Comment: Distinguishing Cohort Effects from Age*Period Effects on Non-Marital Fertility

Abstract

In the article Cohort Effects on Non-marital Fertility, in this issue of Social Forces, Jean Stockard employs a novel strategy for disentangling cohort, period and age effects on the non-marital fertility ratio. In a model with fixed-effect controls for age and for time period, the author documents evidence for three cohort-specific factors affecting the non-marital birth ratio of a given cohort ages 20-44. The first is the non-marital birth ratio at the time a cohort is born. The second is the cohort percent enrolled in school at ages 18-21. The third is the cohort sex ratio at ages 20-24. In models for non-hispanic whites, two of these cohort variables have statistically significant coefficients, and in models for non-Hispanic blacks, all three variables have statistically significant coefficients. Social researchers have often debated the relative importance of period and cohort effects on demographic processes (Namboodiri 1981 ; Pullum 1980). Stockard's finding, if it holds, would clearly strengthen the overall case for cohort effects. Unfortunately, the technique Stockard uses to identify cohort effects is open to an alternative interpretation. The cohorts in this analysis are censored in a manner typical of time-series data, and as a result, age-period interactions can create the appearance of a cohort effect. If the age*period interpretation holds, the cohort variables described by the authors have only a coincidental relationship with trends in the non-marital birth ratio. In this comment, I begin by describing the cohort effect on non-marital birth ratios as it is described by Stockard. Then I explain how the same observed pattern could arise from an age-period interaction. Finally, I offer two arguments that would favor the age-period interpretation over the cohort interpretation. I limit my analysis to the results for non-Hispanic whites; results for non-Hispanic blacks and their interpretation are generally analogous to the results and interpretation for non-Hispanic whites.