Finally, a general press science writer wakes up to the flood of nonsense over sex differences, one of the main instances of the pseudo-scientific woo that our friends at Language Log have been fighting a lonely battle over. One question that the writer does not ask, though, is the following that seems to me obvious to ask. Males and females share genes except those in the Y chromosome, and genes from both parents get shuffled into a single genome during fertilization. Therefore, unless there is a strong selective pressure to make certain traits specifically male or female, the tendency would be for advantageous traits to be shared by both sexes. So, the sex differences advocates need to explain what specific selective pressures are operating to impart sex bias to traits that are obviously advantageous to both sexes like spatial reasoning or verbal competence.