Segregation based on income has become increasingly common in American cities over the past few decades, according to Pew Research Center. But relative to other large U.S. cities, Boston appears to have significantly less segregation between rich and poor.

The research by Pew, released earlier this month, creates an index to measure such segregation by looking at what percentage of high income people live in neighborhoods home to a majority of other high income people, and then by measuring how many low income residents lived in majority low-income neighborhoods.

Those two combine to form what Pew calls the RISI score, a measure of income segregation. And Boston’s RISI score is the lowest of the 10 largest U.S. cities. The city ranks 24th among the 30 largest cities.

The report doesn’t speculate as to why Boston might be comparatively less segregated. Any hypotheses? Check out the income segregation maps for the 10 largest U.S. cities here.