We were drawn to this study by shared interest in an evolutionary enigma: if homosexual sex is caused in part by genes and doesn’t produce children, why haven’t those genes, over thousands of years of evolution, disappeared from the population? To address this question we used data from participants in the UK-Biobank, 23andMe, and AddHealth. In all, our results are based on data from nearly half a million participants for which we had genome-wide genetic data and information on their sexual orientation and number of sexual partners. In our project we wanted to identify genetic variants that contribute to sexual orientation in order to help us understand the biological mechanisms that play a role in sexual orientation. We also used the genetic data to test the evolutionary hypothesis that the genes that predispose to homosexual behaviour are evolutionarily advantageous in heterosexuals who carry those genes.