Cultural factors and language bias are often used to explain the large differences in mean intelligence test scores between ethnic groups, for instance majority Dutch compared to ethnic minorities. A competing explanation comes from Spearman's hypothesis, which states that these group differences in intelligence scores are a function of the cognitive complexity of the intelligence tasks, and that the regression of group differences on IQ subtests (d) on g loadings of subtests (g) leads to a regression line with only a modest amount of scatter. However, when testing Spearman's hypothesis there are two findings that have to be taken into consideration. First, subtests of an IQ battery with a substantial language component partially measure the extent of proficiency of ethnic minorities in the Dutch language, therefore underestimate the level of g of the tested nonnative speakers, which negatively influences the hypothesized correlation between d and g. Second, ethnic minorities score relatively lower on subtests of spatial visualization and relatively better on subtests of short-term memory, and having these 'outliers' in an IQ battery also attenuates the outcome of tests of Spearman's hypothesis, and additionally tilts the regression line of d on g. Spearman's hypothesis was tested using 40 datasets, comparing native Dutch to Turkish, Moroccan, Netherlands Antillean, and Surinamese immigrants. We expected to find a strong positive correlation between group differences on the s