Charter schools - particularly middle and high schools - enroll a larger share of girls than do traditional public schools, in part because boys are more likely to exit charter schools, finds a new study by New York University researchers.

The study, published in the journal Educational Policy, is the first to examine in charter school enrollment.

"As the charter school sector has grown, so has the gender gap with traditional public schools," said study author Sean Corcoran, associate professor of economics and education policy at NYU's Steinhardt School of Culture, Education, and Human Development and co-interim director of the NYU Institute for Education and Social Policy. "This gap seems to be a function of both more choosing to enroll in charter schools and boys being less likely to stay in these schools."

Research has examined whether students in charter schools differ from those in traditional public schools with respect to prior achievement, special education, or English learner status, but until now, it has overlooked gender differences.

In order to study the gender gap in enrollment between charters and non-charter schools, Corcoran and NYU's Jennifer Jennings looked at more than a decade of data for all public schools in the U.S., including charters (from the 1999-2000 school year to 2010-2011).

The researchers found that charter schools enroll a modestly larger share of girls than do traditional public schools serving the same communities - approximately 1.9 percentage points more (50.7 percent in charters versus 48.8 percent in traditional public schools, in 2010-2011). This gap was largest in high school, peaking at 3.1 percentage points in 11th grade. The gender gap also widened over the period studied, more than doubling from 1999-2000 to 2010-2011.

While these gaps appear small, Corcoran noted that they are only slightly smaller than the gaps in English learner and special education status, which have commanded far greater attention from policymakers.

The researchers were unable to fully explain why the gender gap existed between charter and traditional public schools, but highlighted a number of reasons why charter schools might enroll more girls than boys.

"Girls may be more likely to apply to charter schools because of differences in curriculum and program offerings. The finding of larger gaps in middle and high school suggests a student'sown interest in attending charter schools may be an important factor," Corcoran said.

"Gender differences in educational needs and behavior may also play a role," said Jennings, associate professor of sociology at NYU and the study's coauthor. Jennings and Corcoran found the gender gap in charter enrollment was larger in KIPP charter schools, which tend to adhere to strict "no excuses" academic and behavioral expectations, which may present a greater challenge for boys. The gender gap in KIPP schools was nearly 50 percent larger than other charter schools (2.9 versus 1.9 percent).

The researchers also examined whether rates of retention differed between boys and girls and whether this could explain the they saw in the national data. In order to do this, they looked at data for all students in grades 3 through 12 who attended North Carolina public and charter schools between 2005-2006 and 2010-2011. This statewide dataset followed students across grades and between schools, allowing the researchers to get a better sense of attrition patterns.

Both boys and girls were both more likely to leave charter schools than traditional . However, boys were more likely than girls to exit charters at every grade level, by as much as 1 to 3 percentage points more per year, with larger gaps in the upper grades.

The researchers concluded that the higher attrition among boys was not enough to explain the full gender gap between charter and traditional schools in North Carolina, suggesting that the gap begins when students - more girls - initially enroll in .

"Our study suggests that preferences for certain education environments, combined with the ability to succeed in these environments, may drive gender gaps in charter school enrollment," said Jennings.