Like speakers of any nonstandard dialect, from Swiss German to Cypriot Greek, most speakers of African-American English do learn to code-switch naturally, Washington explained. “Some start during kindergarten, then we see a big wave at the end of first grade and another at the end of second. Then you get to third grade and it’s over.” At that point, about a third of them still can’t speak the standard dialect, and “code-switching isn’t going to happen unless you teach it. We know those kids will have trouble.” By the end of fourth grade, “switching” students—that is, students who are proficient in both their home dialect and standard English—score at least a full academic year ahead of their nonswitching classmates in reading.

Why exactly does speaking a nonstandard dialect stymie kids as they’re learning to read? In his seminal 1972 book, Language in the Inner City, the linguist William Labov advanced the reigning theory. A teacher writes a word on the blackboard—something simple, like told or past—sounding it out letter by letter as she does. For a speaker of standard English, the lesson is clear: The four letters represent the four sounds that make up the word. But the rule is more complex for AAE speakers. In the black vernacular, many consonant clusters—such as the -ld in told, and the -st in past—aren’t fully pronounced when they appear at the end of a word. A speaker of African-American English is likely to say told the same as toll (or even toe), and past the same as pass. The profusion of homonyms obscures the fundamental sound-to-letter principle: AAE-speaking kids are presented with an enormous number of words that are all pronounced the same yet spelled in nonsensically different ways. To help kids grasp the dialect of the classroom, Labov wrote, teachers should employ “the methods used in teaching English as a foreign language.”

Labov’s recommendation was largely overlooked outside his field. But last June, Washington completed a four-year study of almost 1,000 low-income elementary-school students in a southern city—the most extensive study ever of the dialect’s role in education—which led her to a similar conclusion. Strikingly, she discovered that African American students’ lagging growth in reading was accounted for almost entirely by the low scores of the students who speak the heaviest dialect. And location mattered: The majority of kids in the city she studied, Washington found, use a regional variety of AAE that is especially far from standard English. This suggests to her that children who speak one of the dialect’s “really dense” varieties are having an experience in the classroom not unlike that of, say, native Spanish speakers.

Compounding these challenges is the fact that most AAE speakers have teachers who are hostile to their dialect. In an illuminating investigation published in 1973 (but, according to several linguists I spoke with, still reflective of classroom conditions today), Ann McCormick Piestrup portrayed the AAE speaker’s experience as one of incessant interruption. Black children at the California schools Piestrup studied often answered questions correctly, only to be pounced on for irrelevant differences of pronunciation or grammar. This climate had a drastic effect: As time went on, Piestrup saw students withdraw into “moody silences”; when they did speak, their voices were soft and hesitant. The interrupted students had the lowest reading scores of any children Piestrup observed.