a1 Ross University, Dominica
a2 University of Ulster, Coleraine, Northern Ireland
Unquestionably, aggregate-level correlations are in most cases higher than individual-level correlations. For example, in the National Longitudinal Survey of Youth (NLSY) in the United States, the correlations are 0·487 between adolescent IQ and log-transformed adult income, 0·449 between years of schooling and adult income and 0·626 between years of schooling and IQ (N=6064). The corresponding country-level correlations are 0·815, 0·816 and 0·788 (N=125 countries).