Master storyteller O. Henry gave a perfectly crafted lesson on marital personal finances in his Christmas allegory “The Gift of the Magi.” You may remember the very poor newlyweds. He hocks his watch to buy a comb for her lush chestnut mane; she sells the very same hair to buy him a fob for the very same watch.

It’s a touching tale about true love not needing big gifts. But in the real world, a major surprise purchase that catches a spouse off guard may break not only the budget; it can threaten the foundation of the marriage itself.

Consider a couple that New York family therapist Susan Burden worked with. The couple married young and in the beginning, money matters were pretty much 50-50. She earned more that he did and put him through graduate school. They lived frugally, saving up for children, a house, and a pedigreed dog. They felt like partners.

But when they moved into the second decade of the marriage, he began to make significant money. More important, he began to feel that because he made the money, he could spend it. Meanwhile, she was still operating under the rules of their original partnership, under which they discussed major expenditures and lived rather modestly.

Then one Saturday morning, he charged into the house and called to her in the shower, “Come see what I’ve got.” She dried off, dressed, and opened the front door to see … a brand-new Jaguar. He had ordered it months before after carefully choosing custom details, all on his own. The kids were dancing around the car, stroking the polished trim and peering at their reflection in the rotating side mirrors.

She was not amused. In fact, she was furious. “She felt demeaned, demoralized, and put down,” remembers Ms. Burden. It provoked her to ask silently, “What else was he hiding?”

For his part, he was equally furious that she wasn’t excited about his new toy.

Many, if not most married people conceal a financial white lie. According to a recent Money magazine survey conducted with Matthew Greenwald & Associates, more than half of the adults surveyed with household income of at least $50,000 fessed up to at least one money secret or lie. Over 40% of those interviewed believed that that a little lie was fair in love.

According to the survey, men’s fibs, whether by commission or omission, tend to be bigger. Twice as many men as women admit that they’ve spent more that $1,000 without their partner’s knowledge or assent.

Money is a lighting rod for our emotions. Anger about a secret purchase, or one made without consultation, runs right along that lightning rod to explode into other areas. “Once you lose trust in your partner – lose the feeling that you are partners – it becomes hard to go out to dinner without rancor, much less be intimate in other ways,” explains Ms. Burden.

Clearly, the range of emotional reaction to money ill spent depends on the partners’ agreement, whether implicit or explicit. If one partner is abiding by an agreement that the other blithely ignores, the marriage will suffer, even if the money spent isn’t terribly extravagant.

Ms. Bailey is a writer and therapist in New York. She can be reached at ebailey@nysun.com.