Sex differences in intelligence and brain size: A developmental theory

In 1992, it was reported by Ankney and Rushton that males have larger avera ge brain size than females even when allowance is made for body size. It is known that brain size is associated with intelligence, and it would theref ore be expected that males would have higher intelligence than females. Yet it has been universally maintained that there is no difference in intellig ence between the sexes. It is proposed that this anomaly can be resolved by a developmental theory of sex differences in intelligence which states tha t girls mature more rapidly in brain size and neurological development than boys up to the age of 15 years. The faster maturation of girls up to this age compensates for their smaller brain size with the result that sex diffe rences in intelligence are very small, except for some of the spatial abili ties. From the age of 16 years onwards, the growth rate of girls decelerate s relative to that of boys. The effect of this is that a discernible male a dvantage of about 4 IQ points develops from the age of 16 into adulthood, c onsistent with the larger average male brain size. This paper presents new evidence on the developmental theory of sex differences in intelligence and discusses alternative attempts to deal with the anomaly by Ankney (1995), Mackintosh (1996), and Jensen (1998).