Surreptitious cash withdrawals and hidden credit card statements may be signs your significant other is cheating—but not necessarily in the bedroom.
Financial infidelity is on the rise, with more people deceiving (or being deceived by) their partner about purchases made, debts incurred, money earned or other issues related to their joint finances.
One-third of people who have combined accounts said they have committed a financial deception, while 35 percent said they have been the victim of one, according to a new study from the National Endowment for Financial Education conducted with Harris Interactive.
Those figures are up slightly from the NEFE's 2011 assessment, when 31 percent of respondents copped to financial infidelity and 32 percent said they had been deceived.
In an American Express Spending & Saving Tracker survey released this month, 40 percent of women admitted to hiding purchases from their partner, up from 27 percent who copped to it last year.
"You would think with the recession that people are talking to each other more about money," said Ted Beck, president and CEO of the National Endowment for Financial Education. "But people are continuing bad habits."
To put it into perspective, more people commit financial infidelity than the more-discussed kind. For example, a 2011 study from Indiana University, the Kinsey Institute and the University of Guelph found that about 21 percent of people in a committed monogamous relationship had had a sexual affair. The breakdown was 23 percent of men and 19 percent of women studied.
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What money secrets people keep have been shifting.
Of those in the latest NEFE study who said they had deceived a partner about money, fewer hid cash, purchases, financial statements and bank accounts than in 2011, while more lied about debts and earnings.
(See chart below for a breakdown of common lies.)
If there's good news, it's that the most common financial deceptions aren't as extreme as a husband betting away his wife's inheritance, or a wife who derailed the couple's retirement with a closet full of designer dresses—both real-life cases from financial advisors.
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The top reported deception was hiding a minor purchase, something 53 percent of offenders confessed doing. In comparison, 11 percent hid a major purchase, while 16 percent hid a bank account.
Keeping secrets usually has its roots in attitudes about money.
"It's 'my' money, and then all of a sudden you're in a relationship and it's 'our' money," said certified financial planner Doug Kinsey, a partner at Artifex Financial Group in Dayton, Ohio.
People can be tempted to withhold some information (and cash)—particularly about expenses that might be perceived as frivolous or embarrassing. Kinsey said he's seen several cases of people hiding money to spend on a hobby they thought their spouse wouldn't approve of.
Lack of communication is what makes deception possible, though, according to certified financial planner Lewis Altfest, CEO of Altfest Personal Wealth Management in New York. It's still common to see one partner take primary control of the finances, he said, which without communication leaves the other in the dark.
"Delegation is OK," Altfest said. "But knowing what's going on, taking an interest, is healthy."
(See chart below for the reasoning behind financial infidelity.)
Reasons aren't all selfish, either.
"It can be out of some misguided sense of protecting the other spouse," said certified financial planner William V. Suplee IV, president of Structured Asset Management in Paoli, Pa. He's said he has seen several instances in which husbands' avoided telling their wives about investment losses to keep them from worrying, and one in which a wife withdrew retirement funds to pay for college for the couple's child.
The cost of deception
Whether small or large scale, financial infidelity can be damaging both financially and romantically. Three-quarters of consumers experiencing it saw some relationship fallout, and 10 percent of them got divorced as a result, according to NEFE.
Sonya Britt, an assistant professor at Kansas State University's Institute of Personal Financial Planning, confirms that fighting about money is the top predictor for divorce.
"When couples argue about money at the very beginning of their relationship, we see they are more likely to have lower satisfaction, and higher divorce rates," said Britt, a certified financial planner who co-authored a 2013 study on the subject.
(See chart of relationship consequences of financial infidelity below).
A couple's finances may also suffer, said Beck at NEFE. For example, surreptitious purchases could easily trigger overdraft fees on a joint checking account. A couple he knew of couldn't get a mortgage because one partner had wrecked his credit score in the process of building a collection of college sports teams' credit cards.
In addition, joint loans equate to shared responsibility for debts, and signing off on a joint tax account could leave one partner liable for assets hidden by the other.
Drastic action is required sometimes.
Suplee recalls a case in which "our recommendation was, 'You really need to sell your house,' " And that wife who used retirement money to pay for tuition? He helped the couple break it to their child that student loans or a work-study program would be necessary.
If divorce is the result, a spouse's financial infidelity may not be reflected in the division of assets, said Andrew Samalin, president of the Association of Divorce Financial Planners.
"Assets acquired during the marriage are generally marital assets," he said. "The same thing holds true for debt." In other words, someone whose partner spent thousands illicitly may still be entitled to only half of what's left.
(Read more: How ID thieves are trying to steal your tax refund)
Samalin said he's seen cases of splitting couples reaching some middle ground on the disparity, with one person giving the other extra assets to make up for hidden spending during the marriage. But that depends on the couple, as well as state laws.
"Keep in mind that these are all taken in the context of settlement negotiations," he said. "There's a lot of variables to it."
Experts say the key to avoiding financial infidelity is to talk about your money principles. Two-thirds of Americans says the right time to combine finances is once you're married, according to a new survey from Chase. But we should start talking about money values well before that.
"You've agreed to a joint life, so start looking at things as partners or a team," Beck said, adding that such an approach also helps set a baseline for communication that makes it easier to ask if so many cash withdrawals were necessary or what prompted an unusual credit charge.
Whether you're combining everything or keeping some accounts separate, a few points bear discussion:
Collaborate on money goals
Even if you have different attitudes, you should have general agreement on priorities, Beck said. (For talking points, try NEFE's Life Values Quiz.)
Setting a must-discuss threshold lessens the chance that one partner may feel deceived about what the other considers fun money.
"Someone might feel like $100 isn't even worth talking about, but ... the other says, 'Hey, you spent $100 without telling me!' " said Kinsey at Artifex.
According to AmEx, the average threshold is about $280.
Budget an allowance
"One of the things that helps couples have fewer arguments is when each spouse has a certain amount each month that they can spend, no questions asked," Britt said. That affords each some freedom.
Schedule money talks
Periodically review together your joint financial situation. If you're not the one handling the money, at minimum, aim for a thorough review before signing off on returns come tax time and each month when bills are paid, Beck said.
"You owe it to yourself," he said.
Even for a couple that's dating seriously rather than married, an unwillingness to divulge details could spell trouble.
" 'That's none of your business' is a red flag," Beck said.
—By CNBC's Kelli B. Grant. Follow her on Twitter and on Google.