Evolution of the shape of the fertility curve: Why might some countries develop a bimodal curve?

Background: Few previous studies have compared how the shape of the fertility curve has evolved in different countries. This study remedies this lack by investigating first- and second-birth fertility curves for 22 highly developed countries. Methods: Using age-specific fertility rate data for first and second births held in the Human Fertility Database, line graphs and contour plots were drawn to illustrate the evolution of the fertility curves of the 1968 to 1980 cohorts of women. Additional period data for the Netherlands and the United States was also used to investigate the more unusual scenarios of these two countries. For Switzerland, the fertility curves for Swiss national and foreign women were investigated. Results: For some countries the transition from an early to a late fertility schedule goes through a phase when the first birth fertility curve is bimodal. In other countries a premodal ‘shoulder’ is apparent. Conclusions: The existence of a bimodal fertility curve suggests the polarisation of women into one group that remains longer with an early fertility schedule and a second group that moves more rapidly on to a later schedule. For most countries this seems to be a transitional phase. Evidence of foreigner fertility contributing to an early bulge in the fertility curve is demonstrated for Switzerland, suggesting it could be common in other developed countries with high rates of immigration. Contribution: This examination of the trends in the shape of the fertility curves for 22 countries reveals several contrasting progressions, which hopefully will prompt further investigation as to their cause.