Jasper Repko’s research while affiliated with University of Amsterdam and other places

Spearman's hypothesis tested at the level of items states that differences between ethnic groups on the items of an IQ test are a function of the g loadings of these items, such that there are small differences between ethnic groups on items with low g loadings and large differences between ethnic groups on items with high g loadings, and it has been confirmed in a fairly substantial number of studies. Most biological variables show correlational patterns that are more similar to the pattern in group differences than non-biological variables. In this paper, we tested Spearman's hypothesis, comparing a group of Sudanese children and adolescents (N = 7226) with other groups of children and adolescents from Denmark, Cyprus, Croatia, Bosnia, South Africa, Estonia, Ukraine, Ireland, Russia, and Chile (total N = 13,105). The analyses were carried out on 19 comparisons between the Sudanese children and the other children. Aggregating all data points from the present study showed that Spearman's hypothesis was strongly confirmed with a sample-size–weighted r with a value of .70; it is argued that applying corrections for statistical artifacts would make this value substantially larger. These outcomes suggest that biological variables, including brain-related variables, are more important than non-biological variables in explaining group differences.