Nurture Is Overrated

Conventional twin studies overestimate the environmental differences between families relevant to educational attainment.

- From here. Specifically:

Estimates of shared environmental influence on educational attainment (EA) using the Classical Twin Design (CTD) have been enlisted as genetically sensitive measures of unequal opportunity. However, key assumptions of the CTD appear violated for EA. 

In this study we compared CTD estimates of shared environmental influence on EA with estimates from a Nuclear Twin and Family Design (NTFD) in the same 982 German families. Our CTD model estimated shared environmental influence at 43%. After accounting for assortative mating, our best fitting NTFD model estimated shared environmental influence at 26%, disaggregating this into twin-specific shared environments (16%) and environmental influences shared by all siblings (10%). 

Only the sibling shared environment captures environmental influences that reliably differ between families, suggesting the CTD substantially overestimates between-family differences in educational opportunity. Moreover, parental education was found to have no environmental effect on offspring education once genetic influences were accounted for.

I'm not surprised despite the fact that it hurts one's pride as a parent to know this fact. There is also a limitation of range effect at work. Really bad environmental circumstances for a family can impair the likelihood that children in the family will receive an education. But essentially no German families enrolled in twin studies are subject to those effects, in part, because Germany has basically free, universal higher education, which many other countries do not.