Biologists embarked upon research into homosexuality in response to an intellectual vacuum created by the failure of other sciences to solve the riddle of sexual orientation. "Other sciences" mostly means psychiatry.
As Michael Bailey and Richard Pillard, the authors of one of the most important genetic inquiries into homosexuality, have observed, decades of psychiatric research into possible environmental causes of homosexuality—that is to say, social and cultural causes—show "small effect size and are causally ambiguous."
As a distinct concept, homosexuality is relatively recent. David Halperin points out in One Hundred Years of Homosexuality that the term itself first appeared in German (Homosexualist) in a pamphlet published in Leipzig in 1869; it entered the English language two decades later.
That some human beings engage in sexual activity with others of the same sex has, of course, been noted since antiquity.
This changed in the nineteenth century, when modern medicine and particularly the science of psychiatry came to view homosexuality as a form of mental illness.
By the 1940s homosexuality was discussed as an aspect of psychopathic, paranoid, and schizoid personality disorders. Among those who looked into the matter was the sex researcher Alfred Kinsey, whose 1948 report Sexual Behavior in the Human Male showed homosexuality to be surprisingly common across lines of family, class, and educational and geographic background.
In his book Being Homosexual, the psychoanalyst Richard Isay writes, Kinsey and his co-workers for many years attempted to find patients who had been converted from homosexuality to heterosexuality during therapy, and were surprised that they could not find one whose sexual orientation had been changed.
When they interviewed persons who claimed they had been homosexuals but were now functioning heterosexually, they found that all these men were simply suppressing homosexual behavior. . . and that they used homosexual fantasies to maintain potency when they attempted intercourse. One man claimed that, although he had once been actively homosexual, he had now "cut out all of that and don't even think of men—except when I masturbate."
Psychiatry not only consistently failed to show that homosexuality was a preference, a malleable thing, susceptible to reversal; it also consistently failed to show that homosexuality was a pathology.
In 1956, in Chicago, a young psychologist named Evelyn Hooker presented a study to a meeting of the American Psychological Association.
Hooker administered psychological tests to her sixty subjects, including the Rorschach ink-blot test, producing sixty psychological profiles. She removed all identifying marks, including those indicating sexual orientation, and, to eliminate her own biases, gave them for interpretation to three eminent psychologists.
One of these was Bruno Klopfer, who believed that he would be able to distinguish homosexuals from heterosexuals by means of the Rorschach test.
As it turned out, none of the three could tell the homosexuals and heterosexuals apart. In side-by-side comparisons of matched profiles, the heterosexuals and homosexuals were indistinguishable, demonstrating an equal distribution of pathology and mental health.
Reviewing Hooker's results from a test in which the subject creates pictures with cutout figures, one of the interpreters, a psychologist named Edwin Shneidman, stumbled onto a particular subject's orientation only when he came across a cutout scene depicting two men in a bedroom.
Shneidman remembers, "I said to Evelyn, 'Gee, I wish I could say that I see it all now, that this is the profile of a person with a homosexual orientation, but I can't see it at all.''
Hooker's research throughout her long career was driven by the belief that for psychiatry to be minimally scientific, pathology must be defined in a way that is objective and empirically observable.
Her study was the first of many showing that homosexuality could not be so defined as pathology.
In 1973 the American Psychiatric Association removed homosexuality from its official Diagnostic and Statistical Manual, signifying the end of homosexuality's official status as a disease.
Today's psychiatrists and psychologists, with very few exceptions, do not try to change sexual orientation, and those aspiring to work in the fields of psychiatry and psychology are now trained not to regard homosexuality as a disease.