Side note: a Cohen’s d of .31 is not small. My opinion is that the rules of thumb used to interpret effect sizes in psychology are messed up, because so much p-hacking in the past produced way overinflated effect sizes. Regardless, 0.3 is typically seen as a moderate effect size. A 0.3 standard deviation increase in IQ would be 4.5 points which would lead to economically meaningful differences in income.
Side note: a Cohen’s d of .31 is not small. My opinion is that the rules of thumb used to interpret effect sizes in psychology are messed up, because so much p-hacking in the past produced way overinflated effect sizes. Regardless, 0.3 is typically seen as a moderate effect size. A 0.3 standard deviation increase in IQ would be 4.5 points which would lead to economically meaningful differences in income.