Vocabulary doesn’t just help children do well on verbal exams. Studies have solidly established the correlation between vocabulary and real-world ability. Many of these studies examine the Armed Forces Qualification Test (AFQT), which the military devised in 1950 as an entrance requirement and a job-allocating device. The exam consists of two verbal sections (on vocabulary size and paragraph comprehension) and two math sections. The military has determined that the test predicts real-world job performance most accurately when you double the verbal score and add it to the math score. Once you perform that adjustment, according to a 1999 study by Christopher Winship and Sanders Korenman, a gain of one standard deviation on the AFQT raises one’s annual income by nearly $10,000 (in 2012 dollars). Other studies show that much of the disparity in the black-white wage gap disappears when you take AFQT scores—again, weighted toward the verbal side—into account.

Such correlations between vocabulary size and life chances are as firm as any correlations in educational research. Of course, vocabulary isn’t perfectly correlated with knowledge. People with similar vocabulary sizes may vary significantly in their talent and in the depth of their understanding. Nonetheless, there’s no better index to accumulated knowledge and general competence than the size of a person’s vocabulary. Simply put: knowing more words makes you smarter. And between 1962 and the present, a big segment of the American population began knowing fewer words, getting less smart, and becoming demonstrably less able to earn a high income.

Why should vocabulary size be related to achieved intelligence and real-world competence? Though the intricate details of cognitive abilities are under constant study and refinement, it’s possible to give a rough answer. The space where we solve our problems is called “working memory.” For everyone, even geniuses, it’s a small space that can hold only a few items in suspension for only a few seconds. If one doesn’t make the right connections within that space, one has to start over again.