inequality in commodity income among men at the price of reducing
aggregate commodity income. Perhaps other restrictions on mating
patterns that reduce inequality would be tolerated, but that does not
seem likely at present.
Since positive assortive mating increases the between-family variance,
it increases the potential for genetic natural selection, by a well-known
theorem in population genetics. 82 The actual amount of selection depends
also on the inheritability of traits, and the relation between the levels of
the traits of mates and the number of their surviving children (called
"fitness" by geneticists). For example, given the degree of inheritability
of intelligence, and a positive (or negative) relation between number of
children and average intelligence of parents, the rate of increase (or
decrease) per generation in the average intelligence of a population would
be directly related to the degree of positive assortive mating by
intelligence.
Moreover, the degree of assortive mating is not independent of inheritability
or of the relation between number of children and parental
traits. For example, the "Cost" of higher-"quality" children may be lower
to more-intelligent parents, and this affects the number (as well as
quality) of children desired. 83 In a subsequent paper I expect to treat
more systematically the interaction between the degree of assortive mating
and other determinants of the direction and rate of genetic selection.