Men make more than women in their side hustles.

On Equal Pay Day, April 2, the disparity between women and men’s pay doesn’t stop at the office or factory floor. Women Uber drivers earn 93 cents on the dollar compared to men, according to a recent study distributed by the National Bureau of Economic Research. “The uniqueness of our data — knowing exactly the production and compensation functions — permits us to completely unpack the underlying determinants of the gender earnings gap,” the researchers wrote.

The authors found that the gender gap is caused by three factors: How long they’ve been driving on the platform, preferences over where/when to work and driving speed. The authors include economists from the Graduate School of Business and Stanford Institute for Economic Policy Research at Stanford University, and Jonathan Hall, chief economist and director of public policy at Uber.

“This suggests that, as the gig economy grows and brings more flexibility in employment, women’s relatively high opportunity cost of non-paid-work time and gender-based preference differences can perpetuate a gender earnings gap even in the absence of discrimination,” they concluded. Earnings are not related to the time of day drivers work and the gender wage gap was not explained by the choice of customers, they added.

Don’t miss: What Rosie the Riveter would make today

“Our results suggest that there is no reason to expect the ‘gig’ economy to close gender differences,” the researchers said. And that gap appears to widen when applied to other side hustles, according to a survey released Monday by personal-finance site Bankrate.com. Men earn an average of $989 a month from their ancillary gigs, almost three times the $361 that women earn.

This may be related to the kinds of freelance gigs women and men choose. Bankrate found the most popular side hustle for men is repair/landscaping and babysitting/child care for women. Men are also more likely than women to drive for a ride-sharing service, an earlier study by careers website Jobvite found. And both companies said men are more likely to take on side hustles.

Jobs involving manual labor that can also double as lucrative side hustles — including plumbers, installers of household electrical equipment, repairs — are more than 98% held by men, according to data from the U.S. Department of Labor. Take this, perhaps extreme, example: One handyman on TaskRabbit told MarketWatch he earns $150 per hour, roughly five times the average hourly wage.

Saudi Women Take to the Road After Driving Ban Is Lifted

“Men tend to be more comfortable at negotiating, and may be setting their rates higher to start with,” said Arielle O’Shea, investing and retirement specialist at finance site Nerdwallet. “Women can apply the same salary advice they do to full-time jobs: Ask what other people are charging for their services and make sure you are charging something fair to you.”

These wage gaps increase exponentially over the years, O’Shea noted. What starts as just a couple hundred dollars each month turns into hundreds of thousands of dollars in missed funds when it comes to investing. “If men invested the extra money they made each month, the difference would be about $600,000 over 30 years,” she said. “That difference can be huge.”

Also see: Women don’t see men with flashy cars as serious husband material

Female personal financial advisers make little more than half (56.4%) of men in the same job, compared with 83% overall, according to a separate analysis by the Institute for Women’s Policy Research, a Washington, D.C.-based research organization. (Critically, that does not account for women who are missing from senior roles in Silicon Valley.)

Indeed, there are only two occupations where women’s median earnings are slightly higher than men’s, while there are 107 occupations in which women’s median earnings were 95% or less than men’s for the same jobs. Women working as “dining room and cafeteria attendants and bartender helpers” and “wholesale and retail buyers, except farm products” slightly out-earned men.

Overall, women’s median earnings are lower than men’s in nearly all occupations, the Institute for Women’s Policy Research concluded. “The highest paid occupations have the biggest gender gaps and the lowest paid occupations that have the smallest gaps, the report found. This reflects median weekly earnings of $770 for all female full-time workers and $941 per week for men.