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. —E. D. Hirsch, Jr., City Journal.