A recent study out of Philadelphia tracked kindergartners who were learning English and found that four years later there were major discrepancies between which groups of students had mastered the language.

Students whose home language was Spanish were considerably less likely to reach proficiency than any other subgroup. And, on the extreme end, Spanish speakers were almost half as likely as Chinese speakers to cross the proficiency threshold.

The study, conducted by the Philadelphia Education Research Consortium, just looked at English learners who entered the district as kindergartners in 2008 and their progress through the end of third grade.

But this phenomenon isn't specific to Philadelphia. "I have never seen any study that has looked at this question and not found this trend," says Ilana Umansky, who studies English acquisition at the University of Oregon.

Earlier this year, the National Academies of Sciences, Engineering, and Medicine published a 415-page report on English learners. The report cited 12 studies — dating back to 2004 — that found this gap between Spanish-speaking English learners and other groups.

But to date, no research has been able to determine why.

So, we emailed or spoke with about two dozen researchers, teachers, and students to hear how they would explain this trend. Predictably, there's no consensus, but here are three basic theories.

Spanish saturation

There are nearly 150,000 Spanish speakers in Philadelphia, according to the American Community Survey. The numbers are even greater in New York City, where Jose Garcia arrived in 2012, at 11 years old, after emigrating from the Dominican Republic.

Garcia moved to the heavily Hispanic Washington Heights neighborhood in upper Manhattan. At home, he spoke Spanish. In school, classmates spoke Spanish. When he watched television, he often tuned into Spanish-language news.

"So it wasn't like a big challenge for me," Garcia says.

Eventually, Garcia moved to Philadelphia and weaned himself off Spanish media by watching American movies like The Fast and the Furious. But he considers his early months in New York wasted time, compounded by the fact that many of his friends didn't seem all that interested in learning English.

"They didn't wanna learn it as fast because they didn't need to use it," he says. "They were speaking Spanish already. So they had a way to communicate with each other."

But Nelson Flores, a professor at the University of Pennsylvania's Graduate School of Education, grew up in an area of Philadelphia with a lot of Spanish speakers, too, and he says that's not always the case. In fact, he doesn't think the Spanish-language achievement gap has much to do with language at all.

Family income and segregation

Flores contests the notion that Spanish speakers aren't learning English — at least in the way we typically understand language acquisition.

"We're not talking about the ability to communicate in English," Flores says. "We're talking about the ability to do grade-level content in English."

Flores believes a lot of the students who score below proficient in English can speak and comprehend the language with ease. Many of them, he says, can speak English better than Spanish.

So why aren't they testing well? Flores believes it's because Latino students are disproportionately living in isolated, high-poverty neighborhoods and learning in isolated, high-poverty schools.

High-poverty schools, Flores points out, tend to receive fewer resources and less-experienced teachers. Plus, these schools have to deal with the compound effect of having so many students who experience trauma, transience and other disadvantages.

It could be true that Spanish-speaking English learners in Philadelphia are generally poorer than, say, Vietnamese-speaking students, but it's unlikely family income totally accounts for the achievement gap.

It's no surprise that researchers studying this trend in the past have used income-based controls — such as whether a child qualifies for free or reduced lunch. Those researchers have still found Spanish speakers lagging.

Family background

If you look only at family income, you might assume many immigrant groups come from relatively similar socioeconomic backgrounds.

"Immigrants often look low-income because they're in transition," says Patricia Gandara, a UCLA professor who has studied this trend in California. "They may have been physicians in their home country, but now they're having to work as a cook."

Many of the proxies we use to measure poverty or disadvantage trace back to how much money a family makes. But in the case of immigrant groups, that may mask some crucial differences.

A 2009 analysis led by Hunter College professor Donald Hernandez found, for instance, large discrepancies in the relative education levels of many immigrant groups. Adult immigrants from East Asia and the Middle East were among the most likely to have a high school or college degree. Adult immigrants from Mexico and Central America were among the least likely to have made it to high school.

Researchers Jennifer Lee and Min Zhou came to similar conclusions when they compared Mexican and Chinese immigrants. They found that, relative to their parents, the children of Mexican immigrants progressed further educationally from one generation to the next. But the children of Chinese immigrants progressed further overall, in large part because their parents started many steps ahead.

The logic here is pretty simple: Parents who attended college are better able to help their children with homework or connect them to resources.

Tip of the iceberg

These were just three theories we heard when we asked about the language acquisition gap, not all of them.

For instance, many people pointed to societal biases against Hispanic students, arguing that teachers and administrators have lower expectations of them than Asian students because of deeply ingrained stereotypes.

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