Second, the opportunity gap exists alongside a substantial ability gap. Most of the graphs in “Our Kids” show the results for parents with at least a college degree versus those for parents with no more than 12 years of school and a high-school diploma. What are the IQs of those two groups? In the 1979 cohort of the National Longitudinal Study of Youth (NLSY), replicating Putnam’s assignment rules, the mean IQ of the college group was 23 points higher than that of the high school group.

In case you’re wondering, that’s not a function of race. Among non-Latino whites, the difference was 22 points. In statistical terms, those are differences of about 1.5 standard deviations. For the population as a whole, the average person in the high-school group was at the twenty-ninth IQ percentile while the average person in the college group was at the eighty-fourth percentile. Since children’s IQ is correlated with parental IQ, it is not surprising to learn that the means of the children of the high school and college groups are also separated—by about 19 points in the same NLSY cohort. Recall the modest role of the shared environment in producing that difference.

Again, my underlying point is simple. IQ has a substantial direct correlation with measures of success in life, and it is also correlated with a variety of other characteristics that promote success—perseverance, deferred gratification, good parenting, and the aspects of personality that are variously called “emotional intelligence” or “grit.” The correlations are not large, but many modest individual correlations produce large differences in life outcomes when the means of two groups are separated by as large a gap as separates both parents and children of America’s working and upper-middle classes.