(a) The concept of race is largely demographic rather than biological. Although blacks and whites may differ in superficial appearance, the differences in their gene structures (where gene frequencies are known) are in most cases far less between races than within them.
(b) Estimates of the heritability of intelligence differ, depending upon the assumptions one makes. Jensen estimates the heritability coefficient to be .81 but other investigators find heritability coefficients as low as .35; the lower estimates assume that more of the variance is attributable to a genetic-environment interaction.
(c) Among black populations there is some tendency for lightness of skin colour (presumably an indication of the degree of intermixture with whites) to correlate positively with IQ. But such correlation’s are very low (typically .15) and can be explained on the basis of environmental differences, a lighter skin colour being associated with less discrimination and greater opportunity. A study of very high IQ black children in Chicago found no indication of greater Caucasian ancestry than for the black population as a whole (Witty and Jenkins, 1936).