1956: American psychologist Evelyn Hooker shares her paper “The Adjustment of the Male Overt Homosexual” at the American Psychological Association Convention in Chicago. After administering psychological tests, such as the Rorschach, to groups of homosexual and heterosexual males, Hooker’s research concludes homosexuality is not a clinical entity and that heterosexuals and homosexuals do not differ significantly. Hooker’s experiment becomes very influential, changing clinical perceptions of homosexuality.

1991: Simon LeVay, a neuroscientist who now directs the Institute of Gay and Lesbian Education in Southern California, published in the magazine Science findings from autopsies of men and women of known sexual orientation. He found that a tiny region in the center of the brain–the interstitial nucleus of the anterior hypothalamus (INAH) 3–was, on average, substantially smaller in nineteen gay men who died from AIDS than among sixteen heterosexual men. The observation that the male brain could take two different forms, depending on one’s sexual orientation, was a stunning discovery.