Biological and sociological interpretations of age-adjustment in studies of higher order birth rates

Several studies of the effect of education on second or third birth rates (e.g. Hoem et al. (2001)) have used the concept of relative age at previous birth (B.Hoem (1996)). B.Hoem's idea was to focus on the social meaning of age at previous birth. We broaden the discussion by considering other interpretations of the explanatory power of the age at previous birth, particularly via known trends in biological fecundity. A mathematical analysis of the approach reveals side effects that have not been taken sufficiently into account. Our recommendation is not to use the relative age approach without supplementing it with the more traditional approach which includes the actual age at previous birth.