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By Allison Linn and Allison Linn

Jillian Phillips and her fiancé had barely begun dating when they began talking about money.

“We were very open about finances from the start,” said Phillips, 29. “I would say within two months we were talking about school loans and checking accounts and where we wanted to be in the next 10 years.”

Yet despite that openness, Phillips admits that she often does not tell her partner the whole truth about her spending habits, and that puts her in good company.

A detailed online survey conducted by TODAY.com and SELF.com found that 37 percent of men and 56 percent of women acknowledge having lied to their partner about money.

Phillips and her fiancé have never fought about money and have had a joint checking account for several years to pay for shared living expenses. It’s one reason she thinks their relationship has worked so well.

Still, Phillips admits that she’s keeping money secrets. Those $150 boots? She’s likely to fib and tell her husband-to-be she got them on sale for $100 –  even though she bought them with her own money.

“I feel guilty, for some reason, when I come home with a new pair of boots or a new bag,” she said. “I always say that I got it on sale even though that’s not true 98 percent of the time.”

More than 23,000 people responded to the TODAY.com/SELF.com survey, and about two-thirds told us that, in a relationship, honesty about money is as important as sexual fidelity.

Yet many admit to keeping money secrets.

The survey found that women are nearly twice as likely as men to hide purchases or receipts from their partner, with nearly one-third of women admitting to the practice.

In addition, about one-fourth of women said they’d pretended something was old that was actually new (“What? This old thing?”) or said they’d bought something on sale when it wasn’t. Only about 8 percent of men had done those things.

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When asked why they keep money secrets, about one-third of women reasoned that they earn money, so they deserve to spend it. About one-third said they disagree with their partner about what to spend money on.

Those were also the most common answers men offered for why they keep money secrets.

Experts say that when it comes to marriage and serious partnerships, financial compatibility is important.

“Since the 1970s, we’ve had this idea of marriage being about a soulmate,” said Bradford Wilcox, director of The National Marriage Project at the University of Virginia, “(but) beneath that idea there’s still this idea, for most people, that money matters in sort of guiding and sustaining a marriage.”

Still, Wilcox said, not all money secrets are equally problematic when it comes to marriage.

For example, he said, fudging the truth on how much you spent at Banana Republic or Best Buy would not likely carry as much weight for most spouses as having a one-night fling.

On the other hand, a long-term pattern of hiding money or having secret credit cards could signal deeper rifts, and perhaps even be a sign that the individual is preparing to leave the marriage or relationship. And that could be as damaging as a one-night stand.

“Financial fidelity is a lot more gray, and sexual fidelity is a lot more black and white,” Wilcox said.

Sex and money do have something in common: They are two areas where one partner has the capacity to devastate the other.

“Those are the two domains where they do fundamentally function based on trust, (and) those are the two domains where, once you’re really hooked on somebody in life, they can completely ruin you,” said Scott Stanley, a research professor at the University of Denver and co-author of the book “Fighting For Your Marriage.”

Stanley said financial issues are often a major cause of stress in relationships, and the economic woes of the past five years, including high unemployment and collapsing home prices, have certainly strained many marriages.

Still, he said, couples may argue about money because money, like child-rearing, is something couples deal with day-to-day.

“I think it gives the impression that money is the cause of divorce, but money comes up every day. Something about kids comes up every day,” he said.

He thinks the fact that couples may disagree about money is less important than how they fight and whether they are able to communicate openly despite their differences.

People with very different philosophies about money may have a hard time making it. Stanley said couples who are planning together for a financial future, such as by sharing accounts or saving money together for retirement, tend to have a better chance of success than those who keep finances separate because they have fundamentally different views about money.

Wilcox said his research also has found that struggling with debt is generally a bad thing for relationships, while saving for the future – whether it’s retirement, vacation or college expenses – is generally a good thing.

“That’s a good thing in part because your assets give you a sense of a future together,” Wilcox said.

Phillips, who lives in Chicago and works as a publicist, suspects that her fiancé actually has an inkling about the money secrets she’s keeping. She also doubts he would care that much about how she chooses to spend her money. Although the couple has a joint account for shared expenses, they keep their own accounts for individual expenses such as clothes.

And yet she honestly doesn’t know if she’ll shed her habit of fibbing about her spending, which she said she picked up from watching her own mother do the same thing.

“I would like to say that it would change,” she said.