Women's wages are the primary source of income for a full 40 percent of American households with children under the age of 18, according to a new report by the Pew Research Center.

These "breadwinner" moms include 8.6 million single mothers, who account for 63 percent of the group, as well as 5.1 million married women who make more money than their husbands, comprising the other 37 percent.

While Hispanic women are represented in both groups, a higher proportion of Hispanic women are single mothers. Of all mothers in the country, 18.9 percent are Hispanic, while 21.7 percent of single mothers are Hispanic. Only 14.8 percent of Hispanic mothers make more than their husbands, but over a fifth of Hispanic mothers -- 21.3 percent -- make as much money as their husband.

The trend toward higher wages for women and higher proportions of household income attributed to women continues. In 1960, only 11 percent of households with children were primarily supported by women's wages.

But while there is growth in that demographic, progress is not evenly distributed. "The income gap between the two groups is quite large. The median total family income of married mothers who earn more than their husbands was nearly $80,000 in 2011, well above the national median of $57,100 for all families with children, and nearly four times the $23,000 median for families led by a single mother," says the Pew report.

"The groups differ in other ways as well. Compared with all mothers with children under age 18, married mothers who out-earn their husbands are slightly older, disproportionally white and college educated. Single mothers, by contrast, are younger, more likely to be black or Hispanic, and less likely to have a college degree," it added.

Women's attitudes about working outside the home have changed since the beginning of the economic recession. In 2007, before the recession, only 20 percent of American mothers said their ideal situation would be to work full-time. That jumped to 37 percent in 2012. In 2007, 29 percent of mothers said ideally they would not work outside the home at all, a sentiment expressed by only 20 percent in 2012.

Single mothers can also be broken down into several groups: women who have been married and then divorced or widowed, and mothers who have never been married. Hispanic women make up 24 percent of mothers who have never been married, and they are less likely to graduate high school or live above the poverty line.

Still, attitudes about single mothers are changing, becoming less harsh. While 64 percent of Americans say the growing number of single mothers is a "big problem," that is down 7 percent from 2007. The sentiment is shared by only 42 percent of people under the age of 30.

Other findings from the Pew study:
* Both groups of breadwinner mothers, married and single, have grown in size in the past five decades.
* The total family income is higher when the mother, not the father, is the primary breadwinner.
* Married mothers are increasingly better educated than their husbands.
* Most people reject the idea that it is bad for a marriage if a wife out-earns her husband.
* Today's single mothers are much more likely to be never married than were single mothers in the past.