Race Differences in Cohort Effects on Non-Marital Fertilityin the United States: Reply to Martin

Abstract

We appreciate opportunity to clarify and provide additional tests of key elements of our age-period-cohort analysis of non-marital birth rates in this March 2009 issue of Social Forces. Where Steve Martin, in his commentary, has suggested specific alternative specifications or interpretations of our findings, we have re-estimated and performed explicit tests of alternatives. In end we have demonstrated robustness of our results and strengthened case for our interpretations. The points Martin raises and our responses should help to clarify nature of cohort effects and our interpretation of them. We thank Martin for his close attention to our paper. As Martin notes, in typical rectangular age-period table there are same numbers of cells/observations for each age, same number of cells/ observations for each period, but different numbers of cells/observations for some of cohorts. This is well known yet cohort analysts persist in using such tables when studying cohort effects. These analysts know that if one does not take into account age and period effects in such a table, then differences between cohorts could be produced by age and period effects. If one does control for main effects of age and period, however, this will not create apparent cohort effects. Martin suggests, however, that there are patterns of age*period interactions that are not related to cohort effects that could produce patterns we observe along cohort diagonals. Certainly such patterns are possible: It is incumbent on critic to point out these patterns and, given that our discipline considers theory important, to produce compelling reasons why such patterns might exist. Martin does both. He suggests that there is an interaction between age and period. He notes that the non-marital birth ratio rose steeply for women in their 20s and slowly for women in their 40s. That is, increases in NFRs over time are different for different age groups. Martin then suggests that [a] plausible explanation for this age*period pattern is that delays in marriage increased unmarried population most rapidly among women in their. 20s, so rise in non-marital birth ratio has been most pronounced among women in their twenties. His Figure 4 displays differences in these slopes, and it is on this figure that much of his argument rests.