WASHINGTON - A record number of American women are now the sole or primary breadwinners in their families, a sign of the rising influence of working mothers, a new study finds. Mothers now keep finances afloat in 40 percent of households with children, up from just 11 percent in 1960.

While most of these families are headed by single mothers, a growing number are married mothers who bring in more income than their husbands, according to a study released Wednesday by the Pew Research Center.

As the numbers have shifted, however, public attitudes have remained mixed regarding the impact of working mothers on families. People are not at all sure that it's a good thing.

Demographers say the change is all but irreversible and is likely to bring added attention to child-care policies as well as government safety nets for vulnerable families.

"This change is just another milestone in the dramatic transformation we have seen in family structure and family dynamics over the past 50 years or so," said Kim Parker, associate director with the Pew Social & Demographic Trends Project. "Women's roles have changed, marriage rates have declined - the family looks a lot different than it used to. The rise of breadwinner moms highlights the fact that, not only are more mothers balancing work and family these days, but the economic contributions mothers are making to their households have grown immensely."

The trend is being driven mostly by long-term demographic changes, including higher rates of education and labor force participation dating back to the 1960s women's movement. Today, women are more likely than men to hold bachelor's degrees, and they make up nearly half - 47 percent - of the American workforce.

But recent changes in the economy, too, have played a part. Big job losses in manufacturing and construction, fields that used to provide high pay to a mostly male workforce, have lifted the relative earnings of married women, even among those in mid-level positions such as teachers, nurses or administrators. The jump in working women has been especially prominent among those who are mothers - from 37 percent in 1968 to 65 percent in 2011 - reflecting in part increases for those who went looking for jobs to lift sagging family income after the recent recession.

At the same time, marriage rates have fallen to record lows. Forty percent of births now occur out of wedlock, leading to a rise in single-mother households. Many of these mothers are low-income with low education, and more likely to be black or Hispanic.

In all, 13.7 million U.S. households with children under age 18 now include mothers who are the main breadwinners. Of those, 5.1 million, or 37 percent, are married, while 8.6 million, or 63 percent are single. The income gap between the families is large - $80,000 in median family income for married couples vs. $23,000 for single mothers.

Both groups of breadwinner moms - married and unmarried - have grown sharply.

Among all U.S. households with children, the share of married breadwinner moms has jumped from 4 percent in 1960 to 15 percent in 2011. For single mothers, the share has increased from 7 percent to 25 percent.

Roughly 3 in 4 adults said the increasing number of women working for pay has made it harder for parents to raise children.