It's no secret that the District's public schools are highly segregated, with a recent analysis showing that nearly three-quarters of black students attend schools where they have virtually no white peers.

But a recent report examines the role that enrollment in private schools, which are disproportionately white, plays in the city's segregation woes. The report was released last month by the Albert Shanker Institute, a nonprofit endowed by the American Federation of Teachers, a major teachers union.

The study found that up to one-third of citywide segregation in the 2011-2012 school year could be attributed to the demographic imbalance at the city's private schools.

There is growing evidence that the nation's public schools are resegregating, with some studies suggesting that U.S. public schools are as segregated by race as they were in the 1960s.

A Government Accountability Office report released last year found that the share of schools that are majority black and majority Latino is growing.

Much of the research on segregation focuses on segregation among public schools, said Matthew Di Carlo, who co-authored the research brief with Kinga Wysienska-Di Carlo. The pair sought to determine how big a role private school enrollment plays in contributing to citywide segregation.

"We were wondering what segregation would look like if we looked at all students in a big city like D.C.," Di Carlo said.

The study has implications for a city that has become a testing ground for school choice — with about 40 percent of students attending charter schools — and for a federally funded private school voucher program that allows a small number of students from low-income families to attend private schools. The researchers said the voucher program is too small to significantly affect school segregation but added that "our results might certainly inform the ongoing debates about vouchers and segregation."

Di Carlo and his colleague examined the demographics of all students who attended school in D.C. in 2011-2012, the last year for which private school enrollment data were available, and found that private schools look radically different from the city overall. They are significantly whiter: While white students made up about 15 percent of all students in the city, they represented nearly 60 percent of private school enrollment. Black students accounted for nearly 70 percent of all students that year but made up just 28 percent of private school enrollment. Hispanic students made up 12 percent of all students but just 8 percent in private schools.

The portrait becomes more dramatic still when looking at public schools in isolation: During that school year, the city's public schools were more than three-quarters black, 13 percent Hispanic and 8 percent white.

"That's a rather large imbalance," Di Carlo said. "That matters for segregation."

The researchers concluded that even if public schools in D.C. were perfectly integrated — if every school reflected the exact demographic makeup of the public school student body — they would still be far less diverse than the city's student body as a whole.

The study counted all students going to schools in D.C., even if their families lived outside the city and would not have been eligible to attend the city's public schools.

Di Carlo said he does not believe that many students fell into that category.

The study recommends that the city's public schools continue to work to attract more private school families and that private schools work to increase diversity, using scholarships and financial aid to recruit more students of color. Without changing the imbalance of demographics between public and private schools, "there may be what amounts to an impermeable ceiling on the citywide impact of big city public school integration efforts," the report concludes.

The city's public schools have been working hard to attract more families, including those who may have opted to send their children to private school, and their efforts appear to be paying off: Enrollment in city schools reached a peak of nearly 90,500 last year, up from about 75,000 when the study was done.

Tomeika Bowden, a spokeswoman for the city's Public Charter School Board, said charter schools are working hard to attract students from across the city by replicating successful charter school models and with marketing.

Charter school enrollment was up to 41,677 last year, up from fewer than 30,000 in the 2011-2012 school year.