nonhuman wealth, religion, ethnic origin, height, and geographical
propinquity of spouses are positive and strong. A small amount of
evidence suggests that the correlations between certain psychological
traits, such as a propensity to dominate, nurture, or be hostile, are
negative. The correlation by intelligence is especially interesting since,
although intelligence is highly inheritable, the correlation between mates
is about as high as that between siblings (Aistrom 1961). Apparently,
the marriage market, aided by coeducational schools, admissions tests,
and the like, is more efficient than is commonly believed.
This evidence of positive simple correlations for a variety of traits, and
of negative correlations for some, is certainly consistent with my theory of
sorting. A more powerful test of the theory, however, requires evidence on
partial correlations, when various other traits are held constant. For
example, how strong is the correlation by intelligence, when years of
schooling and family background are held constant? I do not yet have
results on partial correlations by intelligence, but do have some on years
of schooling, wage rates, and age, for samples of white and black families.43
Even when age and wage rates are held constant, the correlation between
years of schooling is high, + .53 for whites and virtually the same (+ .56)
for blacks. Although the partial correlations between wage rates are
much lower, they are also positive, + .32 for whites and a bit lower
(+ .24) for blacks.
The strong positive partial correlation between years of schooling is
predicted by the theory, but the positive correlation between wage rates
is troublesome since the theory predicts a negative correlation when
nonmarket productivity is held constant. Note, however, that the sample
is biased because it is restricted to women in the labor force in a particular
year. Since the higher the husband's wage rate the higher must be his
wife's wage rate to induce her to enter the labor force, a negative correlation
across all mates is consistent with a positive one for those having
wives in the labor force.44 Indeed, Gregg Lewis has shown45 that a
correlation of about + .3 for mates who are participating almost certainly
implies a negative one (about — .25) for all mates, given the relatively small