Personal Finance

How much do you know about your spouse's finances?

Marriage & money: Know what you own & what you owe
Marriage & money: Know what you own & what you owe

Do you know how much your spouse earned last year?

If you don't, you're not alone. In a new survey from Fidelity that takes a page from "The Newlywed Game," 43 percent of the respondents could not say how much their partner earned, and 10 percent of that group were off by $25,000 or more. (Tweet this.)

That's despite the fact that 72 percent of the respondents said they and their partners communicate "very well" or "exceptionally well."

The communication shortfall was not limited to salaries. Some 36 percent of the couples disagreed on the amount of investable assets they have together, and 47 percent of the couples couldn't agree on the amount they'd need for retirement.

Couples have to "double down" on their efforts, said John Sweeney, executive vice president of retirement and investing strategies at Fidelity. "Transparent communication is the most critical part."

Jamie Grill | Getty Images

Boomers appear to be facing special challenges when it comes to money and communication. They were more likely than younger respondents to disagree about how much money they will need for retirement, even though 10,000 boomers reach age 65 every day.

Sweeney pointed to several reasons why couples are not clear about their partner's finances. For one, he said, the economy is increasingly project-based, so more people have income that fluctuates from year to year. In addition, the strong stock market may be causing income shifts for older respondents living off their investment portfolios.

He also noted that the disconnect was greatest among Gen X couples, and they are at an age where they may be receiving variable income from stock options and bonuses.

Read MoreYour partner might be hiding more than debt from you

But there are also other, darker reasons why partners may have trouble identifying basic information about each other's finances. In a survey obtained by CNBC on Tuesday from insurance site Haven Life, nearly one in five admitted to hiding some of their finances from their significant other, including purchases and investments made without telling their partner.

And in an earlier survey sponsored by the Citi Double Cash card, 24 percent of the respondents said they had a bank account their partner did not know about.

Financial infidelity like that can damage more than wallets, according to research on behalf of the National Endowment for Financial Education. That report found one in three people who had combined their finances with a partner in the past year had committed some kind of financial infidelity, from hiding a purchase or concealing cash to concealing a bank account. And 76 percent of that group of respondents said the deception had had an effect on their relationship.

Read More7.2 million Americans hiding money from their spouses

Ironically, the participants in the Fidelity survey were quick to urge newlyweds to be open with each other about money. Their No. 1 recommendation to newlyweds was to start saving early for retirement, but the second most common recommendation was to "make all financial decisions together."

Read More5 key financial steps to take before saying "I do"

They could be on to something. Another new study, by TD Bank, found that only 27 percent of couples who talked about money less than once a month described their relationship as "extremely happy," but 42 percent of couples who talked about money at least once a week used that description. (Tweet this)

Apparently, while money can't buy you love, love and money do go hand in hand.