Richard Pillard is a professor of psychiatry at the Boston University School of Medicine.

Sheep are another species in which males occasionally show what looks like preferential homosexual mate choice. Biologists Ann Perkins and James Fitzgerald, working at the U.S. Sheep experiment Station ion Idaho, are trying an interesting experiment. They identified rams who show an interest in mounting other rams, electro-ejaculated them to collect sperm, and used the sperm to impregnate ewes. The offspring rams, as they are interbred, will be tested to see if they show a homosexual mounting choice more frequently which they should if there are "gay genes" concentrated and passed on from parent and grandparent ancestors.

This discussion so far still leaves bare the question why an orientation that appears so inimical to reproduction might nevertheless persist in human or animal populations. Homosexuality needs to demonstrate a survival benefit sufficient to offset the reproductive cost that the orientation would be expected to exact. Most reproductively disadvantageous traits occur because of occasional and random mutations which are gradually selected out of the kindred. Huntington's Diseas, for example, occurs in only about four people per million. The gay/lesbian phenotype is far too common to be entirely the result of deleterious mutation. if genetic, it must have undergone some degree of favorable selections. One concept to understand about "reproductive success" is that it can be realized by different strategies. The oak produces thousands of acorns and is lucky if one or two grow to make acorns of their own. The larger land mammals on the other hand, have relatively few offspring and are metabolically expensive in guarding and protecting the relatively few offspring that we are able to have.

This line of thinking, elaborated by biologists Robert Trivers and James Weinrich, among others, is that gay people may have evolved character traits of an altruistic nature that prompt them to work harder for the protection and advancement of closely related family members rather than invest in having children of their own. This idea, called "kin altruism," results from the simple calculus that two nephews or nieces are genetically equivalent to one son or daughter. Reproductive sacrifice explainable by a theory of kin altruism has been demonstrated in some animal species but remains speculative for humans, most of whom, sad to say, seem rather short in the department of altruism.. (Cf., Jim Weinrich, Sexual Landscapes: Why We Are What We Are, Why We Love Whom We Love, 1987.)

Ray Blanchard and his colleagues at the Clarke Institute in Toronto recently analyzed thousands of families and found that gay men have a later birth order than straight. Specifically, the gay men had more older brothers but not more older sisters once the older brothers were taken into account. A psycho-developmental explanation springs to mind: A young boy might develop homosexual attractions more readily if an older brother is in the picture. But a biological explanation is also on the horizon. The placenta supporting every human pregnancy has small protein fingers that engage the mother's bloodstream by digging into the wall of her uterus. When the placenta is shed, some of the placental cells remain in the uterine wall and can be demonstrated to be alive and well years later. These cells came form the fetus and contain its genome. It is possible that some cells of an early pregnancy remain behind and, by an unexplained mechanism, try to capture the resources of subsequent pregnancies, a sort of biological primogeniture? I like this odd (and admittedly remote) possibility because it shows how much is still unexpected in the biology of human sexuality.

Behavior genetics applied to sexual orientation encounters a variety of criticisms. In this debate, I feel like a detective who's presenting his case to the district attorney. I have scraps and clues, hypothesis and sometimes hang together but that also have gaps. But the DA must be a skeptic. He has to put my case before a jury whom he must convince beyond a reasonable doubt. The standard of proof rises as the investigation proceeds. (Thanks to Jimm Weinrich for the analogy.) The most I can say is that no one has yet put forward evidence that is devastating to my case.

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There is one criticism I want at least to touch on, namely that genetics theories pander to homophobia. If "gay genes" are discovered, how much easier will it be to eradicate them? Opinion surveys are unanimous t hat people who endorse a genetic hypothesis are more sympathetic to lesbians and gays than people who hole to a purely environmental theory. Still, technology ahs a way of biting back. No scientific knowledge is risk-free, and this must surely include genetic investigations of sexual orientation. One might take a sort of verse comfort in knowing that homophobia, like racism (an all the xenophobias), exists regardless of whatever might be considered "the facts" of the moment. Research on human sexuality will, by its nature, evoke resistance and fear, to some extent legitimately. Here is where I think social science research has a valuable role. There ought to be funded research programs not only to unravel the genome but to unravel the ethical and psychological dilemmas that accompany the new insights that behavior genetics will generate.

Scientific research is permeated by values. Mine are that it is better to know something than not to know it. I believe that same-sex desire will remain a topic of interest and study for a long time. I'm confident that biological knowledge will prove it to be a valuable trait, selected by evolution precisely because it contributes some quality that was useful, perhaps even essential, to the sudden ascendancy of human beings among all the primates.

	Excerpted with permission from "The Genetic Theory of Sexual Orientation" in the Harvard Gay and Lesbian Review, Winter 1997, pp. 61-67.