Some are genetic accidents, such as Mongolism or Down's syndrome, which are caused by an extra chromosome. Some are caused by physiological problems, such as cretinism, which results from thyroid deficiency. Some are due to physical factors such as not receiving enough oxygen before birth. The answer is unclear. The sexes differ in specific abilities.
Females are typically superior in verbal abilities and in items concerned with social relations.
They also perceive details more quickly and accurately and do better on tests of memory. Males, on the other hand, surpass women on items that involve spatial, numerical, and mechanical performance. These differences, however, are averages. The variance within each sex is much greater than the difference in results between the sexes. In scores of overall intelligence, the two sexes seem to be about equal.