Between 2013 and 2018, ride-hailing driver income has fallen from an average of $1,500 a month to $762 a month, a J.P. Morgan Chase Institute study released on Monday shows. Meanwhile, the number of people who are signing on to become ride-hailing drivers for companies like Uber and Lyft, among others, has soared from practically no households in 2013 to 1% of all American households today.

The findings suggest drivers are seeing increasing opportunities in the ride-hailing business. But in turn, they’re hurting their income and generating less than they otherwise might if fewer drivers were on the road.

J.P. Morgan Chase Institute’s study sheds light on the so-called “gig economy,” in which people sign up for services and generate some income on them. Becoming a ride-hailing driver is generally quite simple, creating few barriers for current drivers to limit the number of people on the road with them and competing for riders.

But the data suggests that ride-hailing drivers aren’t necessarily driving all-year round. Instead, just 12.5% of those drivers are generating earnings from services between 10 and 12 months of the year. An overwhelming number of drivers—58.3%—only take to the road and make money between one and three months out of the year.

For their parts, Uber and Lyft—the two biggest companies in the U.S. ride-hailing market—have contended that earnings are strong and drivers can “earn as much” as they want. Lyft says on its Driver Pay page that it’s paid out more than $200 million in tips and $68 million in Power Driver bonuses.

Neither Uber nor Lyft immediately responded to a Fortune request for comment on the J.P. Morgan Chase Institute study.