In 1957 the first psychological test occurred to determine if there was a biological explanation for homosexuality. This test was undertaken by American psychologist Karen Hooker (The Mother of the Homosexual Movement), who studied the relationship between psychological development and homosexuality and illness. By studying both heterosexuals and homosexuals, and matching their intelligence, educational levels, and ages, all subjects were then given three psychological tests, which were the Make-a-Picture-Story Test (MAPS), the Thematic Apperception Test, and the Rorschach (Boston University, 2012). With finding no major differences in the answers given by the groups of heterosexuals and homosexuals because of the similar scores, Hooker was able to concluded that sexuality is not based on environmental factors (Boston University, 2012). Boston University (2012), "in 1973, based on Hooker’s findings, the American Psychiatric Association removed homosexuality from its Diagnostic and Statistical Manual of Psychological Disorders and in 1975, released a public statement that homosexuality was not a mental disorder" (para. 3).
There have been other studies designed at determining whether homosexuality was or was not a genetic cause. Boston University (2012), "among the most notable were a series of studies Pillard and J. Michael Bailey, a professor of psychology at Northwestern University, conducted in the early 1990s that found that homosexuality is largely biologically determined, not environmentally influenced" (para. 4). In their identical twin, fraternal twin brothers, and adopted nonrelated brothers studies to determine if there was a genetics could explain homosexuality, they found that when one identical twin was gay there was a 52 percent chance that the other was also gay (Boston University, 2012). There was a 22 percent chance for fraternal twins, and only 5 percent for nonrelated adopted brothers (Boston University, 2012).