Although sexual orientation of the human population is variable, most people give priority to the opposite sex (heterosexual orientation). There are only a minority of people who show homosexual orientation. For many years psychological factors were hold responsive for homosexuality. Understanding of homosexuality is today somewhat different. One of the theories believes that homosexuality is influenced by genes. Researches that were conducted on brothers, including twins, have proved that genetic factors play a role in male but not in female homosexuality. Homosexuality is probably transmitted across female line. Since males receive their single X chromosome exclusively from their mothers, any trait that is influenced by an X-linked gene will be preferentially passed through the mother’s side of the family. The main outcome of the research is finding of link between homosexual orientation and markers in the Xq28 region on the X chromosome. But later, some scientists have discovered certain irregularities in the theory. In addition to these theories, there is one suggested a connection between influence of hormones and sexual orientation. The influence of prenatal androgen steroids on the sexual orientation of adults was demonstrated first on animal models, and later in humans. Numerous findings indicate that in some species, aromatization (conversion of testosterone to estradiol) is a critical step for brain masculinization with testosterone. Since hormones dump the prenatal development affect brain development, they also affect the development of behavior. Perinatal castration of male rats and application of testosterone to females caused a preference for the same sex. There are some indications that perinatal hormones affect sexual orientation in humans, but there is still little evidence to support that. According to the third theory, brains of homosexuals and heterosexuals are structurally different. The differences are reflected in the size of the third interstitial nucleus of hypothalamus, size of suprachiasmatic nucleus and anterior commissure. After various opinions about the origin and causes of homosexuality, scientists have shown that homosexuality is not a choice, but an unchangeable orientation.