Online work platforms took off after Uber's founding in 2009. They benefited from a weak economic recovery that had left millions of Americans unemployed and desperate for work.

The initial popularity of gig work prompted a rash of speculation that independent workers — freelancers as well as gig workers and contractors — would soon occupy a steadily larger portion of the workforce. In October, a study by the Freelancers Union and Upwork, a freelancing website, predicted that a majority of U.S. workers would be freelancers by 2027.

Yet the JPMorgan Chase Institute's report casts doubt on that likelihood. It found that among drivers, 58 percent work just three months or less each year through online economy websites. These include ride-hailing services such as Uber and Lyft as well as delivery drivers and movers who find work through online apps. Amazon, for example, now uses independent drivers to deliver some packages.

The study also reviewed online platforms that provide home improvement work, including TaskRabbit and dog-walking, home cleaning and other sites. Two-thirds of those workers perform gig work for only three months a year or less.

Most participants cycle in and out of gig work to supplement their incomes from other jobs, the institute found. In any given month, one in six workers on online platforms are new — and more than half will have left the gig economy after a year of entering it.

"People aren't relying on platforms for their primary source of income," said Fiona Grieg, director of consumer research for the institute and co-author of the study.

A government report in July concluded that the proportion of independent workers has actually declined slightly in the past decade.