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Want Smarter Kids? Space Them (At Least) Two Years Apart


A new study (PDF here) by University of Notre Dame economist Kasey Buckles and graduate student Elizabeth Munnich finds that siblings spaced more than two years apart have higher reading and math scores than children born closer together. The positive effects were seen only in older siblings, not in younger ones.

The authors attribute at least part of the difference to older children getting more of their parents’ time during the first formative years of their lives before a younger sibling comes along. The paper is set to be published in an upcoming issue of the Journal of Human Resources.

Here’s the abstract:

This paper investigates the effect of the age difference between siblings (spacing) on  educational achievement. We use a sample of women from the 1979 NLSY, matched to reading  and math scores for their children from the NLSY79 Children and Young Adults Survey. OLS  results suggest that greater spacing is positively associated with test scores for older siblings, but not for younger siblings.  However, because we are concerned that spacing may be correlated  with unobservable characteristics, we also use an instrumental variables strategy that exploits  variation in spacing driven by miscarriages that occur between two live births. The IV results  indicate that a one-year increase in spacing increases test scores for older siblings by about 0.17  standard deviations—an effect comparable to estimates of the effect of birth order.  Especially close spacing (less than two years) decreases scores by 0.65 SD.  These results are larger than the OLS estimates, suggesting that estimates that fail to account for the endogeneity of spacing may understate its benefits.  For younger siblings, there appears to be no causal impact of spacing on test scores.

The paper fills in one of the few remaining gaps in the large body of economic research that’s been done on family structure and children’s outcomes. We already know that children from larger families generally have lower educational attainment and IQ scores, worse employment outcomes, and are more likely to engage in risky behavior. But birth spacing has received less attention.

The authors sampled a nationally representative panel of 12,686 young people ages 14 to 22 in 1979 and their children, and used  the Peabody Individual Achievement Tests for math and reading to measure performance. The largest effect they observed suggested that a one-year increase in spacing improves reading scores for older children by 0.17 SD—which would be three times the effect of increasing annual family income by $1,000.