The cornerstone of the same-sex marriage debate is the question of whether or not sexual orientation is a fixed and immutable characteristic. If it is, there is some justification for arguing that homosexuality is a “suspect class” which should be granted “extraordinary protection from the majoritarian political process”. 

But is it?

Professor Paul McHugh, a distinguished psychiatrist from Johns Hopkins University, addresses this issue in an amicus brief to the US Supreme Court as background for its deliberations on same-sex marriage. He concludes that the current state of scientific knowledge indicates that sexual orientation is neither a clearly definable (discrete) category or a fixed and immutable characteristic, like race and gender.

Read the complete amicus brief at this link. Below are some excerpts. The questions have been added by MercatorNet.

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Homosexuals are a clearly defined group and they need special protection in a homophobic society.   

“There is currently no scientific or popular consensus . . . that definitively ‘qualify’ an individual as lesbian, gay, or bisexual.” … “Much of the confusion about sexual orientation occurs because there is no single agreed upon definition of the term …

Most definitions of sexual orientation “include components of at least one of three” of the dimensions of behavior, attraction, and identity. Some definitions include all three and, additionally, membership in a community defined by sexual orientation.

The problem with sexual orientation, so defined, is that many people are not consistent across all three dimensions… “The more carefully researchers map these constellations, differentiating, for example, between gender identity and sexual identity, desire and behavior, sexual versus affectionate feelings, early-appearing versus late-appearing attractions and fantasies, or social identifications and sexual profiles, the more complicated the picture becomes because few individuals report uniform intercorrelations among these domains.”

But there are huge numbers of gays and lesbians. They can’t all be wrong.

Different definitions produce substantially different estimates of the size of the homosexual population. “Sizable numbers of people reporting only same-sex attraction and/or behavior self-identify as heterosexual or bisexual. Similarly, sizable numbers of those who identify as gay or lesbian report some sexual partners of a different sex and/or some level of attraction to different sex partners.” The “Chicago Sex Survey,” considered one of the most reliable scholarly efforts to determine sexual practices in the United States, reported that of the portion of the population exhibiting at least one of the three components of sexual orientation, only 15% of the women and 24% of the men exhibited all three…

Its very capaciousness might lead one reasonably to “conclude there is serious doubt whether sexual orientation is a valid concept at all. Social constructionism suggests that there is nothing ‘real’ about sexual orientation except a society’s construction of it.”

Because scientific experts cannot agree on how to define it with substantial certainty, this Court should reject the category of sexual orientation as incapable of being “identified or defined in customary equal protection terms.”

Pardon the French, but this is utter crap. Everyone knows that you are born that way.

Studies of identical twins have confirmed that same-sex attraction is not solely determined by heredity or other biological factors. [A 2010 Swedish study found] concordance rates of 18% for male identical twins and 22% for female identical twins. [An American study found] concordance rates of 31.6% for identical twins. Because there is not 100% concordance among identical twins, genetic factors are not the sole cause of sexual orientation…

Other studies have found strong correlations between sexual orientation and external factors, such as family setting, environment, and social conditions, which are difficult if not impossible to explain under exclusively biological theories…

In brief, available evidence casts serious doubt on the simplistic, popular notion that sexual orientation is biologically determined. There is simply no firm evidence to support that conclusion, only unproven theories. As the American Psychiatric Association’s latest statement on the issue summarizes: “Currently there is a renewed interest in searching for biological etiologies for homosexuality. However, to date there are no replicated scientific studies supporting any specific biological etiology for homosexuality.”

Many gays and lesbians refuse to acknowledge the deepest truth about their identity because of the homophobic society in which they live. But once they come out as gay and proud, they feel liberated and stay that way.

Research suggests that a person’s sexual orientation is not entirely fixed and may be influenced by individual preference or choice. “Contrary to the notion that most sexual minorities undergo a one-time discovery of their true identities, 50% of [a study’s] respondents had changed their identity label more than once since first relinquishing their hetero- sexual identity.” Sexual orientation appears to be especially plastic for women…

Is there any research to support this?

Indeed, a 10-year study of 79 non-heterosexual women reported that 67% changed their identity at least once and 36% changed their identity more than once…

Extensive studies of the real-world experiences of men and women sharply rebut the notion that sexual orientation is unchanging. The Chicago Sex Survey found that of those who had at least one same-sex partner in the last five years, over half of the men and two-thirds of the women also had an opposite-sex partner in the same time period. Scholarly surveys also show that a significant portion of individuals in same-sex relationships had previously been married to someone of the opposite sex…

If human sexual preference were generally fluid rather than fixed, one would expect that some individuals would fall at the tails of the bell curve where behavior, attraction, and identity remain exclusively homosexual or heterosexual, but that most would exhibit at least some variation along the sexual orientation continuum with respect to attraction, behavior, and identity. Research confirms this expectation… These studies show that even if the concept of immutability were extended to mean a substantial resistance to change, available evidence tends to show that sexual orientation is more plastic than commonly supposed.

Dr. Paul McHugh, M.D. is the University Distinguished Service Professor of Psychiatry at the Johns Hopkins University School of Medicine. From 1975 until 2001, Dr. McHugh was the Henry Phipps Professor...