A new open access British twin study at PLoS One that also contains a fairly comprehensive review of the prior literature on the subject examines the genetic and environmental contributions to female sexual orientation and also examines the extent to which childhood gender typicality and adult gender identity assessment are related to sexual orientation.
This fills a major gap in the study of sexual orientation because lesbians have been the subject of less intensive research than gay men, despite indications that the genetic and environmental factors, and the nature of the sexual orientation phenotype may not be strictly parallel or identical in men and women. The sample size was 4,066 women who were British twins.
* The study finds that a common genetic source is involved in both childhood gender typicality and sexual orientation (as measured by attraction), with a hereditary component of about 32% of variation in childhood gender identity and 24% of the variation in sexual orientation (measured by attraction) (each of which the study views as continuous rather than yes or no variables). Some other studies using different measurements to identify the phenotype have found up to twice as much of a hereditary component (up to 60%), while others have found less (as little as 17%), all serious studies have found a substantial hereditary genetic component to female sexual orientation. The stronger genetic component of childhood gender identity suggests that this phenotype might be more useful in identifying one or more "lesbian genes" than actual self-understood sexual orientation.
* Childhood gender identity and sexual orientation as measured by attraction as an adult are strongly correlated with each other due to both shared genetic factors and shared non-genetic factors (including, but not limited to, in utero androgen exposure). They appear to be basically different manifestations of the same thing.
* The genetic effect appears to be additive rather than involving a dominant gene pattern.
* The study finds few shared environmental effects on sexual orientation or childhood gender typicality: "shared factors such as home environment and parenting styles have little impact on human sexual orientation."
* The study reviews the literature regarding in utero androgen exposure which it finds persuasive but not definitive. Thus, exposure to male hormones in the womb may have an important effect on sexual orientation in women. (This is an example of a cause which would be congenital, i.e. permanently present from birth, but not hereditary.)
* The instrument it uses to measure "adult gender identity", which measures self-perceptions of masculinity or femininity, in the study is found to be only weakly linked to the genetic factor and sexual orientation and childhood gender typicality and suggests that other instruments may be better. This instrument also had lower internal consistency than the other measures. Put another way, sexual orientation and childhood gender identity are not strongly associated with a butch v. a femme self-identification as an adult.
* The study finds evidence of "sex-atypicality" as a possible intermediate phenotype in women between a strictly heterosexual phenotype and a strictly homosexual phenotype, which is probably more driven by in utero androgen exposure than by genetics.
* The study does not appear to have really evaluated bisexuality as a potentially separate construct from a lesbian sexual orientation and "sex-atypicality" despite at least one prior study showing this sexual orientation to be stable and distinct from a lesbian sexual orientation (in a larger share of women who are not clearly heterosexual than in men). A bisexual individual in this study would probably be treated as more homosexual than heterosexual but not completely homosexual.