The Intrinsic Total Fertility Rate: A New Approach to Tempo-Adjusted Fertility

Peter McDonald, Australian National University
Rebecca Kippen, Australian National University

Various parameterised approaches have been developed to adjust the Period Total Fertility Rate (PTFR) for birth timing distortions. These tempo-adjusted Total Fertility Rates have been criticized and their interpretation is unclear. The PTFR can be considered as the product of a matrix of birth probabilities by age, parity and interval since last birth applied to a population composition matrix with the same characteristics. Our contention is that past impacts of changes in the timing of births are reflected in the current population composition matrix and it is this that distorts the PTFR. This distortion gradually disappears if a constant matrix of birth probabilities is applied across time. After about 25 years, the population composition matrix and the PTFR become constant. We refer to the final PTFR emerging from this process as the Intrinsic Total Fertility Rate. Using Australian data, we illustrate the benefits of this measure in interpreting fertility trends.