Many Americans are not being completely truthful about their finances and spending with their partner or spouse, according to a new CreditCards.com report.

The survey found that 13 million Americans are committing financial infidelity with a hidden bank account or secret credit card.

And those are not the only secrets many are keeping.

Forty-one percent have spent over $100 without their partner's knowledge, including 19 percent who spent more than $500.

"That means there are a lot of couples out there with some pretty big secrets between them," said Matt Schulz, CreditCards.com's senior industry analyst.

The data also showed that men are nearly twice as likely as women to have spent more than $500 without notifying their significant other.

"That's the kind of money that if you spend regularly, without telling your spouse, you can do a lot of damage to family budget," Schulz said.

While the secrecy can sometimes stem from wanting to hide sinister or shady activities, it is often just a couple simply avoiding what can be a difficult and awkward conversation.

Honesty and openness are still the best policies when it comes to managing money in a relationship, believes Schulz.

"If you are hiding some big financial secret and you get found out, it's only natural for the other person to ask, 'I wonder what else they're hiding," he said. "It's incredibly important for people to have conversations about money with those they love, even if they're really uncomfortable."

Some other key findings from the CreditCards.com report include:

  • Millennials and seniors are less likely to spend over $25 without their spouse or partner's knowledge than people between the ages of 30 and 64.
  • Surprisingly, out of all the income groups, the middle class were the most approving about having their spouse or partner spend $500 or more without having to know about it.
  • Northeasterners were the most likely to have spent more than $500 without their spouse or partner knowing about it.

Schulz recommends couples creating an accurate household budget to help track spending, which can help keep down debt and create a lower-stress environment for your relationship.

"When it comes to financial infidelity, those issues can take a toll on your personal relationship, not just your finances," he said. "You'd rather be uncomfortable having that conversation now than find out something that you really didn't want to find out about later on, and have it ruin both your finances and your relationship."

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