Are differences in IQ scores also differences in general intelligence? Four psychometric meta-analyses on the relation of lead level, breastfeeding, and prenatal cocaine and smoke exposure with general intelligence
Jan Smit, Master thesis, 2011.
The central question addressed in this study is whether some potential biological and environmental factors might cause true differences in general mental ability (g) between groups, or just “hollow” score differences between groups.
Four bare-bones psychometric meta-analyses (MAs) were performed to test these premises. We predicted strong positive correlations between vectors of lowered IQ scores following lead exposure, prenatal cocaine exposure, and prenatal smoke exposure on the one hand; and vectors of g loadings on the other hand. We also predicted a strong positive correlation between the vector of increased IQ scores following breastfeeding on the one hand and vectors of g loadings on the other hand.
In line with our hypotheses, the meta-analyses showed correlations of .79 on breastfeeding (total N=7847) and .91 on prenatal cocaine exposure (total N=391). However, the variance explained by artifactual errors is too low to draw strong conclusions about the link between cocaine exposure and g, and about the link between breastfeeding and g. Contrary to our hypotheses, the meta-analyses showed correlations of -.19 on lead exposure (total N = 702) and -.19 on prenatal smoke exposure (total N = 443). We were not able to come up with strong theoretical explanations for these values of the correlation between lead exposure and g.
In sum, two of the four meta-analyses showed mixed support for the theory: high correlations between g loadings and effects, but little variance explained in the data points in the meta-analysis. The other two meta-analyses showed no support at all for the theory: an absence of correlations between g loadings and effects and virtually no variance in the data points explained. The amount of support for the theory in these four meta-analyses is therefore modest.