Women’s mean age at first birth steeply increased by as many as 4–5 years during the second half of the twentieth century throughout Europe and the United States, and was accompanied by an overall increase in educational attainment. It has been argued that the educational expansion is largely responsible for fertility postponement for a number of reasons. Being enrolled in education may, for example, lead to postponement of childbearing because combining student and mother roles is difficult, given that they both entail time-intensive tasks. Women might delay childbearing because of the high costs of child rearing and limited resources while enrolled in school, and social norms might discourage parenting before the end of education. Education may increase people’s aspirations and ability to pursue a career and women might further postpone childbearing until they are well established in their careers and so forth.
However, this explanation can also be criticized. First, on a population level the association between education and age at first birth changed during the past century; the simultaneous rise in age when leaving education and age at first birth occurred only after WWII. Second, in addition to the overall rise in education, a number of competing explanations exist for the rise in (female) age at first birth in developed countries. Third, the relationship between education and age at first birth may be partly spurious due to family background effects – genetic and non-genetic.
In a study published in Demography, we quantified to what extent fertility postponement by women in the United Kingdom born between 1944-1967 can be directly attributed to longer educational enrolment. For the trends on the population level, we used data from the Office for National Statistics. Complementarily, we estimated the effect of educational enrolment on the age at first birth on the individual level using 2,752 twins from the TwinsUK register.
Our analysis proceeded in three steps. First, we estimated standard linear regression models over all individuals, correcting the precision of the estimates for the fact that we have siblings in our data. Our findings show that one year of additional schooling is associated with about one-half a year later age at first birth in ordinary least squares (OLS) models.
Second, we applied a within-twin (fixed-effects) approach to our data. The within-twin approach controls for all factors shared among siblings, including genetic material, by examining whether a twin who stays longer in the educational system than her sister has her first birth later than her sister. We see that, within families, one year of longer enrolment leads only to a 1.5-month later age at first birth. Biometric analyses reveal that it is mainly influences of the family environment—not genetic factors—that cause spurious associations between education and age at first birth.
Last, using data from the Office for National Statistics, we looked at how far this relationship on the individual level can explain the trend in the age at first birth that we observed on the population level. The figure below shows the simultaneous rise in age at first birth (solid line) and age of leaving education (dashed line, starting point adapted to age at first birth) for cohorts born between 1944 and 1967. These cohorts started childbearing after the 1960s and can therefore be considered the main drivers of the fertility postponement. Age at first birth in 1967 in the UK (26.00) was approximately 2.74 years later than in 1944 (23.26).
Figure: Can educational expansion explain postponement of childbearing since 1945? The graph depicts the observed trends in mean age at first birth and education (age at leaving) for successive birth cohorts, and the predicted trend in age at first birth by education using the OLS and MZ estimates. The trend for educational level was rescaled to that of age at first birth. Predictions were based on estimates from Model 2, Table 2, and data from Table 1. Source: General trends in education were derived from British General Household Surveys (2000–2006) and age at first birth from the Office for National Statistics, Cohort fertility, Table 2 (Office National Statistics 2013).
The figure also shows the simultaneous rise in age of leaving full-time education of approximately 1.3 years across these birth cohorts. A population-based explanation might consequently conclude that 48% (= 1.3 / 2.74) of the 2.74 years of fertility postponement can be attributed to a rise in educational enrolment. However, as shown in the regression models, for each year of additional education, individuals postpone ~5.3 months in the standard model and ~1.5 months in the within-twin models. Across the birth cohort born between 1944 and 1967, therefore, only between 21% (= 1.3 ´ 0.44 / 2.74; OLS models) and 4.8% (= 1.3 ´ 0.10 / 2.74) of the observed postponement in age at first birth can be directly related to the educational expansion.
The Figure shows the predicted average age at first birth based on the OLS (short dash-dot) and MZ twin fixed-effects results (long dash-dot), as well as the explained (green) and unexplained (red) trends in age at first birth across the second half of twentieth century and in yellow the difference between OLS and MZ twin fixed-effects model. The OLS specification can be considered the upper bound of the explained trend in AFB by education, and the MZ fixed-effects model can be considered the lower bound. From these findings, we conclude that the rise in educational enrolment alone cannot explain differences in fertility timing between cohorts.