There’s no evidence that a single ‘gay gene’ exists – Science News
Publication of the largest-ever study of
the roles of genes in homosexual behavior is fanning the debate over whether
being gay is due to genes or environment.
First reported at a genetics conference in
2018, the study found five
genetic variants associated with having a same-sex sexual partner (SN: 10/20/18). But those variants,
called SNPs, don’t
predict people’s sexual behavior, researchers report in the Aug. 30 Science.
“There is no ‘gay gene’ that determines
whether someone has same-sex partners,” says Andrea Ganna, a geneticist at the
Broad Institute of MIT and Harvard and the University of Helsinki.
Family studies have suggested that
genetics account for about 32 percent of heritability of homosexual behavior. But
each SNP, or single nucleotide polymorphism, has a very small effect on whether
someone has ever had a same-sex sexual partner, the new research found.
Taking into account all the SNPs measured in the study,
including those that weren’t statistically significantly associated with same-sex
behavior, explained only 8 to 25 percent of heritability of same-sex behavior. When
considering just those five statistically significant SNPs, that number drops
to much less than 1 percent.
But those variants could point to
biological processes that are involved in choosing sex partners, the researchers
say. For instance, one variant identified in the study has been linked to male-pattern
baldness, and another to the ability to smell certain chemicals, which may
affect sexual attraction.
“The study is a big step forward because
of its huge size,” says J. Michael Bailey, a psychologist at Northwestern
University in Evanston, Ill., who has worked on sexual orientation genetics but
was not involved in the work. It included more than 470,000 people, dwarfing
previous research.
“This is the first study that we can be
pretty sure that they’ve identified genetic variants associated with an aspect
of same-sex behavior,” Bailey says. “I’ve been a coauthor on some previous
molecular genetic studies that were much more tenuous. I believe these results
will replicate.”
But Bailey disagrees with some of the study’s
conclusions. For instance, Ganna says that people who have exclusively same-sex
or exclusively opposite-sex partners are genetically distinct from people who
have partners of both sexes. That means that sexuality may not be a continuum
from completely heterosexual to homosexual after all. So the Kinsey scale,
which scores people’s sexual behavior along a spectrum with bisexuality in the
middle, may need to be rethought, the researchers say.
But the Kinsey scale accurately predicts
men’s arousal when shown erotic pictures of either men or women, Bailey says.
That makes it a far better tool than the genetic score for predicting sexual
preference, he says.
Qazi Rahman, a psychologist and sexual
orientation researcher at King’s College London, has bigger quibbles with the
study. “I should be really excited about this,” he says. But “despite being an
ardent believer in the biological basis of sexuality, I’ve found this study
problematic, and I’m not at all sure what was found and whether that holds up.”
Rahman points to what he sees as inconsistencies
in the data and possible bias in the people who volunteered to participate in
the study. The study drew volunteers from two big genetic databases, the UK
Biobank and the consumer DNA testing company 23andMe, and from three smaller
studies. Participants answered questionnaires about how many sexual partners of
each sex they had ever had. 23andMe customers also responded to questions about
attraction, sexual identity and fantasies.
But only 5.5 percent of UK Biobank
participants and about 1.5 percent of 23andMe’s customers joined the study. Such
low participation rates could skew the results, or point to genetic variants
that make people more likely to sign up for a study. “What you’re getting is
genetic influences on self-selection into a study, not genetic influences on
same-sex behavior,” Rahman says.
It’s legitimate to question where study
participants come from, but there’s no way to know whether that bias is
affecting the results, says coauthor Benjamin Neale, a geneticist at Massachusetts
General Hospital in Boston and the Broad Institute.
The study wasn’t designed to address
sexual orientation or identity, but the same variants associated with same-sex
behavior were also associated in 23andMe participants with attraction, sexual
identity and fantasies. The small contribution of genetics to sexual behavior
is in line with genetic contributions to other behaviors, such as level of education
attainment. “There’s a lot of room for nongenetic effects,” Bailey says.
The authors don’t disagree. The study
underscores that elements of both biology and one’s environment may play roles
in shaping sexual behavior, Neale says. Environmental influences may include an
array of developmental, social and cultural factors that all could affect
behavior, he says.
That’s true, says coauthor J. Fah
Sathirapongsasuti, a computational biologist at 23andMe in Mountain View, Calif.
But, he says, “just because something is not completely genetic or something
has an environmental, or what we call nongenetic, component doesn’t mean it’s a
choice.”