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 serotonin, and you can restore their preference for females. The obvious conclusion is that serotonin affects the sexual preferences of mice. The obvious question is whether it also affects the sexual preferences of humans. And, as is often the case, it's a little more complicated than that.