(Illustration by Gwen Keraval)

Four years ago near Valentine’s Day, Erika Torres made a very unromantic discovery: Her husband, Eric, was racking up credit card charges.

About a year into their marriage, the two were on a tight budget as they worked to aggressively pay down $45,000 in student loans, car loans and credit card debt.

They had postponed their honeymoon to Costa Rica and moved from a one-bedroom apartment in Orange County, Calif., to a studio – which they shared with their Wheaten terrier, Bentley – so that they could save $500 in rent. At times, they each had only $20 extra to spend per month.

She thought he was sticking to the budget, but instead he was piling on credit card debt to treat himself – and her.

Torres was upset. She also felt guilty: One $75 charge covered flowers for her.

About a third of people who have combined their finances with a significant other say they’ve hidden cash, a purchase, a bank account or a credit card from their partner, according to a survey by the National Endowment for Financial Education, a nonprofit. Some 13 percent have lied about how much they make or how much debt they have. Those in the money management business have a name for it: financial infidelity.

Of those who admitted to lying, 35 percent said they felt that they should be able to keep some aspects of their finances from their spouse. One in four said they worried their spouse would disapprove.

“They know that it’s wrong, but they would rather lie to their partner than talk to them beforehand,” says Patricia Seaman, a senior director for the nonprofit. “That fear of judgment and shame and disapproval is so strong that they are committing financial infidelity rather than talking with their partner.”

Many couples do share financial accounts. The majority, or 82 percent, of people surveyed this winter said they share financial information like salary, debts and credit scores with their significant other, according to a poll sponsored by Citi Cards.

Still, problems arise when people merge lives and bank accounts without talking openly about their spending habits. One in five people living with a spouse or partner have spent $500 or more without telling their significant other, according to a survey by CreditCards.com. Those impulse buys can be harmless, but they can also cause trust issues, especially when they disrupt the household budget.

Pamela King, 42, says she and her husband, Stephon, began to set real boundaries for their finances on the day 15 years ago when she drove to their Atlanta home in a new Mazda 626 – her fantasy car. In her previous relationship, King had been the primary breadwinner and used to controlling all of the finances.

So when she test drove the car at the dealership that day and the salesman offered what seemed to be a good deal, she snapped it up. “I never picked up the phone,” she says. “I would never do that today.”

They didn’t need the shiny red car; they already had two. And their combined family, which by then included four kids and a niece, didn’t even fit in the two-door sedan. It took a week for her husband to cool down enough that they could talk about how they were going to pay for the car. The new car payment and higher insurance costs added $500 a month to the bills.

King says she realized that the two of them, who were both working and used to making their own spending decisions, needed to start thinking of their finances together.

“He’s very independent, and I was very independent,” says King, who had been married about a year when she bought the car. “We really had to understand each other’s strengths.”

To start with, she agreed to cover most of the new costs by cutting back on shopping and by making fewer trips to the hair salon. Eventually, they traded the Mazda in for a bigger ride that was a better fit for the family, which now lives in Memphis.

It took some time, but they figured out ground rules. Now they don’t spend more than $300 or $400 without talking first, Pamela King says.

As it did with the Kings, couples that overcome financial secrets can come out stronger. Some people may feel more comfortable speaking openly about money after learning more about what is important to their partner, says Rachel Sussman, a psychotherapist.

“When you overcome a stressor, it creates a collaborative model,” Sussman says. “On the other hand, if you can’t overcome it, maybe that’s a red flag.”

More than three-quarters of the people who had deceived a partner about money said it damaged the relationship, according to the nonprofit report. Ten percent said it eventually led to divorce.

Some grievances, like second mortgages, hidden gambling problems or massive debts, are hard to reconcile, says Bruce McClary, a former credit counselor in the Washington area.

He remembers a case from about 10 years ago when a husband called a meeting to talk about finances with his wife. When they sat down in his office, McClary says the wife took the lead, laying out statements to show that they were in good standing and saying that she was confused about why they were there.

At that point, the husband pulled out a stack of envelopes and asked McClary to give them a few minutes alone. Inside were statements for credit cards the wife didn’t know about, totalling up to $30,000 in debt he had accumulated over the past year after losing his job. “Apparently, he was heading out the door everyday,” pretending he was still at work, says McClary, who is now a spokesman for the National Foundation for Credit Counseling.

The husband was able to find short-term work, but the pay wasn’t enough to keep up his share of the bills. The deceit derailed his wife’s plans of saving to return to graduate school. Instead they went to couples therapy. Then they returned to his office and came up with a plan for tackling the debt.

Last he heard, they were separating. “By the time this thing unraveled,” he says, “there were other issues in their relationship.”

Covert dealings can be disastrous, but full transparency about money can back-fire, too, says Lisa J.B. Peterson, a financial planner in Boston who runs a course to help engaged couples and newlyweds talk about money. Making equal contributions into separate accounts can give each person the freedom to indulge without recrimination, be it on pedicures, season tickets or even gifts for their partner.

“There is some healthiness to having some independence around your financial life,” Peterson says.

Erika Torres says she thinks lack of independence drove her husband to be secretive about his credit card use. “That was his way of saying ‘this isn’t working for me. I really need more flexibility,’ ” says Torres, 30, who was running the finances and scrutinizing even the smallest purchases. “I had to learn to be more lenient.”

They still kept their spending down. Instead of meeting friends at fancy restaurants, they invite them over for potlucks. When they stayed in on Friday nights, they ate store-bought pizza instead of ordering delivery. Torres, who works in public relations, took on side jobs, selling ads on her blog and freelancing articles.

Now instead of handling all of the finances herself, they talk regularly about the budget and adjust it as needed.

Last year, their finances went from negative to positive. Eric, who previously earned little more than the minimum wage as an emergency medical technician, landed a job as a firefighter, nearly tripling his income. The change came just as they made their final loan payments – amounting to about $1,500 a month, which suddenly started to pile up in their savings.

They also stashed most of Eric’s paycheck, at one point saving up to 50 percent of their take-home pay. By November, they had $55,000 to put down for a two-bedroom, two-bathroom house in a neighborhood that is close to friends and family. Bentley, their dog, has a yard to run around in.

This Valentine’s Day weekend, they are vacationing in Napa with some friends. “Shows how far we’ve come,” Torres says.

Read More:

The 10 rules a couple needs to manage their money

When money opposites attract

How couples cope when money is an unequal yoke

For richer or poorer: a financial plan for newlyweds