On the average, they`re both equally smart, however, the male populations distribution is wider than the female ones (ie, there are more genius males at the cost of more moron males to balance it out), apparently its an evolution-genetics thing; after all only a small fraction of the available male population is required to maintain a races survival (like, they can just duck multiple women to reproduce) so genetics tend to experiment with them instead of the females.
At the very least thats what I`ve read sometimes ago