This is one secret millennials desperately want to keep.

Nearly one in four (23%) of millennials say they have hidden purchases from their spouse or partner, according to a survey of roughly 2,700 adults released Thursday by Northwestern Mutual. Meanwhile, just 17% of Gen Xers and 8% of Boomers say they’ve done that.

This may come down to the guilt and shame they feel in how they spend their money, says psychologist Elizabeth Lombardo, author of “Better than Perfect: 7 Strategies to Crush Your Inner Critic and Create a Life You Love.” Nearly one in three (32%) millennials say they are prone to excessive or frivolous spending, compared to 26% of Gen Xers and 19% or Boomers. And 31% of millennials have spent money budgeted for others things on themselves, compared to just 15% of Gen Xers and 4% of boomers.

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It may also have roots in the deep stress and anxiety millennials — who worry about everything from savings to losing their jobs to not earning enough — feel about money; it’s to the point that their anxiety is bleeding into other aspects of their lives. More than four in 10 say financial anxiety causes them to miss social events and opportunities — 2.5 times the rate for the U.S. population on average. And 28% say that financial anxiety impacts their job performance at least month, and sometimes as much as daily.

“With stress levels elevated, many people are looking for ways to have less stress and greater happiness. And they think that ‘shopping therapy’ will bring that joy,” says Lombardo. “In reality, though, while happiness levels may increase temporarily after a purchase, that elevation is short-lived. This results in the person buying more and more in an effort to feel happier. And in doing so, they may be quite cognizant that their partner will not concur.”

Whatever the reasons, experts say repeatedly hiding purchases can harm your marriage or relationship. “It scratches the surface of the foundation of trust in a relationship,” says Beverly Hills psychotherapist Fran Walfish, costart of WE tv’s “Sex Box.” “And if you scratch the surface over and over, eventually it makes an indelible mark that is irreparable.”

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It can also reveal things about the state of your relationship. For Kelly — a 36-year-old, now-divorced brand advisor who asked that we withhold her full name — money was a weekly source of conflict in her former marriage. Her former husband struggled financially and “complained all the time about not having enough money or what was happening at his job” so Kelly often ended up paying the bulk of the couple’s expenses. The stress that money put on their relationship caused a number of arguments — and one time it led to her hiding a pair of shoes that she’d bought. “At times I felt like I had to sacrifice my needs for his, so one time I said forget it, I need shoes I’m buying them,” she says. “It made me feel like I had made the wrong decision with marriage and that I was sorta being held hostage.”