By Drew Kann, CNN

(CNN) -- The number of married women earning more money than their husbands has grown. But you won't hear them boasting about it.

A new study finds that many spouses would prefer to keep that fact on the down-low. Researchers analyzed Census Bureau data and discovered that in heterosexual marriages where the wife out-earns her husband, both spouses misreport in census surveys how much they actually make.

In these cases, both spouses tend to exaggerate the husband's income while deflating the wife's earnings.

The study, co-authored by Marta Murray-Close and Misty Heggeness, found that husbands who earn less than their wives reported their income to be 2.9% higher than what was on their tax returns. However, researchers found that wives inflated their husband's earnings even more than the husband did.

When it comes to reporting the wife's income, the opposite was true.

While both spouses downplayed the wife's salary in census surveys -- reporting her income to be 1.5% lower than what she actually earned -- they found husbands tended to underestimate their wife's income more than she did.

Of the couples the authors analyzed, almost 1 in 4 were in marriages where the wife was the primary breadwinner.

So why are spouses fibbing about how much they make?

According to Murray-Close, there's a psychosocial explanation for the behavior.

"Survey researchers have long known that peoples' desire to present themselves in a positive light may lead them to understate socially undesirable behaviors and overstate socially desirable behaviors," she said. "Some people who give inaccurate answers on surveys may know they're distorting the truth, but many people probably do it unconsciously."

The researchers say the findings show how stubborn societal norms -- in this case, the idea of a "traditional" marriage, where the husband is the primary financial provider -- can seep into how we self-report on surveys.

"Our findings suggest that social norms and expectations haven't completely caught up with contemporary work and family arrangements," Murray-Close said.