We develop a theory of fertility, distinguishing its intensive
margin from its extensive margin. The deep parameters are
identified using facts from the 1990 US Census: (i) fertility of
mothers decreases with education; (ii) childlessness exhibits a
U-shaped relationship with education; (iii) the relationship
between marriage rates and education is hump-shaped for women and
increasing for men. We estimate that 2.5 percent of women were
childless because of poverty and 8.1 percent because of high
opportunity cost of childrearing. Over time, historical trends in
total factor productivity and in education led to a U-shaped
response in childlessness rates while fertility of mothers
decreased.