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Cheating on a significant other isn’t always about sex. There’s another type of infidelity that is less talked about and harder to spot, yet has the same, damaging effect as any other type of betrayal: financial infidelity. And, according to a new Smart About Money survey, the number of people who are financially unfaithful is on the rise. Among those couples who have ever combined finances, two in five (42 percent) have committed some sort of financial deception.

“That’s up noticeably from two years ago,” Ted Beck, president of the National Endowment for Financial Education, told Fox News. “When people in relationships are financially unfaithful, it can include things like hiding purchases, hiding bank account statements, taking things from the mail so their partner or spouse does not see, keeping cash, or even having a credit card the other party doesn’t know about.”

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Financial infidelity can be as severe as lying about the amount of debt one partner brings into the relationship or about the income he or she makes. It usually goes unnoticed until the unassuming partner finds out on their own, whether through a discovered bank statement, a questionable purchase, or the like.

“It’s important to remember that money is the number one stress factor for most people and most relationships,” Beck said. “In a two-income family, there can be a mentality where one partner thinks, ‘I want to use the money I earned as I see fit.’ Very often, they’re afraid that their partner won’t approve of how they spend money or they’re embarrassed or fearful that their partner will find out about a bad habit.”

Beck added that, in financial infidelity, it’s the secrecy that really harms the relationship — not only do partners set money aside secretly, but often they hide the fact the money exists at all.

Among those who have committed financial deception, and/or whose partner did, 75 percent said the financial deceptions affected their current or past relationships, with over a quarter (28 percent) saying it caused less trust in the relationship.

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Like many problems that arise in relationships, if you find out your partner is being financially unfaithful, it’s important to communicate about it without being accusatory or confrontational. Although 32 percent of the survey recipients said they’d prefer if some aspects of their finances remained private from their partner, Beck advised couples to still keep those financial lines of communication open.

“Especially in married couples, the thing you have to remember is that your credit rating is affected by your spouse’s actions,” he pointed out. “So, you're really talking about your own life when these things happen — especially if it deals with any sort of credit or bad behavior like overdrawing accounts or bouncing checks.”

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Being open and honest about your finances doesn’t mean you have to pool every penny earned, or consult each other on every purchase made, though.

“It’s okay if you both have incomes to agree that there is a certain amount each of you can spend without question,” Beck said. “This way, you avoid the desire to hide things. Watch for red flags, and set ground rules going forward.”