How did homosexuality evolve? There might be a clue in our genes

Genetic effects associated with same-sex sexual behavior are also associated with a mating advantage among people who engage only in opposite-sex sexual behavior, according to a study involving participants from the United States and United Kingdom published in Nature Human Behaviour. However, the authors caution that the genetic differences studied here are small, are spread throughout the human DNA sequence and capture only a small portion of the heritability of same-sex sexual behavior. Further research is needed to confirm whether these findings apply to the wider human population.

Brendan Zietsch and colleagues analyzed genetic effects for same-sex sexual behavior (versus never having had sex with someone of the same sex) in a genome-wide association study (GWAS) of 477,522 people from the UK and the US. They also estimated genetic effects for opposite-sex sexual behavior in a GWAS of 358,426 people from the same countries, who had only ever had opposite-sex partners and who also reported how many partners they had had in their lifetime. The authors found that genetic effects associated with same-sex sexual behavior were also associated with having more opposite-sex sexual partners among individuals who had only ever had opposite-sex partners. The authors suggest that number of opposite-sex sexual partners is an indicator of mating success, which during evolution would have led to more children. The genetic effects Zietsch and colleagues identify may help to explain why same-sex sexual behavior has persisted throughout the evolution of the human species: these genetic effects may have been favored by evolution as they are associated with more children.

The authors note that there are a number of limitations to this study and their findings should be interpreted with caution. First, the data used in this study are only from individuals of European ancestry in the UK and the US and therefore capture a fraction of human genetic and behavioral diversity. This is likely to affect the results, as both same-sex sexual behavior and number of opposite-sex sexual partners are highly societally regulated behaviors. Second, the number of opposite-sex sexual partners reported in individuals today may not be associated with a reproductive advantage in our evolutionary past.