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* Cliffordson , C and Gustafsson , J ( 2008 ) Effects of age and schooling on intellectual performance: estimates obtained from analysis of continuous variation in age and length of schooling . Intelligence 36 , 143 – 152 .

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* Lee , ES ( 1951 ) Negro intelligence and selective migration: a Philadelphia test of the Klineberg hypothesis . American Sociological Review 16 , 227 – 233 .

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