Our own research has shown that male sexual orientation is substantially genetic.
Over the last two years we have studied the rates of homosexuality in identical and non-identical twin brothers of gay men as well as adoptive brothers of gay men.
Fifty-two percent of the identical twin brothers were gay, as against 22 percent of non-identical twins and 11 percent of the adoptive, genetically unrelated brothers.
In contrast, research on social factors has been fruitless.
Despite many attempts, there has been no clear demonstration that parental behavior, even a parent's homosexuality, affects children's sexual orientation.
Cultures tolerant of homosexuals do not appear to raise more of them than do less permissive societies.
Homophobes sometimes justify their prejudice against homosexuals by alleging that homosexuality is contagious - that young homosexuals become that way because of older homosexuals and that homosexuality is a social corruption.
Such beliefs form the core of the organized anti-homosexual movement.
If homosexuality is largely innate, this would prove these claims are groundless.
Given these implications, it may seem surprising that the biological studies disturb many gay and lesbian advocates.
Misunderstanding them, the advocates often suggest that the search for a biological cause is motivated by an assumption that homosexuality is an illness.
The advocates worry that biological findings may be misused to try to alter or prevent homosexuality.
But no scientific theory or finding by itself can lead to a proper attitude or policy toward homosexuality. Here, moral values must be primary.
This leads to a more pertinent fear of gays and lesbians, that people will assume that answers to moral questions hinge on the results of scientific study.
Should a benevolent view of homosexuality depend on the assumption it is innate?
Are gays and lesbians to be tolerated only if they are "born that way?"
Regardless of what causes sexual orientation, there is no plausible justification for oppressing homosexuals.
Reasons that have long been offered - that homosexuals disproportionately molest children, convert heterosexuals to homosexuality, are mentally ill, betray their country - have been shown to be false.
But homophobia remains the one form of bigotry that respectable people can express in public.