Would you prefer to have sex with males or females? It’s such a simple question, but a loaded one. Biologically, we know very little about what goes on behind such sexual choices, in humans or other animals. Socially, it’s a question that can provoke fierce debate, social stigma, and psychological anxiety. Into this minefield steps a new study from Yan Liu and Yun’ai Jiang at Beijing’s National Institute of Biological Sciences.

Here’s the simple version. They found that male mice lose their normal preference for female mice if they have low levels of serotonin – a hormone that carries signals in the brain. Instead, they try to mate with individuals of both sexes, in equal measure. Inject them with more