Lindsey cited research showing that “by the time they reach age 3, children of professional parents have heard some 45 million words addressed to them — as opposed to only 26 million words for working-class kids, and a mere 13 million words in the case of kids on welfare.” So, class distinctions in vocabularies are already large among toddlers. Parental choice of neighborhoods and schools mean that children of college-educated parents hang out together. Such peer associations may have as much effect on a child’s development as do parents. These factors, Lindsey said, explain why “people raised in the upper middle class are far more likely to stay there than move down, while people raised in the working class are far more likely to stay there than move up.”

In a historical blink, Lindsey said, humanity has moved from lives rooted in a remembered past to lives focused on an imagined future. This orientation favors the intellectually nimble. “Who gets ahead, who struggles to keep up, and who gets left behind are now determined primarily by how people cope with the mental challenges of complexity.” And coping skills are incubated in families.

Today, the dominant distinction defining socioeconomic class is between those with and without college degrees. Graduates earn 70 percent more than those with only high school diplomas. In 1980, the difference was just 30 percent.