Torche and Echevarría compare twins in fourth grade in Chile to see whether differing birth weights predict differing test scores. "We merge birth registry information on birthweight with standardized Math and Spanish test scores for all fourth graders in Chile to create a prospective data set. Twin fixed-effects models are used to estimate the causal effect of intra-uterine growth on test scores."
What do they find?
"Birthweight differences within twin pairs have a substantial effect on test scores. A 400-g increase in birthweight results in a 15% standard deviation increase in Math scores. The effect is larger among (estimated) monozygotic than dizygotic pairs, reaching >20% standard deviation. The effect varies across family socioeconomic status. It is strong among disadvantaged families but it nearly disappears among advantaged ones."
So it seems that advantaged Chilean families are able to compensate for these early differences, but not poor ones! Another bit of evidence in favor of intervening to help poor children, especially those with a biological disadvantage to boot, to catch up.
The study is Florencia Torche and Ghislaine Echevarría, The effect of birthweight on childhood cognitive development in a middle-income country, Int. J. Epidemiol. first published online February 28, 2011.