Gender incongruence is the state of having a gender identity that does not correspond to one's sex assigned at birth. This is experienced by people who identify as transgender or transsexual, and often results in gender dysphoria. The causes of gender incongruence have been studied for decades. The most studied factors are biological, especially brain structure differences with relation to biology and sexual orientation. Environmental factors have also been proposed. Transgender brain studies, especially those on trans women attracted to women (gynephilic), and those on trans men attracted to men (androphilic), are limited, as they include only a small number of tested individuals. Studies conducted on twins suggest that there are likely genetic causes of gender incongruence, although the precise genes involved are not known or fully understood. One study published in the International Journal of Transgender Health found that in 20% of identical twin pairs, if one twin was trans, the other was as well, compared to only 2.6% of non-identical twins where this was the case; researchers attribute this to their shared genetics. Sexologist Ray Blanchard created a taxonomy of male-to-female transsexualism that proposes two distinct etiologies for androphilic and gynephilic individuals; this taxonomy has become controversial, supported by J. Michael Bailey, Anne Lawrence, James Cantor and others, but opposed by Charles Allen Moser, Julia Serano, and the World Professional Association for Transgender Health. Biological factors Genetics A 2008 study compared the genes of 112 trans women who were mostly already undergoing hormone treatment, with 258 cisgender male controls. Trans women were more likely than cisgender males to have a longer version of a receptor gene (longer repetitions of the gene) for the sex hormone androgen, which reduced its effectiveness at binding testosterone. The androgen receptor (NR3C4) is activated by the binding of testosterone or dihydrotestosterone, where it plays a critical role in the forming of primary and secondary male sex characteristics. The research weakly suggests reduced androgen and androgen signaling contributes to trans women's identity. The authors say that a decrease in testosterone levels in the brain during development might prevent complete masculinization of trans women's brains, thereby causing a more feminized brain and a female gender identity. A variant genotype for the CYP17 gene, which acts on the sex hormones pregnenolone and progesterone, has been found to be linked to transsexuality in trans men but not in trans women. Most notably, transmasculine subjects not only had