Evidence for one or more loci on the human X chromosome influencing social cognition was recently presented by Skuse et al.(1). The imprinted locus is only expressed from a paternally inherited X chromosome, which means that boys do not express it because their only X chromosome comes from their mother. This raises the possibility of genetic as well as cultural influences on sex differences in behaviour and cognition. It may also offer some explanation for why boys are more vulnerable to (...) developmental disorders that affect social behaviour, such as autism. (shrink)