While most of us depend heavily on our paychecks to cover monthly bills, it’s surprising how many of us aren’t sure what our partners bring home. According to a new study by Fidelity Investments, 43% of couples couldn’t identify how much their partner makes, despite the fact that most of them (72%) say they communicate well.

“We know couples don’t always agree when it comes to money, but we were surprised how many missed the mark on the question of their partner’s salary,” said John Sweeney, executive vice president of retirement and investing strategies at Fidelity.

In fact, Sweeney said the disconnect was up dramatically from 2013, when only 27% couldn’t correctly identify their spouse’s salary. Even more surprising is that among Generation X, those born 1965-1978, nearly 55% didn’t know what the other spouse made.

Sweeney said there are a couple of reasons why that number has gone up recently, including the fact that the economy has moved toward more project-based work, and away from straight salaried positions. Also, as stock market performance is increasingly tied to some corporate compensation packages, it can be more difficult to predict.

Some experts say that talking about money with your spouse has always been tough. Here are a few reasons why it’s such a touchy subject:

Talking about money is forbidden

“Talking about money is the last remaining taboo and is far more ‘off-limits’ a topic than even sex,” said Dr. Daniel Crosby, a behavioral finance expert and president of IncBlot Behavioral Finance. He said if you’re highly compensated, you may be viewed as privileged or out-of-touch. Or, if you’re not making that much, you may be pitied or viewed as underachieving.

“It just seems that for most people, the only good salary figure is the one you don’t discuss,” Crosby said.

Money is a very emotional topic for couples

Money can be an uncomfortable, especially if couples feel that it’s tied to their self-worth, according to Farnoosh Torabi, author of When She Makes More, and correspondent at Nerdwallet.com.

“If you’re insecure about it, you don’t want to talk about it,” she said.

Also, sometimes the topic of money doesn’t come up until there’s a problem, according to Dr. Mary Bell Carlson, a financial therapist based in Washington, D.C. Dr. Carlson says it’s ironic that even with so many ways to communicate, couples still don’t talk about it until there’s something to fix, and problems, by nature, are emotional.

Also, as the concept of marriage evolves, many couples are choosing to keep their finances separate, according to Dr. Erika Rasure, personal finance expert and assistant professor of business and financial services at Maryville University in St. Louis, Missouri.

“It seems kind of crazy that you wouldn’t know what your spouse makes,” Rasure said. “But for a lot of couples, it never comes up.”

Attitudes about money depend on how you grew up

Dr. Rasure says whether or not a couple can have a productive conversation has a lot to do with how each spouse has experienced money in his or her life.

“Those who have experienced money in a negative way, will often become agitated, tense or indifferent,” she said. “When conversations arise they often experience feelings of fear or anxiety, effectively shutting down productive conversations regarding the role of money within the marriage.”

Recognizing differences in the way he or she deals with money issues is vital to starting the conversation, according to Dr. Susan Heitler, clinical psychologist and author of PowerOfTwoMarriage.com, a website that promotes healthy marriages.

“The key is to be able to talk calmly about his style and hers, and then put these together to create our way,” she said.

Why it’s important to talk about money and what each other makes

“Knowing each other’s salaries is part of a sharing, intimate relationship,” said Dr. Heitler. “It’s equivalent to knowing each other’s health situations and sexual preferences.”

Plus, it’s tough to develop a sound savings and investment plan without a clear picture of household income.

“If there is no transparency, there can be no plan, and if there’s no plan, there can be no confidence,” Dr. Crosby said. “Forthrightness is an important first step toward financial security.”