Biology of sexual orientation

The relationship between biology and sexual orientation is a subject of research. A simple and singular determinant for sexual orientation has not been conclusively demonstrated; various studies point to different, even conflicting positions, but scientists hypothesize that a combination of genetic, hormonal, and social factors determine sexual orientation.

Biological theories for explaining the causes of sexual orientation are favored by experts and involve a complex interplay of genetic factors, the early uterine environment and brain structure. These factors, which may be related to the development of a heterosexual, homosexual, bisexual or asexual orientation, include genes, prenatal hormones, and brain structure.

A number of twin studies have attempted to compare the relative importance of genetics and environment in the determination of sexual orientation. In a 1991 study, Bailey and Pillard conducted a study of twins recruited from "homophile publications", and found that 52% of monozygotic (MZ) brothers (of whom 59 were questioned) and 22% of the dizygotic (DZ) twins were concordant for homosexuality. 'MZ' indicates identical twins with the same sets of genes and 'DZ' indicates fraternal twins where genes are mixed to an extent similar to that of non-twin siblings. In a study of 61 pairs of twins, researchers found among their mostly male subjects a concordance rate for homosexuality of 66% among monozygotic twins and a 30% one among dizygotic twins. In 2000 Bailey, Dunne and Martin studied a larger sample of 4,901 Australian twins but reported less than half the level of concordance. They found 20% concordance in the male identical or MZ twins and 24% concordance for the female identical or MZ twins. Self reported zygosity, sexual attraction, fantasy and behaviours were assessed by questionnaire and zygosity was serologically checked when in doubt. Other researchers support biological causes for both men and women's sexual orientation.

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