It used to be said that men married for sex and women for money. But marriage is proving an increasingly profitable ­enterprise for a growing number of ­American men who earn less than a ­growing number of better educated spouses – without enduring the social stigma that once afflicted husbands with working wives.

A study of married couples in the US has revealed that the proportion of American wives earning more than their husbands has risen more than five-fold since 1970. With that has come greater control by women of household finances.

The study, by the Pew Research Centre in Washington DC, shows that the shift ­follows a reversal in patterns of education. Today, a majority of wives are as well, or better, educated than their husbands.

"This is a portrait of gender role ­reversals in marriage," said one of the report's authors, D'Vera Cohn.

"The larger earner is increasingly likely to be the wife, the better educated partner is likely to be the wife.

"Marriage has been a way for men to raise their economic standing in recent decades when traditionally marriage has been a way for women to increase their economic standing."

In 1970, just 4% of American wives earned more than their husbands. That figure rose to 22% in 2007. Men are still the bigger earners in one in four marriages but with a far smaller proportion of wives not working, the income gap is smaller. Most married men did not have a working wife 40 years ago. Now they do.

Andrew Cherlin, a sociology professor at Johns Hopkins University who ­specialises in the study of marriage, said the findings showed a fundamental change in marriage over recent decades.

"It shows a shift in the basic marriage bargain. Under the old bargain men earned the money and women took care of the house," he said. "Now the bargain is that they both work and marriage is a pooling of income. The two-earner ­couples are the winners. Increasingly, they are better off."

From about 1990, it made economic sense for men to get married as the median income for those who got hitched for the first time rose above those who remained single. The median income of married women and men, and unmarried women, is about 60% higher than similar groups in 1970. But unmarried men, who once had the financial benefit of not having to support a family, have fallen behind. Their median income is only up by 16%.

The report said the economic crisis was "reinforcing these gender reversal trends, because it has hurt employment of men more than that of women".

The Pew study, based on US census ­statistics for married couples aged 30 to 44, shows that the changing financial ­fortunes of married couples reflects rising levels of education among women. Over the past 40 years, there has been a reversal in the pattern of education within marriage. The number of couples who have equal levels of education has stayed constant at about 53%.

But in 1970, 28% of husbands were ­better educated than their spouses while 20% of wives had the more advanced ­education. Now those numbers are reversed.

"Women had more access to ­education in the 60s and 70s," said Cohn. "The 70s began an era of great change for women in education and working. Marriage changes reflect those changes."

The report added that there was ­evidence from other research that women's growing economic clout gave them more power within marriage.

A Pew study two years ago found that wives who earned more than their ­husbands were more likely to have decision making power, especially over major purchases and household finances.

In couples where the wife made more money she was then more than twice as likely to make most decisions on ­household finances. But the report said there was an ­important exception to the rule that ­married people have fared better than unmarried ones.

"Married women without a high school diploma did not make the same gains as more educated women," it said.

"The stagnant incomes of married women without high school diplomas reflect the poor job prospects of less ­educated men in their pool of marriage partners. These less educated married women now are far less likely than in the past to have a spouse who works: 77% did in 2007, compared with 92% in 1970."

In addition, less educated Americans are less likely to get married. Forty years ago, education levels had little impact on who got married.

That is no longer true, with university graduates significantly more likely to marry – and each other.

"The university-educated people have been marrying each other and separated from the rest of the population. We've seen a shift between the educated and less well off," said Cherlin.

The report also found that the economic downturn was reinforcing the gender reversal trends as men were more likely to become unemployed. "Males accounted for about 75% of the 2008 decline in employment among prime-working-age individuals," the report said.

How are men taking all this? Opinion polls show that there is an expectation among a clear majority of couples that both partners will work, something that was not true in polls as recently as 1988.

"Certainly there are still some men who are insecure about women who earn more than they do," said Cherlin. "But increasingly they are pleased and less pressured than a generation ago. Fifty years ago a man would be ­embarrassed and ashamed of a woman working. It was a sign he couldn't support his family. No one thinks that any more."