The magic number for marital financial bliss is a mere $400.

That's how much, on average, that couples said it's all right to spend without first talking to a partner, according to a new survey of 1,500 couples by Ameriprise Financial. That threshold grew with age, rising from $100 among millennial couples to $500 for boomers. A third of the couples surveyed didn't set a number.

Almost 70 percent of the partners said their communication around finances was good. Across generations, boomers were "most likely to say that their communication is perfect," said Marcy Keckler, vice president of financial advice strategy at Ameriprise. (Hey, they've had more time to practice.)

Not everyone was perfect. Five percent of couples copped to having a hidden account that partners doesn't know about. And 30 percent argued about money at least once a month—often about a major purchase or spending habit—though 82 percent said that they could resolve the disagreement and move on.

Maybe conflict is inevitable, since many of the couples included one person who identified as a saver and one claiming to be a spender. Unsurprisingly, 73 percent of pairs said they approached financial decision-making differently than a partner did.

Related: Why more women are putting off retirement

While most people aren't so extreme, it can get ugly when a spendthrift marries a tightwad, said Scott Rick, associate professor of marketing at the University of Michigan's Ross School of Business, who has studied both types.

Each type is unhappy with it relates to money, he said. "They're more likely to marry each other than someone like themselves, which is kind of rare, because usually we marry ourselves—but if you don't like something about yourself, you don't look for that in someone else; you look for something different," said Rick.

Early on in a relationship, that's probably a good thing, he said, but "the more different spouses are in this dimension, the more they fight about money—and the more they wish they had married someone else."

About 70 percent of the couples said they spend money without telling their partners. It makes sense—imagine how annoying it would be if your partner checked in with you on every bit of spending—and 59 percent said they didn't tell their partner about a purchase because it wasn't that big. Others assume their partner is simply "too busy" to care.

Same-sex couples—which made up 6 percent of the couples surveyed—were more likely than heterosexual couples to say they shared financial responsibility equally and were more likely to say that their respective roles and responsibilities were something they had intentionally discussed early in their relationship, rather than something that evolved over time, said Keckler. They were also more likely than heterosexual couples to have a financial adviser, set spending limits, and a combination of separate and joint bank accounts.

All those surveyed weren't always clear on how much they had saved for retirement. Twenty-three percent weren't sure how much they had saved, echoing a survey that found that 21 percent of couples don't even have a ballpark idea of their partner's retirement savings. In the Ameriprise survey, more than 90 percent of couples agreed on how much they need to save for retirement: $1.7 million.

While 51 percent of couples said they weren't saving enough, 45 percent thought their savings were just right. About 3 percent of those surveyed had a "problem" you rarely read about, said Keckler: "They think they have saved too much."

Let's hope, for those people's sake, they don't go around boasting about this. The legions of Americans way behind on retirement savings might just whip out tiny violins.

Instant Insights /

The secret to a good marriage might be keeping small financial secrets

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