We used information from nearly half a million research participants who contributed DNA and answered questions about their sexual behavior to the following large-scale research projects or direct-to-consumer genetic companies:
- UK Biobank, a population-based cohort of approximately 500,000 participants.
- 23andMe, Inc., a personal genetics company founded in 2006 that, as of May 2018, had genotyped more than 4 million customers who have consented to participate in research.
- The National Longitudinal Study of Adolescent to Adult Health (Add Health) , which originated as an in-school survey of a nationally representative sample of US adolescents.
- Molecular Genetic Study of Sexual Orientation (MGSOSO), a study specifically focused on sexual orientation in males.
- The Child and Adolescent Twin Study in Sweden (CATSS), a study targeting all twins born in Sweden since July 1, 1992.
Click here for more information about these datasets.
How did we identify genetic markers in each participant’s DNA?
Each person’s DNA was genotyped — that is, we determined which of the four DNA bases (A, C, G, and T) was present at each of hundreds of thousands of defined locations in the human genome that are common across people. Click here for videos about this topic.
How did we define same-sex sexual behavior?
In this study, we examined “same-sex sexual behavior,” which is defined as having ever had sex with someone of the same sex. “Same-sex sexual behavior” is related to, but not the same as sexual orientation and identity. Individuals in our study may have engaged in “same-sex sexual behavior”, and they may have a range of identities and personal reasons for engaging in this behavior. While our study includes many gay and lesbian individuals, it may also include those who sexual identify as bisexual, pansexual, straight, or one of many other identities. These and other limitations in our data and study, outlined in this section, mean that we are unable to fully explore the rich complexity of sexual identity and orientation. In particular, because our research analyzes existing information and data, our scope and findings are limited by the way these datasets have been structured. Moving forward, additional work in this field requires thoughtful approaches to linking genetics to better measures of identity and behavior to ensure that research considers the full diversity of human sexuality, identity, and experience.
The questions relevant to our study were embedded in large questionnaires assessing a wide range of characteristics. Each project which we included in our analysis approached this question in slightly different ways. Again, we use 'same-sex sexual behavior' as a catchall term, given the different ways the different databases captured this outcome (READ MORE)
- UK Biobank participants were asked: Have you ever had sex with someone of the same sex? and were classified according to if they answered this question as having only opposite-sex partners or having had any same-sex partners. See the question here. Participants who reported having had no sexual partners were excluded from our analysis, as our study focused on sexual behavior rather than identity or experience.
- Participants from 23andMe filled out a ‘Sexual Orientation Survey’ that included questions about sexual identity, sexual attraction, sexual experience, and sexual fantasies.
- Participants in the Add Health study were asked about sexual experience, romantic attraction, and sexual identity.
- In the MGSOSO study, sexual orientation was based on participants’ self-reported sexual identity and sexual feelings.
- In the CATSS study, sexual orientation was based on participants’ self-reported sexual attraction and sexual experience.
We also explored other indicators of sexuality. In particular, we examined the proportion of same-sex partners among people that, at least once, had sex with someone of the same sex in order to explore whether there were differences in the genetic associations between different aspects of sexual behavior.
To identify any links between genetic markers and same-sex sexual behavior, we relied on a well-established approach known as a genome-wide association study (GWAS). GWAS allows scientists to systematically and statistically test millions of genetic markers across the entire genome for associations with a trait — for instance, to determine whether or not having a particular DNA base (an A, for example) at a given location is associated with same-sex sexual behavior.
Using the Genome-Wide Association Study, or GWAS, approach, we:
- Identified genetic markers associated with same-sex sexual behavior.
- Explored the extent to which the genetics of same-sex sexual behavior is the same for females and males.
- Looked for evidence of overlap between the sets of genetic variants that influence same-sex sexual behavior and those that influence the proportion of same-sex partners among people that, at least once, had sex with someone of the same sex.
- Looked for evidence of overlap between the genetic markers that influence same-sex sexual behavior and those that influence other personality, reproductive, and psychiatric traits.
We preregistered the study at the Open Science Framework and made available our summary statistics, which allow interested researchers to explore the variant-level associations — available.