On the quantum and tempo of fertility

Demographers have known since the 1940s that standard measures of period fertility, such as the widely used total fertility rate, are distorted by changes in the timing of childbearing. Period fertility rates are depressed during years in which women delay childbearing and inflated in years when childbearing is accelerated. This distortion is usually ignored because there has been no generally accepted method correcting for it. This study proposes a method for removing the distortions caused by tempo changes from the total fertility rate. The key assumption of the method is that period effects, rather than cohort effects, are the primary force in fertility change, an assumption supported by past research. An application of the adjustment procedure to fertility trends in United States shows that concern over below-replacement fertility in the past 25 years has been largely misplaced. Without the distortion induced by the rising age at child; bearing, the underlying level of fertility was essentially constant at close to two children per woman throughout this period. That conventionally measured fertility in Taiwan was below replacement since the mid-1980s is also largely attributable to tempo effects.