Whether you fight like a girl or like a boy is hardwired into your nervous system—at least if you are a fruit fly. A research team at Harvard Medical School and the Institute of Molecular Pathology in Vienna discovered that a fruit fly gene named fruitless, known to be involved in courtship behavior, also plays an important role in the biology of aggression, directing sex-specific fighting patterns.

Male and female fruit flies fight with distinctly different styles. Female fighting involves head butting and shoving, whereas males prefer boxing and lunges. But when the team swapped the male and female versions of the gene, the flies switched roles. With a feminine fruitless gene in their brains, male flies adopted more ladylike fighting tactics and females carrying the male version of the gene started fighting the way boys do.

“The fact that a single gene is involved in programming several different sex-specific behavior patterns rather than just one is a novel discovery,” says co-author Edward Kravitz of Harvard Medical School. The next step is to identify the neural circuits that are unique to each of the two instincts, he adds.

Even though humans do not have a gene analogous to fruitless, the findings could help elucidate the biological basis of human behavior, Kravitz says: “Anything we can learn about how behavior is wired into the nervous system is ultimately going to be relevant.”