While romance may be brewing ahead of Valentine's Day, so are some dirty secrets.

About 44% of U.S. adults admit to hiding a bank account or debt, or to spending more money than their partner would be comfortable with, according to a new study from CreditCards.com, which surveyed 1,378 adults who are married, in a civil partnership or living with their partner. That number was included in a survey of 2,501 adults from CreditCards.com.

So why are people committing financial infidelity? More than one-third say they do it for privacy or a desire to control their own finances.

“Money is such a taboo,” says Ted Rossman, an industry analyst at CreditCards.com. “People would rather talk about other uncomfortable topics like religion or political views than money. This is really unfortunate because hiding that kind of a secret can hold back your financial future and undermine trust.”

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Roughly 34% of the 1,378 respondents say they believe they’ve spent more than their significant other would tolerate.

About 17% of those surveyed have kept a secret account (10% credit card, 9% savings, 8% checking) while 12% have carried some amount of hidden debt.

Americans between the ages of 24 and 39 in relationships are much more likely than Gen Xers and baby boomers to have committed financial infidelity with their partner, the study shows.

One reason why: Millennials are more likely than Gen Xers and boomers to have divorced parents, which has likely caused them to be more protective of their own finances, according to experts. Overall, 57% of these millennials have committed financial infidelity compared to 45% of Gen Xers and 37% of boomers.

Over one-quarter of adults say that financial infidelity is worse than an affair. That was out of a larger pool of respondents regardless of relationship status.

Men (47%) were slightly more likely than women (43%) to admit to keeping a financial secret.

About 21% of respondents say they have kept their finances hidden in case a relationship ends poorly.

Those in the highest income bracket, making at least $80,000 a year, were much more likely (39%) than those in lower-income brackets (20%) to say they kept secret finances because they were “embarrassed” about the way they handled their money.

“An awful lot of people are keeping financial secrets from their partner,” Rossman says. “But these topics don't have to be scary, intimidating or embarrassing. If you're sharing a life with somebody, it's something you need to talk about.”