The sibling correlation is a lower bound estimate of the total effect of social origins, and thus a measure of a society’s inequality of opportunity. Its sensitivity to both observed and unobserved family and community influences also makes it an attractive choice for comparative research. We gather and summarize all available estimates of sibling correlations in educational attainment (M = .46, SD = .09), before employing meta-regression to explore variability in these estimates. We find significantly lower sibling correlations in Sweden, Norway, Finland, and Denmark than the US, with US correlations on the order of .1 -- or 25% -- higher. Most of the other (primarily European) countries for which we have estimates fall between the US and the Nordics. Correlations between sisters are modestly higher on average than between brothers or all siblings regardless of sex. Sibling correlations are lower in contexts of greater educational expansion, both at the upper secondary and (especially) tertiary levels. We find no overall temporal trend in the size of sibling correlations across cohorts. Finally, we find a novel Great Gatsby Curve-type positive association between income inequality in childhood and the sibling correlation. This implies that greater equality of educational opportunity likely requires a reduction in economic inequality.