By contrast, in a study doing genetic analysis of 409 pairs of homosexual brothers, including twins, strong evidence was found that some homosexual men are born homosexual.The study, including approximately three times as many people as the previous largest study on this subject, indicates that it is significantly more statistically reliable.Some researchers think this may indicate that childhood family experiences are important determinants to homosexuality, From their research on 275 men in the Taiwanese military, Shu and Lung concluded that "paternal protection and maternal care were determined to be the main vulnerability factors in the development of homosexual males".

Mc Conaghy (2006) found no relationship between the strength of the effect and degree of homosexual feelings, suggesting the influence of fraternal birth order was not due to a biological, but a social process.Gender-nonconforming children, on the other hand, will feel different from children of their own sex.In either case, this feeling of difference may evoke physiological arousal when the child is near members of the sex which it considers as being "different", which will later be transformed into sexual arousal.A 2006 Danish study compared people who had a heterosexual marriage versus people who had a same-sex marriage.Heterosexual marriage was significantly linked to having young parents, small age differences between parents, stable parental relationships, large numbers of siblings, and late birth order.