It used to be a tradition among anti-feminists that the smaller size of the female brain was proof of woman’s inferior intelligence. It is true that the average circumference of male heads exceeds that of female heads at all ages, but head circumference has no known intellectual correlates. Brain size does, however, bear a direct relationship to body size, and the female’s brain is smaller than the male’s because females are on average smaller than males. (Actually, ounce for ounce, the female has a slightly bigger brain than the male.)
If we want to investigate the relationship between intelligence and biological sex we must discard the idea of intelligence as a single quality, because existing tests of general intelligence have been standardised to minimise the known sex differences in composite abilities. Items which differentiate consistently between the sexes are never included. (This is the reverse procedure from the one used in the Masculinity-Femininity Test described in Chapter 2.) One significant fact which does emerge from tests of general intelligence is the tendency for girls to score higher in the early years (particularly before six) and boys higher later on (particularly after puberty). The results of tests administered over a period of time in late adolescence and adulthood to the same sample show a fairly consistent tendency for men to improve their scores relative to the women in the sample.