If a school offers separate classes for gifted students, one of the most difficult questions is who should be allowed in and who can benefit. Should students be picked based on their IQ alone, given that IQ scores fluctuate and correlate heavily with race and family income? Or should other factors play a role?

A new National Bureau of Economic Research working paper found that the students who benefit the most from gifted classrooms are students who aren't gifted at all — at least, not as measured by intelligence tests. Instead, they're students who scored well on standardized tests in previous years.

What the study found

The researchers, David Card from the University of California-Berkeley and Laura Giuliano from the University of Miami, studied classrooms in a large school district with an unusual method of educating gifted students. (They don't name the district, but it's likely in Florida, based on the description they give of the district's size and its policies.)

Beginning in fourth grade, elementary schools in the district have separate classrooms for gifted students, rather than occasionally pulling them out of class for enrichment activities. But the bar is high to be classified as gifted: Students who aren't from disadvantaged backgrounds have to have an IQ of at least 130; students who are learning English or who are from poor families can meet a slightly lower bar, an IQ of higher than 116.

And so gifted kids are rare. Most schools have fewer than 10 students in the fourth grade who meet those criteria, and classes need to have at least 20 students. So schools have to find other kids to fill the remaining seats in gifted classrooms. They turn to "high achievers," students with high scores on statewide standardized tests the previous years.

It turns out that those high-achieving kids, who don't score as gifted at all on an IQ test, benefit the most academically from being in a gifted classroom.

The effects (or lack thereof) of gifted education

The gifted classroom had little to no effect on the standardized test scores of students admitted based on their IQ. For the students with IQs of at least 130, that's not surprising. Their test scores were high to begin with. They didn't usually need help with basic reading and math. And they were more likely to say they were satisfied at school after they moved to the new classroom — suggesting that the gifted class had a positive effect by helping students who were bored or disengaged before.

Students with disadvantaged backgrounds who were admitted based on IQ tests also didn't see a huge bump. The researchers found this more troubling, because those students' scores weren't as high coming in as the first group's. And those students said they were less satisfied at school after testing into the gifted class, so it's not clear that they were getting other benefits from being grouped with smarter peers.

Students with high test scores but lower IQs — the kids who got the leftover seats — saw a significant improvement in their standardized test scores. The impact was larger for students who are racial minorities or from disadvantaged backgrounds, the students least likely to be admitted to a gifted program based on IQ alone.

In other words, the gifted program ended up providing the biggest test score boost to kids who weren't really supposed to be there in the first place.

Separating students could raise test scores, but it's controversial

The authors argue for a broader definition of "gifted" that includes test scores, not just IQ — since it's the students with the high test scores who were benefiting the most. Establishing separate classrooms for high achievers, whether or not any of them are "gifted" as measured by intelligence tests, could lead to a test score boost at low cost, they write.

Whether students should be sorted into classrooms based on academic ability at all, though, is far from a settled question. The researchers found that students left behind when their higher-achieving peers were sorted into a separate classroom didn't suffer.

Some researchers, though, argue that this type of sorting perpetuates academic inequality. The best students, whether they're chosen based on their IQ or their standardized test scores, get the richest curriculum; the others are left behind.

Grouping students together based on ability for one subject, particularly in elementary school, is fairly common. But the idea is that students can move between ability groups as they progress. That's very different from picking students out and assigning them to a separate classroom based on their score on a standardized test.