Genetic variations linked to human bisexual behavior

ANN ARBOR, Mich. – In a groundbreaking study published Wednesday in Science Advances, scientists have identified genetic variations associated with human bisexual behavior for the first time.

Led by Professor Jianzhi “George” Zhang at the University of Michigan, the research delves into the intricate relationship between genetics, behavior, and societal factors, shedding light on the long-standing evolutionary puzzle of why natural selection has not eradicated the genetics underpinning same-sex attraction.

The study, based on data from more than 450,000 people of European descent who participated in the UK Biobank, a long-term genomics project, builds on growing research, including a seminal 2019 paper in Science that found genetic variants influencing same-sex behavior, with environmental factors playing a significant role.

Zhang explained that the motivation for the new research was to differentiate between various forms of homosexual behavior, recognizing that there is a spectrum. “In the past, people lumped together all homosexual behavior…but actually, there’s a spectrum,” he said.

Analyzing participants’ complete sets of DNA and combining that information with survey responses, Zhang and co-author Siliang Song confirmed that the signatures associated with same-sex behavior and bisexual behavior were distinct. This allowed them to analyze these behaviors separately, leading to intriguing discoveries.

Male heterosexuals carrying markers associated with bisexual behavior (BSB)-associated alleles were found to father more children than average, suggesting reproductive advantages. Moreover, men identifying as risk-takers tended to have more children and were more likely to carry BSB-associated alleles.

“Our results suggest that male BSB–associated alleles are likely reproductively advantageous, which may explain their past persistence and predict their future maintenance,” the authors wrote.

Nature’s complexity was underscored by Zhang, noting that a single gene can influence multiple traits, a phenomenon known as “pleiotropy.” “Here we’re talking about three traits: number of children, risk-taking, and bisexual behavior—they all share some genetic underpinnings,” he said.

Exclusive same-sex behavior (eSSB) associated alleles, however, were correlated with having fewer children when carried by heterosexual men, indicating that these traits may diminish over time.

The UK Biobank data revealed a significant trend—the proportion of people reporting both bisexual and homosexual behavior has been rising for decades, likely due in part to growing societal openness.

In an additional report, scientists announced on Wednesday that they have identified genetic variations associated with human bisexual behavior. The study builds on the University of Michigan research and further emphasizes the link between these genetic variations and risk-taking behavior, as well as the impact on offspring numbers when carried by heterosexual men.

The authors of both studies estimated that approximately 40 percent of a person’s behavior regarding bisexuality is influenced by genetics, while 60 percent is influenced by the environment. Emphasizing the contribution to understanding human sexuality, the researchers stressed that their results are not intended to endorse discrimination based on sexual behavior.

This comprehensive research provides a nuanced perspective on the interplay between genetics, behavior, and societal factors in shaping human sexuality, opening the door to further exploration in this complex and evolving field.