The Causes of Group Differences in Intelligence Studied Using the Method of Correlated Vectors and Psychometric Meta-Analysis

The Causes of Group Differences in Intelligence Studied Using the Method of Correlated Vectors and Psychometric Meta-Analysis

Daniel Metzen, Master thesis, 2012.

Abstract

The huge IQ gap between non-Western immigrants and ethnic Dutch has emerged as one of the primary explanations for the large differences in school and work achievement between these groups. Is there a genetic component in the IQ gap between immigrants and ethnic Dutch? Meta-analyses have shown that the group differences on IQ subtests correlate almost perfectly with the cognitive complexity of these subtests; moreover, the cognitive complexity correlates perfectly with heritability and strongly with physical characteristics of the brain. If no other causes for IQ differences show a strong correlation with g loadings, this would point to a strong genetic component in IQ differences between immigrants and ethnic Dutch. In the present study, we first seek support for the hypothesis that only variables under genetic influence show a strong positive relationship with general intelligence. These are group differences, heritability, and physical characteristics of the brain. Second, we test whether differences in IQ due to variables not under genetic influence, namely biological-environmental factors, aging, and autism show a negligible to weak correlation with general intelligence. Support for both hypotheses would suggest that group differences are primarily driven by genetic factors and only to a minor extent by non-genetic factors. Therefore, group differences between non-Western immigrants and ethnic Dutch should be regarded as stable over time.

Concerning the first analysis, we first conducted a full-fledged meta-analysis on reaction time differences between Whites and higher-IQ groups, and Whites and lower-IQ groups, and we conducted several bare-bones meta-analyses and analyses of individual studies on differences in IQ profile between groups of different ethnicity. Second, we explored subgroups on school type, and religion. Third, we carried out a meta-analysis on the question whether g-loadedness of reaction time measures predicted the heritability of these measures. Fourth, we carried out a meta-analysis on the link between g loadings and brain volume. Concerning the second analysis, we first conducted several bare-bones meta-analyses and analyses of individual studies on biological-environmental variables. Second, we conducted bare-bones meta-analyses on the psychological phenomena autism and aging.

The hypothesis was strongly supported: heritabilities and most group differences showed moderate to strong positive correlations with g, but the correlation of brain volume with g was quite modest. All other phenomena showed no strong positive correlation with g.

It is concluded that these findings are strongly in line with a substantial genetic component in group differences in intelligence. This suggest that the large group differences in school achievement and work achievement are stable and that I/O psychologists should find ways to deal with them instead of ways of trying to change them.

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