Marriages are just as likely to be broken up by financial hanky-panky as adultery, and the current economy makes hiding mounting bills as big a problem as overspending.
Of 2,000 people polled by American Express, 91 percent said they avoid discussing money with their spouses or partners, half made a purchase that their spouse was against and 30 percent admitted to actually hiding what they bought.
Financial counselors say the bad economy has brought on more of the flip side of being unfaithful about funds --spouses who keep serious debt to themselves until it's too late.
A problem needn't be as severe as pathological gambling or addictive shopping to wreak irreparable havoc on a marriage. It can be as simple as "I don't like how you manage money, so I'm putting some of mine away so you can't get to it."
If you keep personal purchases a secret from your spouse or partner, or stash cash that your significant other is unaware of, you're cheating.
Susan Zimmerman, an Apple Valley, Minn., marriage and financial counselor, said she sees three times more couples who are grappling with money issues than clients who are dealing with sexual infidelity. "There's a greater feeling of ethical wrong with sexual infidelity, while the multiple ways people can keep financial secrets isn't seen as so bad. You start with one little secret and if you get away with it, it turns into a strategy."
People may find illicit money affairs easier to rationalize than physical affairs -- but not easier to understand. The core problem is not so much money itself as the fact that spouses haven't learned how to bridge their differences.
"Fights about money are really about the couple not having figured out how to make decisions fairly and listen to each other without defensiveness," Zimmerman said.
Money is one of the most avoided topics in a marriage, even though it's also one of the most important. That avoidance is usually what turns a small issue into a significant one. Many couples don't talk about money at all until after they're married, and then start learning things about their spouses that they don't like.
"The money secrets become a method of protection that the cheater justifies because they've learned a certain distrust," sad Zimmerman.
Role models are a huge influence, something that Wendie Pett learned the hard way. Pett, a small-business owner in White Bear Lake, Minn., has been divorced six years. Financial infidelity wasn't the only reason for the split, but it was one of the ex-couple's biggest problems, she said.
"I grew up in a family where the kids always got what we wanted; it didn't matter if my mom had to take three jobs and put things on layaway. So I was a spender. My ex was very frugal, like his family, and wanted to save," she said. "He was so detailed about the finances that once there was a quarter missing and he wanted to know where it went."
The couple's money war "kept getting brushed under the rug and finally the pile of dirt was so big we were tripping over it. I was buying clothes he didn't know about and I would lie and say I've had it forever. I would hide receipts and break down boxes and stuff them down deep in the trash. I would hide new clothes until my birthday so I could say they were gifts."
After her divorce six years ago, Pett was determined not to repeat her mistakes, and has taken steps to avoid it, including money management classes.
"If I get married again, I don't want to have the same patterns, and it's also helped me understand money better for my business."
Just as shifty as not owning up to spending -- and more common in a bad economy -- is keeping growing household debt under wraps. If the spouse in charge of family finances gets laid off or finds that an entrepreneurial business isn't so great anymore, he or she may not share the shaky state of their finances until it's too late. Serious credit-card debt can build up quickly when one spouse is not aware.
Amy Wolff, a financial adviser in Edina, Minn., cites two recent cases, one involving a couple in their late 50s and the other in their 30s, both following a traditional pattern of the husband being the main breadwinner and managing more of the money. In both cases, the husbands hid family debt from their wives to the point where one's house was in foreclosure without the wife knowing, and the other's home was in danger of a similar fate.
"In both these situations, they were initially thinking, 'I just need a little more time, and things will get better,'" she said. "At first, they deny it's happening, then they feel so embarrassed they think they can't bring it up."