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A 2000 study examined whether sexual orientation is fixed or changes with time through environmental influence or the effects of aging. J. Michael Bailey recruited a community sample of twins from the Australian Twin Registry and assessed sexual orientation, childhood gender nonconformity (atypical gender behavior), and continuous gender identity (an individual's self-identification as "male" or "female"). Familial factors were important for all traits, but less successful in distinguishing genetic from shared environmental influences. Only childhood gender nonconformity was significantly heritable for both men and women. Statistical tests suggested that causal factors differed between men and women, and for women provided significant evidence for the importance of genetics factors.

Birth-order studies found homosexual males were not usually first born, having older siblings. Extremely feminine homosexual men had a higher than expected proportion of brothers, not an equal numbers of brothers and sisters.

hormone and nervous systems

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