The logic of these two results—low heritability and high polygenicity—clearly demonstrate that the dominant cultural narrative about sexual orientation—which sees homosexual persons as a distinctly bounded biological class of people who were “born that way”—simply cannot be true.
A new study adds to a growing body of evidence demonstrating that the dominant narrative about sexual orientation—that it is genetically determined—simply cannot be true. Instead, the science shows that a person’s sexual orientation and choice of partners depends heavily on the development and expression of personal autonomy regarding one’s own sexual possibilities. People with same-sex attractions should be legally and culturally free not to identify with or act on them.
Late last month, a team of MIT and Harvard scientists published a landmark study of the genetic basis for sexual orientation in the journal Science. The study, which was based on an examination of the genetic material of almost half a million individuals, definitively refutes the idea that being gay is an innate condition that is controlled or largely compelled by one’s genetic makeup.
The study contained two key findings. First, it found that the effect of the genes we inherit from our parents (known as “heritability”) on same-sex orientation was very weak, at only .32 on a scale from 0 (none) to 1 (total) heritability. This means that a person’s developmental environment—which includes diet, family, friends, neighborhood, religion, and a host of other life conditions—is twice as influential on the probability of developing same-sex behavior or orientation as a person’s genes are.
Second, rebutting decades of widespread belief, the study established that “there is certainly no single genetic determinant (sometimes referred to as the ‘gay gene’ in the media)” that causes same-sex sexual behavior. On the contrary, “the variants involved are numerous and spread across the genome.” Each of these genetic variants increases a person’s propensity for same-sex behavior by an infinitesimally small amount. In scientific terms, same-sex orientation and behavior are highly polygenetic.
The logic of these two results—low heritability and high polygenicity—clearly demonstrate that the dominant cultural narrative about sexual orientation—which sees homosexual persons as a distinctly bounded biological class of people who were “born that way”—simply cannot be true.
Low heritability, a consistent finding of prior genetic studies, has always suggested that determinism may not be true. But high polygenicity does much more: it affirmatively precludes the possibility of determinism. A genetic arrangement based on dozens of markers across the genome means that virtually all human beings have this arrangement, or large portions of it. In other words, gay people have a perfectly normal human genotype; they are not genetically distinct from all other human beings in any meaningful sense. Consequently, the development of sexual orientation and choice of partners cannot consist primarily in the elaboration of some controlling genetic disposition but, to a much greater degree, consist instead in the development and expression of personal autonomy regarding one’s own sexual possibilities.
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