In this paper it is argued that models for completed fertility have to take
into consideration that childless couples and couples with an only child a
re qualitatively different from couples with two or more children. Indeed,
these differences may be the cause of the underdispersion that characterize
s completed fertility data. An empirical illustration using Portuguese data
suggests that accounting for the qualitative difference between having zer
o, one, or more children leads to considerable improvements over a model of
the type generally used to describe this sort of data. JEL classification:
C25, J13.