Roughly 40% of American adults who live with their significant other are unable to correctly identify how much money their partner earns, according to data from a recent study of 1,713 couples conducted by Fidelity.

Participants in the annual “Couples & Money” survey, which asks married and long-term couples who are at least 25 about their financial lives, struggled to identify their partner’s salary when given several options, each with a $25,000 range. The difficulties came despite 71% of partners saying they communicate “very well” with their other half, and 1 in 4 saying they communicate about money “exceptionally well.”

Even when given a larger salary range, 1 in 10 were still unable to identify their partner’s salary, Fidelity finds.

Many couples, it seems, are still hesitant to have full, honest discussions about money. “Life is busy and people don’t necessarily take the time to talk about their finances,” Stacey Watson, senior vice president of Life Event Planning at Fidelity, tells CNBC Make It. “Money can be an uncomfortable topic.”

Indeed, 44% of respondents tell Fidelity that they argue about money with their spouse occasionally, while 1 in 5 say money is their biggest relationship challenge.

Shame is often a factor that can keep people from being forthcoming with their partner, whether it be about how much money they make or the amount of debt they have, says Shannon McLay, founder and CEO of The Financial Gym.

“People are more comfortable getting physically naked with somebody than financially naked,” McLay says. “We’ve seen couples who have been married for years, who have children, and don’t know about each [other’s finances]

There are many different kinds of conversations about money, but many couples limit themselves to only discussing financial decisions like how much they spend on vacations or their children’s education, McLay says.

“There’s a difference between making financial purchase decisions and sharing financial details,” she says. “It’s great that you can have those conversations about purchases, but if you don’t know the digits, then you’re making decisions from a flawed perspective.”

At a minimum, McLay recommends that couples talk about their respective salaries, savings, credit scores, investments and debt. It’s crucial for couples to share information, because it could be a major factor in larger decisions, such as taking out a loan or buying a house.

For couples wondering how to start talking about money, Fidelity’s Watson recommends arranging a monthly date night where partners agree to discuss financial topics without judgment. And if one partner still feels uncomfortable, she says that bringing in an expert, like a financial planner, could be helpful.

“Both partners being involved the decisions is going to lead to a better financial future,” Watson says.