Children with additional siblings appear to fare worse on a variety of developmental and educational outcomes across social contexts. Yet, the causal relation between sibship size and later attainment remains dubious, as factors that influence parents’ fertility decisions also shape children’s socioeconomic prospects. We apply instrumental variables methods that treat multiple births (e.g., twins, triplets) and same-sex composition as natural experiments to test whether increases in sibship size have a causal effect on the educational attainment of older siblings in the U.S. We pool several nationally representative datasets, including the Child and Young Adult Cohorts of the NLSY79 and the Panel Study of Income Dynamics, to obtain adequate sample sizes for these methods. Although results indicate that the presence of an additional sibling has a trivial effect on the attainment of older siblings for most families (those with two to four siblings), a large penalty arises with the introduction of a fifth sibling. Our findings imply that the costs associated with sibship size are likely concentrated among the largest families.
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Our primary analytic sample focuses on children mostly born in the 1970s and 80s, though some were born in earlier years. Estimates from the Current Population Survey suggest that this range of sibship size generalizes to a nontrivial number of families (Downs 2003). The share of women aged 40–44 who had three or more children was around 43% in 1985 (with nearly 10% having five or more children), and was 32% (and 5%) by 1990.
Among women who would have had more than three children regardless of a multiple second birth event, multiple births do not contribute to variation in sibship size. This is not problematic because such women should be equally represented in the multiple birth and singleton birth groups.
The estimates of interest do not change after adjusting for temporal shifts in the average age at childbearing (e.g. tempo effects). Authors’ calculations are based on U.S. Vital Statistics.
Although delayed childbearing, which is also associated with MAR, is an important contributor to the increased rate of twinning, it cannot solely account for increases in multiple births.
Nearly all multiple births in our analytic sample (96.1–99.2%, depending on the nth birth event) occurred before 1997—the year in which the Centers for Disease Control (CDC) began systematically collecting data on assisted reproductive technologies.
Results are available upon request.
PSID treats race as a household variable, so each family is coded as the race of the head of household.
To increase sample sizes, we include all observations with educational data reported at age 19 or older for high school completion, at age 20 or older for college attendance, and at age 23 or older for bachelor’s degree attainment.
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Diaz, C.J., Fiel, J.E. When Size Matters: IV Estimates of Sibship Size on Educational Attainment in the U.S.. Popul Res Policy Rev (2020). https://doi.org/10.1007/s11113-020-09619-2
- Sibship size
- Educational attainment
- Instrumental variables