WASHINGTON, DC -- AUGUST 6: The Louis and The Harper are high-end apartment buildings on 14th Street in the heart of one of DC's hottest neighborhoods. (Andre Chung for The Washington Post)

When we think about the reasons behind the movement of younger, higher-income people into center cities -- reversing the decades-long trend of suburbanization -- lots of things come to mind. Urban crime decreased from the 1990s through the 2000s. The fertility rate sank, lessening the need for three-bedroom houses with big backyards. A new generation started to value walking and coffeeshops and communal public spaces, rather than out-of-the-way cul-de-sacs.

But according to a new working paper from researchers at Columbia University, one big factor looms larger than all others: The decreasing amount of leisure available to educated people.

"Long hours render non-work time scarce, planting low-utility activities such as commuting in the cross-hairs," the authors write. "One of the simplest ways to control commuting is to live close to work, which for skilled workers may mean the city center. There, by definition, land is scarce and higher demand translates into higher land rents. In time, local amenities adjust, boosting the attractiveness of the locality, further fueling the gentrification process."

To make that a little more explicit: The growth of the knowledge economy has centralized high-skilled jobs in city centers. Holders of advanced degrees tend to work more hours, and also command better salaries, which they can afford to spend on housing, fueling the growth of high-end development that in turn pushes out the lower-income people who'd long made their homes in the urban core.

The authors document this bifurcation in the availability of leisure time, which has grown for less-educated workers and shrunk for those with more skills. A key graph from the study describes how people working longer hours with higher degrees started shifting closer to the central business district over the past 40 years:

In a parallel trend, more educated people tend to marry later and postpone, if not opt out, of parenthood entirely. That decreases the need to move to the suburbs for higher-quality schools, allowing these high-earning urbanites to remain in expensive apartments indefinitely.

Of course, there's a downside to this phenomenon. Those less-educated workers who hold service jobs in the city might have more leisure time, since they're getting fewer hours. But as they're pushed further and further outside the urban core, at some point there aren't enough hours in the day for them to commute back and forth -- starving the city of its labor supply, and poor people of their access to income.