The role of cohort overlays of evolving childbearing patterns in shaping period fertility trends

Tomas Frejka, Independent consultant

Quantum and tempo effects in shaping total period fertility rate (TPFR) trends have been thoroughly investigated in the literature. An analysis of the effect of the overlap of changing cohort childbearing patterns of successive cohorts on TPFRs, which so far has been overlooked, is the focus of this paper. The fertility history of Western countries, Southern Europe, Central and Eastern Europe, and of East Asia during the past half century is analyzed in the light of this relationship. Our research concludes that period fertility descents and troughs, for instance, “lowest-low” fertility, as well as increases and peaks are predominantly the outcome of changing cohort childbearing patterns due to fertility postponement and recuperation combined with overlays of successive birth cohorts. Period fertility troughs occurred in Western countries during the 1980s, in Central and Eastern Europe around 2000. TPFR increases in the early 21st century are largely generated by relatively high numbers of recuperated births in older birth cohorts outweighing smaller numbers of births among young women in overlaying younger cohorts. By elaborating on the mechanisms of interacting fertility trends of age groups of overlapping birth cohorts over time these empirical investigations are thus an extension and a complement to the findings and conclusions of Bongaarts and Feeney (1998, 2006 and Bongaarts 2002).