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Many researchers and theoreticians have advanced hypotheses in an effort to explain what causes sexual orientation and transgender identity. Some believe that the biology of an individual determines heterosexuality, same-sex attraction, and gender identity. Some studies have shown chemical and brain structure differences for gay men and transgender persons. But it is impossible to determine causation from the results of these studies. A comprehensive study of sexual orientation was undertaken by Bell, Martin, Weinberg, and Hammersmith of the Alfred C. Kinsey Institute for Sex Research. They studied 979 gay men and lesbians and 477 heterosexual men and women, gathering a large amount of information about their lives in an effort to determine critical and statistically significant differences between the two groups. They found that (a) by the time that boys and girls reach adolescence, their sexual orientation is likely to be determined even though they were not yet sexually active; (b) the gay men and lesbians were not lacking experience in heterosexual experiences during their childhood and adolescent years, but they found the experiences ungratifying; and (c) among the men and women in the study there was a strong link between gender unconformity and the development of same-sex sexual orientation. Gender nonconformity refers to children who prefer engaging in activities generally associated in this culture with the other gender-for example, boys playing with dolls. These findings suggest that sexual orientation is established early in life, perhaps before adolescence; one's sexual orientation may even be set at birth. The reason(s) for this have not been established.
Studies have also suggested that the brains of heterosexual and gay men are different and that certain brain structures in gay men are more like those in women. Twin studies have suggested that in pairs of twins both twins would most likely be gay if they were identical twins (they share exactly the same genes) than fraternal twins. After reviewing the studies conducted on sexual orientation, Hyde and DeLamater concluded that the causes of sexual orientation are not yet known.
A number of theories of sexual orientation and the evidence supporting or refuting them have been examined. It is not known which theory is correct and what causes sexual orientation. The same may be said of gender identity, but a recent study by UCLA researchers has suggested that male-to-female transsexuals have different brain structures than those males who are biologically male and have a congruent gender identity. These differences occur mainly in the frontal and orbito-frontal cortex (involved in decision-making), central sulcus, perisylvian regions helps to process language), and paracentral gyrus; and within the right hemisphere included pre-/post-central gyrus (involved in sense of touch), parietal cortex (integrates sensory information), temporal cortex (involved with visual information), precuneus (concerned with reflections upon self and aspects of consciousness), fusiform, lingual, and orbito-frontal gyrus. This led the researchers to conclude that there was a biological difference between these participants and males who felt compatible with their gender.
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