Some women may be born gay because of the amounts of male and female sex hormones they were exposed to in the uterus, according to a new study.
Based on a review of 460 scientific studies, New Zealand and European researchers argue that the quantities of testosterone and oestrogen may be crucial in understanding the full range of female traits - from those that are typically masculine, to those that are typically feminine.
The researchers believe that arguments suggesting same-sex sexual behaviours are contrary to the order of nature are implausible when seen in the context of their findings.
Sex hormones play a key role in the development of reproductive organs and other characteristics. Testosterone is found in men and less so in women. Oestrogen, too, is produced in the bodies of both sexes, but plays a bigger role in women.
The review article by the researchers, one of whom is Severi Luoto, a PhD student of evolutionary psychology at the University of Auckland, has been published in the journal Archives of Sexual Behavior.
The review identified clusters of sex-typical traits which vary in their degree of masculinity.
Lesbian and bisexual women tended towards being more masculine on physical traits such as facial structure, the length of leg and arm bones and hearing. Their behaviour inclined towards the riskier, greater alcohol use and more "sexual sensation seeking", the university said.
"While these traits vary between heterosexual and non-heterosexual women, the current findings suggest the traits also vary between different types of non-heterosexual women."
Luoto said women have increasingly masculine traits across the range of sexual orientation: from heterosexual, mostly heterosexual, bisexual, femme lesbian to butch lesbian women.
"Butch lesbians show a composite of masculine biological, psychological, and behavioural characteristics.
"Higher bodily masculinity is an indication of higher exposure to testosterone in prenatal development.
"Femme lesbians and bisexuals do not have similarly masculinised bodily traits, but they do show psychological and behavioural masculinisation.
"So, we infer that bodies of femmes and bisexuals have not been masculinised in prenatal development but parts of their brains have. Increased masculinisation of psychological and behavioural traits may have resulted from moderate exposure to testosterone, or high exposure to oestrogen."
"We propose that the sex hormones testosterone and oestrogen present at different times of fetal development might account for differences in masculinisation of the body and psychological traits between types of non-heterosexual women.
"Our neurodevelopmental theory can provide a framework for understanding non-heterosexual women's body morphology [or type], psychological dispositions, behavioural outcomes and lower general health.
"Distinguishing between different types of non-heterosexual women leads to an improved understanding of their different developmental trajectories and behavioural outcomes.
"Advances in the scientific understanding of diversity in human sexuality should help direct social policy, and provide impetus to abolish laws across nations which still restrict freedoms of expression and association, or punish same-sex sexual behaviour."