Perspectives on the complex genetics of same-sex sexual behavior

In a peer-reviewed study published in Science, researchers found evidence that many genetic variants contribute to same-sex sexual behavior, but each has a small influence. When taken together, the variants explain only a minority of a person’s likelihood of ever engaging in sexual behavior with a person of the same sex. 

The study concludes that both genetics and non-genetic factors play important roles. 

Five locations in the human genome were associated with this trait at a statistically significant level, but these five loci capture only a tiny fraction of the genome's overall contribution (far less than one percent). The analysis further revealed that thousands of other variants also make tiny contributions that, together with the five loci, account for between 8 and 25 percent of the variation in self-reported same-sex sexual behavior. Much of the remainder is likely due to non-genetic factors. 

These results do not make any conclusive statements about the degree to which “nature” and “nurture” influence sexual orientation or behavior, but indicate that both are likely to play a role.

These genetic results — likely thousands of variants, each with a very small effect — are similar to those for many other complex traits, like height, and indicate that same-sex sexual behavior is a normal part of human variation.

According to the study, there is no “gay gene” that determines whether a person will have same-sex partners in their lifetime. The findings indicate that it is impossible to meaningfully predict an individual’s same-sex sexual behavior from genetics.

The study and the data

While the relationship between genetics and human same-sex sexual behavior has been studied in the past, this previous research has often lacked a high degree of precision. For this study, the researchers, representing six institutions, wanted to use rigorous genetic and statistical methods to study existing data that have been made available for scientific research worldwide. Beginning in 2017, they performed genome-wide association studies (GWAS) using survey responses and genetic data from more than 470,000 participants in UK Biobank and 23andMe.

The data used in this study, including data on self-reported sexual behavior, were previously anonymized and de-identified and had been provided to UK Biobank and 23andMe for the purpose of helping researchers answer a wide range of scientific questions about ourselves — including identifying genetic links that can help explain aspects of health, disease, behavior, and personality. These individuals provided consent for their information to be used for approved research purposes. 23andMe participants also gave a “secondary consent” specifically for studies involving genetics and sexual activity. 

The researchers received permission from UK Biobank and 23andMe to analyze the datasets.

The study’s limitations — and what it doesn’t say

This study is about sexual behavior, not sexual orientation or gender identity.

• This study relied on self-reported data gathered as part of several different research projects, which then had to be aligned across disparate datasets. Therefore, the authors were limited in what they could examine with statistical rigor.
• The study’s central question (whether participants had ever had a same-sex partner) reflects only a small aspect of the complexity of human sexual behavior. The authors note this important limitation in the paper.
• The data in the analysis came from individuals of European ancestry, because there was not enough data in the existing datasets to draw meaningful conclusions for other populations.

The findings

• Thousands of genetic variants each make small contributions to a person’s likelihood of ever engaging in sexual behavior with a person of the same sex. These genetic results are similar to those for many other complex human traits. Of these variants, five locations in the human genome were associated with this trait at a statistically significant level. While statistically significant, these five variants together account for much less than one percent of the variation of this behavior between people.
• Taken together, all of the thousands of genetic signals account for only a minority of the variation in self-reported same-sex sexual behavior. In particular, the proportion of the variation that is explained by genetics is estimated at between 8 and 25 percent. The remainder is attributed to something else.
• The results definitively show that there is no single “gay gene.”
• The results also indicate that it is not possible to meaningfully predict an individual’s same-sex sexual behavior from genetics.
• These results do not make any conclusive statements about the degree to which “nature” and “nurture” influence sexual orientation or behavior, but indicate that both are likely to play a role.
• The locations of the handful of significantly-associated variants in the genome hint at some of the biological pathways that may be involved in this behavior. One variant was located in a stretch of DNA that houses genes related to the sense of smell. Smell has strong ties to sexual attraction, but its links to sexual behavior are not clear. Another was near a gene associated with whether or not men become bald, which is closely linked to how the body regulates sex hormones. This suggests a relationship between sex hormone regulation and same-sex sexual behavior. However, these observations provide only a tiny glimpse into the vastly complicated biology of human sexual behavior.
• The researchers also looked for correlations between the thousands of genetic factors that are associated with different traits. They observed some genetic correlation between whether people reported ever having a sexual experience with a person of the same sex and a variety of personality and behavioral traits, such as risk-taking and openness to experience.
• The genetic influences on ever reporting a same-sex sexual partner showed little overlap with the genetic influences on the proportion of same-sex sexual partners. This means that  a single continuum, such as that reflected in the Kinsey scale (a research tool ubiquitously used to measure sexual orientation), does not fully reflect the diversity of behavior people are reporting.
• The findings reinforce the idea that the diversity of sexual behaviors across humanity is a natural part of our overall diversity as a species.

Suggestions of social and cultural influences

The authors note that some of the participants became adults at a time and place where same-sex sexual behavior was criminalized, which other studies indicate can have a profound effect on mental health. This study finds some correlations that appear consistent with this.

The findings also show some differences between men and women in the genetic associations with same-sex sexual behavior. Similarly, there is evidence of some differences between the two largest samples, in the US and the UK. These differences pose questions about how various cultural and societal norms can shape complex human behaviors.

Perspectives

The authors — and the Broad Institute of MIT and Harvard — note that the research raises important social, ethical, and scientific issues that are worth considering and discussing.

This study represents the work of a group of scientists, including faculty and students, who were interested in this topic. Scientists are independent and free to consider research questions, provided they have appropriate approvals to use the data. Here, the researchers received permission from UK Biobank and 23andMe to analyze these datasets, and individuals included in these datasets provided consent for their information to be used for approved research purposes. 23andMe participants also gave a “secondary consent” specifically for studies involving genetics and sexual activity. The paper was subject to rigorous peer-review and was published in a world-leading scientific journal.

The study’s authors — some who identify as heterosexual, and some who identify as gay — engaged a number of experts in LGBTQIA+ advocacy and research organizations in the US and around the world as they prepared the study and the publication. These experts offered important feedback, including concerning the language used in the paper and related materials, to help ensure that the study was done rigorously and the findings were clearly and responsibly communicated.

At the Broad Institute, several members of the Out@Broad group, which includes members of the LGBTQIA+ community and their allies, worked with some of the study authors to raise concerns about the project itself — including whether the authors should have undertaken the research. They also made suggestions around language and scientific figures, and flagged possible misinterpretations of the findings.

Because we believe it is important to represent a range of perspectives about this work, we invited members of the Broad community and others to provide their thoughts on the study, the process, the implications, and lessons we might learn.

Here are their perspectives, which we hope will inform a needed discussion:

Community engagement strengthens science
A Broadminded blog opinion piece by Broad institute member and study co-senior author Benjamin Neale

Seeking justice in the age of genomics and a call for higher ethical standards for research involving human populations
A Broadminded blog opinion piece by Broad Institute research associate Bryan Ferguson

Unintended, but not unanticipated: the consequences of human behavioral genetics
A Broadminded blog opinion piece by Broad Institute bioinformatics analyst Carino Gurjao

Discovery or discrimination? Starting the conversation about the potential outcomes of a LGBTQIA+ targeted study
A Broadminded blog opinion piece by Broad Institute operations specialist Meagan Olive

For all this science, what did we learn?
A Broadminded blog opinion piece by Broad Institute postdoctoral fellow Steven Reilly

Weighing the positive and negative impacts of studies regarding sexual minorities
A Broadminded blog opinion piece by Broad community members Liam Spurr, Julian Avila-Pacheco, Meagan Olive, and Denisse Rotem

Big data scientists must be ethicists too
A Broadminded blog opinion piece by Broad Institute postdoctoral fellow Joseph Vitti

What Genetics Is Teaching Us About Sexuality
A New York Times opinion piece by Broad Institute and Harvard University postdoctoral fellow and study co-author Robbee Wedow and UT Austin integrative biologist Steven Phelps

How do genes affect same-sex behavior?
A Science perspective by University of Oxford sociologist Melinda Mills

Same-sex sexual behavior and genes: like love, the answer is complicated
A STAT opinion piece by 23andMe vice president of business development Emily Drabant Conley