Would a hypothetical "gay gene" produce a disposition to behavior or identity? What factors would account for the difference between the two overlapping groups outlined in Kinsey's study? And would a disposition to either behavior or identity at the genetic level necessarily guarantee the manifestation of either at the phenotypic level (that is, able to be otherwise observed in the constitution of the organism)?

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A movement toward the integration of biological and social science research approaches to understanding human behavior has grown up contemporary with (and heavily dependent on) the study of genetics. Sociobiology is one among various approaches that differ in the extent to which they perceive biological and environmental influences to be determinative of human behavior, but concur insofar as they believe that neither set of variables can be ignored.

In the sociobiological model, social behavior in all organisms is orchestrated primarily around maximizing the amount of genetic material that each individual is able to spread to others; in short, this means having as many offspring as possible. Social behavior is thus understood to be primarily a function of self-interest, of the persistence of one's own genetic material over generations. Perhaps paradoxically, sociobiology has explained a failure to produce offspring in terms of altruism.

In sociobiological theory, celibacy or homosexuality on the part of one organism permits its close relatives to have more offspring who will in turn have greater access to resources in the environment. That this supposition is generally true of life on earth is believed to be evidenced by the widespread incidence of homosexual behavior in non-human animals. Yet the idea of apparently unmotivated altruism does much damage to an otherwise coherent theory.

Is reproductive altruism a conscious or unconscious response? If reproduction is such a strong imperative, what could inspire an even stronger response against it?

One possible answer is that homosexuality is a response exhibited by males who are unsuccessful in their competition for access to women, a proposition that is vaguely redolent of Freud. Another is that homosexuality is characteristic of genetically deficient individuals who are programmed not to reproduce so as not to spread undesirable traits through the population. Neither is an explanation that will be palatable to many gay and lesbian people.

Both the genetic and sociobiological conclusions enumerated here are at best premature; at worst, they have been characterized by some as "bad science." As we learn more about the human genome and the extent of its influence on human behavior, the debate over the causation of homosexuality will doubtless continue, and its content will be changed.

As the political situation of gay and lesbian people changes, certain kinds of etiologic explanations will necessarily be perceived as better serving the interests of this constituency than others; likewise, certain etiologies of homosexuality will play larger roles in shaping individual and group identity and politics than others.

Genetic models of causation have reawakened the specter of eugenics in the possibility of genetic engineering, inspiring the not-unjustified fear that homosexuality may one day be systematically eliminated from human populations. Like the other etiologies of homosexuality identified in this article, genetics has offered gay and lesbian people as much a threat to their existence as it has a vindication of it. If and when the definitive answer is known, it is equally likely to produce as much unease as relief.

Anthropology, the first of the social science disciplines to take sexuality--and particularly homosexuality--seriously as a field of intellectual inquiry in the late nineteenth and early twentieth centuries, has achieved a new impetus in the post-Stonewall era.

A form of behavior modification that employs unpleasant and sometimes painful stimuli, aversion therapy was one of the more popular treatments for homosexuality and cross-dressing in the 1950s and 1960s.

Butch-femme identities are controversial and difficult to define with precision, but both roles subvert prescribed gender and sexual expectations; ultimately, the butch-femme dynamic is a unique way of living and loving.

Ethnography, the description of indigenous non-European peoples by Euro-Americans, has been a safe way for writers to discuss homosexuality as a normal, non-pathological behavior.

The term "homosexuality," coined in 1869, with "heterosexuality" as its opposite, has led to a binary concept that oversimplifies the complexity of human sexual behavior.

As part of its agenda to preserve an "Aryan master race," Nazism persecuted homosexuals as "asocial parasites"; more than 100,000 men were arrested on homosexual charges during the Nazi years, with 5,000-15,000 gay men incarcerated in concentration camps.

Psychoanalysis, which began as a therapeutic procedure, ultimately became one of the most powerful methods of cultural analysis and critique of the twentieth-century.

Reparative therapy is a dangerously misguided attempt, supported by homophobic religious organizations, to change a person's sexual orientation from homosexual to heterosexual.

One of the finest English novelists of the twentieth century and a tireless defender of humane values, Forster deserves a special place in the gay and lesbian literary heritage.

The founder of psychoanalysis and the discoverer of the unconscious, Sigmund Freud initiated a fundamental transformation in the self-understanding of Western men and women, including especially the role of sexuality.

Formed soon after the Stonewall Riots of 1969, the short-lived but influential Gay Liberation Front brought a new militancy to the movement that became known as gay liberation.

German-born Magnus Hirschfeld deserves recognition as a significant theorist of sexuality and the most prominent advocate of homosexual emancipation of his time.

American psychologist Evelyn Hooker's pioneering studies on male homosexuality in the 1950s and 1960s challenged the "sickness" model of homosexuality then prevalent.

Ferdinand Karsch-Haack's most significant contribution to the sexual emancipation movement in Germany consisted of demonstrating the occurrence of same-sex sexual activity throughout the animal kingdom, among the so-called primitive peoples, and in all non-Western cultures.

The most important sex researcher of the twentieth century, Alfred C. Kinsey contributed groundbreaking studies of male and female sexual behavior in America.

The Kinsey Institute at Indiana University, established by Alfred Kinsey in 1947, has pioneered in the study of American sexual behavior.

The carefully detailed case studies of nineteenth-century psychiatrist Richard von Krafft-Ebing shed light on the sexual habits of a wide spectrum of men and women.

One of the greatest painters in the history of art and an outstanding empirical scientist, Leonardo was haunted by his illegitimacy and rumors of homosexuality.

Although she was one of the most prominent and widely admired American anthropologists of her generation, Margaret Mead chose to keep her own bisexuality a secret.

Best known for his genius in art and architecture, Michelangelo was also an accomplished author of homoerotic poetry.

The most famous artist who ever lived, Michelangelo left an enormous legacy in sculpture, painting, drawing, architecture, and poetry; while the artist's sexual behavior cannot be documented, the homoerotic character of his drawings, letters, and poetry is unmistakable.

Among Greek writers on homosexual themes, Plato is preeminent not only as a major philosopher but also as the greatest master of Greek prose.

Although sometimes reviled by heterosexuals and homosexuals alike, the sissy has historically helped define gay culture, and has questioned the dominant constructions of sex and gender.

Nineteenth-Century German activist Karl Heinrich Ulrichs was both the first modern theorist of homosexuality and the first homosexual to "come out" publicly.