Despite massive efforts, the gap between students at the top and bottom of the socioeconomic scale hasn’t narrowed in 50 years—and may have increased significantly. While there’s been no shortage of attempted explanations, it was only after years within the education reform movement that Natalie Wexler stumbled across a fundamental and pervasive root cause that no one was talking about.

Elementary schools spend hours every week on decontextualized reading comprehension “skills,” like “finding the main idea,” leaving little or no time for social studies and science—especially in schools where test scores are low. But evidence from cognitive science shows that reading comprehension depends far more on how much knowledge the reader has about the topic than on abstract skills. The more general knowledge you have, the better you do on reading tests—and often, in life. Hence the phenomenon we’ve come to call the achievement gap: students who acquire more knowledge about the world—usually outside school, from their better-educated and higher-income families—have an advantage on the tests.

To narrow the gap, we need to immerse all children, and especially those from low-income families, in content-rich subjects, building their knowledge beginning in kindergarten if not before—exactly the opposite of what schools have been doing. But an increasing number of schools and districts are beginning to adopt new elementary literacy curricula that focus on knowledge rather than skills, with promising results.