What causes homosexuality

Genome study with almost 500,000 people : Homosexuality cannot be read from genes

The sober question of why around two to ten percent of people feel attracted to the same sex, while the rest are more likely to look for partners of the opposite sex, is essentially a behavioral biological one.

But even though same-sex partnerships and marriages are now socially accepted, there is still heated debate about whether researchers should even address this question. Is there a search for a “defect” in the genetic make-up, for a “flaw” in development and thus possibly pathologized what society in this country only accepted after a long struggle as part of the diversity of being human? Or would it rather create more social recognition and understanding for gays and lesbians if researchers found reasons for the small difference in love?

There will also be this discussion about the most recent and so far largest study, presented in the journal “Science”. Using genetic information and information on partner choice of 477,522 people, a team of scientists led by Andrea Ganna from the Broad Institute in Cambridge, Massachusetts, examined whether there are gene variants that are associated with same-sex partner choice. Anticipating the social debate surrounding their project, the researchers developed their own website to present and explain the results.

Five genome segments are correlated with same-sex partner choice

In a nutshell: Yes, there are five gene segments in the human genome that have an influence on same-sex partner choice. Two of them are involved in the regulation of sex hormones such as testosterone and estrogen. But the influence of these gene variants - two are only found in men, two in both sexes and one only in women - is very slight. Taken together, the five genes explain barely one percent of same-sex behavior in the population.

The effect is so marginal that the researchers expressly emphasize that no prediction about a person's sexual orientation is possible based on these genes - for example, by genetic testing at birth.
A number of studies - including twin studies - have already shown that homosexuality occurs more frequently in some families than in others and that heredity also plays a role in addition to environmental factors. However, to date researchers have not been able to find any clues as to which of the approximately 23,000 human genes might be involved.

The reason given was the reference to the insufficient number of those examined. With the present study, this argument is now off the table. Ganna's team used data sets from 408,995 people who also answered the question “Who have you had sex with in the past?” From the UK Biobank for a genetic test, and from six different possible answers from “exclusively with the same sex” to “mostly with the opposite sex ”. The US company "23andMe" contributed the data of a further 68,527 study participants. Anyone can send in their genetic material there for analysis and release it for study purposes.

"The genetic material is not suitable for discriminating against people"

“This study clearly shows that the heritability of sexual orientations is low and that nothing can be 'read off' from a person's genetic make-up,” says Jan Korbel from the “European Molecular Biology Laboratory” in Heidelberg. “There is no single gene that determines sexual orientation.” The genetic make-up is not suitable for discriminating against people of different gender tendencies.
Markus Nöthen from the University of Bonn also classifies the results as a “milestone in research into the biological causes of homosexual behavior”. However, the human geneticist considers the “superficial assessment of the extremely complex behavioral dimension of sexual orientation” to be problematic.

Multiple causes of same-sex orientation

There are probably many factors that shape same-sex orientation anyway. Ray Blanchard, for example, researched whether the statistical discrepancy that homosexual men have older brothers more often than the population average could be a key to understanding same-sex orientation. According to this, around 15 to 29 percent of homosexual men could be gay because their brain development during pregnancy was exposed to an immune reaction of the mother against certain proteins of the embryo that are important for brain development. This immune response is triggered during the previous pregnancy, but then affects the younger brother. "Andrea Ganna's results do not contradict that," Blanchard told Tagesspiegel. It is even likely that homosexuality is influenced by both genetic and prenatal environmental factors.
How research into homosexuality is received in public is a matter of "fashion," says Blanchard. "There was a time when activists interpreted this as insulting and marginalizing." But that changed when research showing the immutability of homosexuality made a good case for same-sex partnerships and marriages. “Scientists should certainly not do research that harms humanity or certain groups,” says Blanchard. But it is not desirable to align research with current opinion. (with smc)

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