The heritability of scholastic achievement
In an earlier review of evidence on the heritability of scholastic achievement, I stated: 'In general, individual differences in scholastic performance are determined less than half as much by heredity than are individual differences in intelligence. The largest source of individual differences in school achievement is the environmental differences between families. Variance in achievement due to differential environmental effects within families is extremely small' (Jensen, 1967, p. 153). I now believe this statement is too broad and too simple. No such general statement about the magnitude of the heritability of scholastic achievement seems warranted in view of the large number of factors that are now known to affect the magnitude of h2 for achievement measures. This fact is reflected in the wide range of values of h2 found in various studies. The values of h2 estimated from correlations of MZ and DZ twins on six sets of achievement tests ranged from 0·05 to 0·82, with a mean of 0·40 (Jensen, 1967, p. 152). But the variations are not entirely haphazard and certain generalizations do seem warranted concerning the conditions affecting the magnitude of h2 for achievement measures.