This paper, authored by John Bongaarts and Griffith Feeney, addresses the issue of how changes in the timing of childbearing affect period fertility rates, such as the total fertility rate (TFR). The authors propose a method to adjust TFR for these tempo distortions, which are caused by variations in the mean age at childbearing. The key assumption is that period effects, rather than cohort effects, are the primary drivers of fertility change. The method involves dividing the observed TFR by the factor $(1 - r)$, where $r$ is the change in the mean age at childbearing for each birth order during the year. This adjustment helps to remove the distortion induced by changes in the timing of births, providing a more accurate measure of the underlying level of completed fertility. The authors apply this adjustment to fertility trends in the United States and Taiwan, showing that concerns over below-replacement fertility in the US were largely due to rising age at childbearing, and that in Taiwan, below-replacement fertility since the mid-1980s is largely due to tempo effects. The paper concludes by emphasizing the importance of tempo-adjusted TFRs in assessing fertility behavior and its implications for future population growth.This paper, authored by John Bongaarts and Griffith Feeney, addresses the issue of how changes in the timing of childbearing affect period fertility rates, such as the total fertility rate (TFR). The authors propose a method to adjust TFR for these tempo distortions, which are caused by variations in the mean age at childbearing. The key assumption is that period effects, rather than cohort effects, are the primary drivers of fertility change. The method involves dividing the observed TFR by the factor $(1 - r)$, where $r$ is the change in the mean age at childbearing for each birth order during the year. This adjustment helps to remove the distortion induced by changes in the timing of births, providing a more accurate measure of the underlying level of completed fertility. The authors apply this adjustment to fertility trends in the United States and Taiwan, showing that concerns over below-replacement fertility in the US were largely due to rising age at childbearing, and that in Taiwan, below-replacement fertility since the mid-1980s is largely due to tempo effects. The paper concludes by emphasizing the importance of tempo-adjusted TFRs in assessing fertility behavior and its implications for future population growth.