Which factor reduces family size the most? Below are the standardized OLS regression coefficients for a sample of whites ages 40-59:
Standardized OLS regression coefficients
Church attendance .10*
Belief in God .06*
Political conservatism .06*
*p < .05, two-tail test
Education reduces fertility more than any factor. IQ has no net impact. Look at how income has a significant positive effect on family size. Not surprisingly, religiosity and conservatism are associated with more children.