seen between the ages of 3-14 when they displayed symptoms of effeminate behaviour. The children were seen again approximately 27 years later and were questioned on their sexual preferences. The results were that 73% were homosexual, 6% were heterosexual and 21% were bisexual.
Another study by Green (1985) compared two groups of boys on measures of gender identity in childhood and adolescence. One group consisted of 66 clinically referred boys whose behaviour showed clear signs of gender identity disorder, and 56 boys who were demographically matched. During childhood, extensive data was gathered on the boy's sex-typed behaviours, relationship with other children, and relationships with parents. In adolescence, a sexual orientation score was determined, and 68% of boys in the referred group demonstrated significant homosexual / bisexual orientation, whereas none of the boys in the control group did. These studies indicate that homosexuality is not gradually learned but is present from a very early age.
Why does homosexuality persist?
Genes persist when they improve their owner's ability to reproduce, forming attachments to the same sex would decrease one's chances of reproducing, and we would thus expect that such behaviours would die out over a few generations. Some possible explanations:
1. Some individuals do not become aware (or do not accept) their homosexual orientation until later in life, often following reproduction. According to LeVay (1996) around 67% of female homosexuals have children (similar to that of heterosexual women) but only 27% of homosexual produced children (60% of heterosexual men do so).
2. Does a homosexual gene confer a reproductive benefit on a heterozygous carrier (i.e. who only have one copy of the gene)? Males are disadvantaged in that they have a single copy of the X or Y chromosome, if a single disadvantageous gene is present then it may be sufficient to be fully expressed. Is this why homosexuality is more prevalent in males? For females this is not a problem as a deficient gene carried on the X chromosome will have a ‘good’ version on their other X chromosome. Some researchers have suggested that the sisters of homosexual men have more offspring, or that they may be more physically attractive but this has not been confirmed. Miller (2000) argued that sexual orientation is polygenic and during development these genes alter the sensitivity of the male brain to hormones which shift certain regions in a female direction. Single alleles confer sensitivity, empathy and kindness (traits which females prefer in males) while possessing several alleles would produce homosexuality.
3. McKnight (1997) points out that the basis of our social organisation is co-operation amongst relatives. Theories of the evolution of homosexuality have therefore focused on possible advantages of this behaviour for relatives i.e. by helping one's relatives with raising their children rather than reproduce. However, this kin selection hypothesis is not confirmed as there is no evidence that homosexual’s near relatives have more children, and that homosexuals favour their kin more than heterosexuals.
4. Baker & Bellis (1995) point out that all humans have the capacity for bisexuality and that many children’s first sexual encounters are with their own sex. Homosexual sex provides an opportunity for sexual novices to practise for later heterosexual behaviours. Those who experiment in this way will become more competent heterosexual lovers thereby enabling them to seduce more females and reproduce earlier. "
http://psychology.unn.ac.uk/nick/HBlec04.htm
