SAME-SEX ATTRACTION is found in many animals—but, like all animal behaviours, it can be complicated and difficult to study. The underlying biological mechanisms, however, are of great interest for understanding human health, genetics and evolution. Researchers know there is no such thing as a “gay gene”; in fact genetics can explain less than a third of the variation in people’s self-reported same-sex behaviour. Non-genetic factors, therefore, play an enormous role. Scientists can’t agree on exactly what those factors are, though.Abby Bertics, The Economist’s science correspondent, visits a Royal Society conference in Cookham on the genetics and evolution of same-sex attraction, to meet some of the researchers applying scientific rigour to these intriguing questions. Lisa Diamond, a psychologist at the University of Utah, explores some of the theories related to brain development in babies. Robbee Wedow, a sociologist at Purdue University, and Andrea Ganna, a data scientist at the Univ