Given that IQ is positively correlated with income, a negative correlation between income and the number of children desired may reduce average IQ over time, other things being equal.
http://economix.blogs.nytimes.com/2011/07/01/richer-people-want-fewer-children/July 1, 2011, 12:54 PM
Richer People Want Fewer Children
By CATHERINE RAMPELL
New York Times
...
[S]ince 1977, most Americans have said having no more than two children is ideal. I should note also that Americans have been making good on those desires, and the fertility rate has fallen greatly over the last half-century.
Even within the populace today, preferences for brood size correlate with income, Gallup found.
Only a third of people with annual household incomes over $75,000 say they want families with three or more children. Of all Americans with income levels below $75,000, 44 percent say they want families with at least three children.
