Each year, banks reap billions of dollars from overdraft and other fees. A new analysis of more than 500,000 U.S. users on investing app Stash found women pay a disproportionate amount of those penalties.

Male Stash users, the survey found, pay $182 per year in fees, while women pay $214, or 18% more. That includes late fees as well as charges for ATMs, minimum balances, commissions and more, but overdraft charges make up the lion’s share. Women pay almost 30% more in overdraft fees compared with men, the survey found.

“The main reason we think this is happening is the pay gap,” said Stash CEO and co-founder Brandon Krieg. Women earn about 20% less than men on average, he said, and therefore have less money in their bank accounts, making them more likely to overdraw and incur penalties.

A decade ago, the federal government passed the Overdraft Protection Law to protect consumers from excessive banking fees, and for a while, overdraft fee revenue dropped. It’s since bounced back, according to a 2018 survey from Moebs Services.

In 2017, Americans paid $34.3 billion in overdraft fees, a high since the regulation went into effect. That’s partly because the average fee went up from $20 per transaction in 2000 to $30, the researchers found. Last year, the four largest banks collected $5.1 billion in overdraft fees alone, a number that has held relatively steady over the last three years.

This year, the Consumer Financial Protection Bureau opened a review of the rule. Banks and consumer advocates alike have urged the agency to keep it as is.