There is no study that has found if either gender is smarter than the other, but there are some other interesting facts that scientists have detected. For one, it appears that earlier in life, girls develop faster than boys, and this faster development also has an effect on IQ scores - which is why young girls usually score higher on such tests than boys from the same age group.
On the other hand, as the children mature and the years pass, the pendulum swings in the other direction. In the end, there are several different schools of thought on the subject, with no one coming to a complete conclusion. At the earlier ages, girls excelled at language skills (which they often acquire earlier), and memory skills (such as remembering where objects are).