No 'gay gene,' sexuality formed in womb

The study's findings were published by Science on August 29, 2019. According to a report by Nature, the study, which was conducted on 493,001 participants, found five markers on the human genome that were linked to same-sex behavior, yet none were reliable enough to predict one's sexuality.

 

Andrea Ganna, a geneticist at the Board Institute of MIT and Harvard, used the analysis to estimate that up to 25% of sexual behavior can be explained by genetics.

 

 

Melinda Mills, a sociologist at the University of Oxford, said the study is "a solid study," but she also said it might not be representative of the overall population.

The vast majority of the genomes used in the study have come from the databases of UK biobank and 23andMe, with mostly European participants between the ages of 40-70.

The authors also pointed out that they followed convention, omitting individuals whose biological sex did not match their self-identified gender, such as transgender  and intersex (previously referred to as hermaphrodites) people, Nature reported.

As genetics explain only 25% of sexual behavior, the rest of sexual behavior is influenced by environmental and cultural factors, affirming the findings of smaller studies, according to Ganna.

A 2011 study conducted by Dr. Jacques Balthazart of the university of Liège, Belgium, suggested that gonadal hormones, also known as sex steroids, have a significant influence on the development of one's sexual orientation.

"These organizing actions of sex steroids on behavior are paralleled by irreversible changes in brain structure, embryonic sex steroids differentiate the size of several brain structures," Balthazart wrote in the study.

Researchers from the laboratory of Roger Gorsky at the University of California discovered an area of the hypothalamus that is significantly larger in men than in women, Balthazart noted.

This area, according to the findings, is significantly smaller in homosexual men than in heterosexual men.

In his study, Balthazart looked at the effects of diethylstilbestrol, which he says was used to treat 2 million women in Europe and the US between 1939-1960 to prevent miscarriages.

As the treatment turned out to be ineffective, but one of its unexpected outcomes was that girls born from treated mothers showed a significant increase in bisexual and homosexual behaviors.

"Clinical conditions associated with significant endocrine changes during embryonic life often result in an increased incidence of homosexuality. It seems therefore that the prenatal endocrine environment has a significant influence on human sexual orientation," Balthazart wrote.