Extra female genes make men more masculine

NOW here's a brain bender: an extra set of female genes appears to make males more masculine. The surprising discovery suggests that sex chromosomes play a role in directing behaviour that extends beyond the effects of hormones.

"The predominant idea is that the difference between male and female behaviours is down to hormones," says Emilie Rissman at the University of Virginia in Charlottesville. This starts early in life - male fetuses are exposed to testosterone from 4 weeks old, while females are not.