A stock adjustment model of U.S. marital fertility.

An attempt is made to explain the changes over time in the marital fertility of certain age groups in the US, 1947-1974, using a stock adjustment model in conjunction with a version of the relative income hypothesis. The stock adjustment model shows that the initiation of the fertility decline in the late 1950s was not because of a decline in desired completed fertility but due to its leveling off, along with an increase in children ever born. This reduced the additional desired fertility and hence the flow of marital fertility. The desired stock, however, did not begin to fall until considerably later. The stock adjustment was also used to refine the measure of desired completed fertility by correcting for births that were unwanted when conceived due to contraceptive failure and for births that may become unwanted later due to the emergence of unforeseen socioeconomic conditions. The empirical results support the position that desired completed fertility is strongly influenced by permanent relative income, with positive elasticities in the range of 3-7. The rate at which desired completed fertility is realized, measured as the birthrate to couples wanting additional children, was found to be insignificantly related to transitory economic conditions. As the analysis is far from conclusive, the results must be viewed cautiously. The uncertainties include the following: there are several data limitations; the amount of variation in the data series over this period probably does not allow a decisive test of rival hypotheses; and although the stock adjustment model is an advance, it would be more useful if direct measures of the stock variable and the timing variable were available for individual estimation and testing.