Document Type

Working Paper

Publication Date



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 problem is usually ignored because there has been no generally accepted method for solving it. This study proposes a method for removing the tempo distortions from the total fertility rate. The key assumption of the method is that period effects, rather than cohorts 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 childbearing, the underlying level of fertility was essentially constant at very close to two children per woman throughout this period. Below-replacement fertility in Taiwan since the mid-1980s is also largely attributable to tempo effects.