A while back I promised to blog about sex differences in IQ but never did because I couldn’t find good data (many IQ tests assume the sexes are equal a priori, thus eliminating subtests or items that show big sex differences).

Anyway, here’ s some interesting data comparing the sexes in brain size and g (general intelligence) (hat-tip to James Thompson for blogging about this).


The first thing I notice is how HUGE younger American adults (age 22 to 37 circa 2011 to 2013) are in brain size.  Scientists have been claiming for years that brains are shrinking, but as Richard Lynn noted back in the 1990s, they only shrunk because of the malnutrition/disease that was agriculture,  but they rebounded over the 20th century.  Indeed if you average the intracranial volume of these “young” American men and women, you get 1,586,683 cubic mm, which is virtually identical to Cro-Magnon’s 1600 cc crania though methods of estimating cranial capacity likely differ and participants in the study were pre-screened for health issues.

As for sex differences in IQ (or g), they’re expressed as Z scores but if we convert them to the IQ scale where all Americans average 100 with a standard deviation (SD) of 15, we find that men average 102 (SD = 13.4), while women average 98 (SD = 13.2).