At the age of 17, cognitive performance does much to determine the fate of American youth. That is the age at which they take the SAT and sort themselves out among various universities. I am going to ask you (for now) to accept two promissory notes: that we know how much family affects vocabulary at various levels of achievement; and that we have a rough estimate of the percentile gap between levels of achievement and the cognitive quality of the family typical at that level. For example, those at the 98th percentile of vocabulary come on average from homes just below the 70th percentile of cognitive quality. Assuming we have this knowledge, let us look at the consequences.

Vocabulary and family quality

When students sit the SAT, universities take scores on the SAT for reading (SAT-R) as the best measure of the viability of their students. Vocabulary is highly predictive of those scores. I will average Vocabulary results from all six of the data sets the leading IQ tests give us: Stanford-Binet tests from 1985 and 2001; Wechsler tests from 1950–55, 1975, 1992, and 2004–05 (these dates average the years when the Wechsler Intelligence Scale for Children and the Wechsler Adult Intelligence Scale, the WISC and the WAIS, were actually normed). By combining their results, I hope to eliminate the vagaries of any particular set.

Table 1 shows that family has different effects at various levels of performance. For example: students whose vocabulary puts them at +2 SD above average, which is the 97.73 percentile (they are better than almost 98 percent of 17-year-olds), suffer from a typical disadvantage thanks to their families of about 1 IQ point; those at +1 SD (or at the 84th percentile – better than 84 percent of their peers) have a typical disadvantage of 3 IQ points; typical students at −1 SD (the 16th percentile) are advantaged by just under 3 points: while those way down at the −2 SD (the 2.27 percentile) are advantaged by over 7 IQ points.