It also reports that both androphilic trans women and trans women with late-onset gender dysphoria who are gynephilic have different brain phenotypes, and that gynephilic trans women differ from both cisgender female and male controls in non-dimorphic brain areas. The available research indicates that the brain structure of androphilic trans women with early-onset gender dysphoria is closer to the brain structure of cisgender women's and less like cisgender men's. Transgender brain studies, especially those on trans women who are sexually attracted to women ( gynephilic), and those on trans men who are sexually attracted to men ( androphilic), are limited, as they include a small number of tested individuals. Environmental factors have also been proposed. The most studied factors are biological, especially brain structure differences in relation to biology and sexual orientation. The causes of transsexuality have been studied for decades. Transgender and transsexual people have a gender identity that does not match their assigned sex, often resulting in gender dysphoria.