Parity-decomposition of the change in the mean age at childbearing. Lessons for the timing of the second demographic transition

Adrien Remund, University of Geneva

In the theory of the second demographic transition the mid-1960s were identified as a turning point in terms of family and demographic behaviors that encompass ever-decreasing fertility levels and rapid postponement. However, evidence shows that the Mean Age at Childbearing (MAC) only started increasing in the mid-1970s in Western Europe and North America. The question is then to determine whether the statement of timing per se or the measure of the MAC should be revised. In this paper I show how the change in the MAC is influenced by the parity-unbalanced decrease in fertility, and propose a decomposition of this change between the effects of pure postponement and the parity-composition of the TFR, using Kitagawa's technique. Although it has often been identified by demographers, this compositional effect has never been precisely measured. After having isolated the share of the change in the MAC directly attributable to postponement, I show that the timing of the turning point in the postponement of childbearing must be relocated to the mid-1960s for USA and Canada, just as predicted by the theory of the second demographic transition. I also show that this parity weights-adjusted measure avoids the traditional pitfalls of using the yearly change in the MAC as a measure of postponement. I finally propose an alternative to Bongaarts and Feeney's tempo-adjusted TFR' that makes use of this parity-purged measure of MAC rather than the usual MAC.