Sex differences in intelligence and brain size: a developmental theory

In 1992, it was reported by Ankney and Rushton that males have larger average 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 therefore be expected that males would have higher intelligence than females. Yet it has been universally maintained that there is no difference in intelligence between the sexes. It is proposed that this anomaly can be resolved by a developmental theory of sex differences in intelligence which states that 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 differences in intelligence are very small, except for some of the spatial abilities. From the age of 16 years onwards, the growth rate of girls decelerates relative to that of boys. The effect of this is that a discernible male advantage of about 4 IQ points develops from the age of 16 into adulthood, consistent 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).