Researchers know a lot about how various factors associated with income level affect a child’s learning: parents’ educational attainment; how parents read to, play with and respond to their children; the quality of early care and early education; access to consistent physical and mental health services and healthy food. Poor children’s limited access to these fundamentals accounts for a good chunk of the achievement gap, which is why conceiving of it instead as an opportunity gap makes a lot more sense.

But we rarely discuss the impact of concentrated poverty—and of racial and socioeconomic segregation—on student achievement. James Coleman’s widely cited 1966 report Equality of Educational Opportunity has drawn substantial attention to the influence of family socioeconomic status on a child’s academic achievement. However, as Richard Kahlenberg, Senior Fellow at the Century Foundation, notes: “Until very recently, the second finding, about the importance of reducing concentrations of school poverty, has been consciously ignored by policymakers, despite publication of study after study that confirmed Coleman’s findings.”

It’s time that we stop ignoring it. The past few decades have seen increasing income polarization ......