Perhaps the least surprising aspect of Pew Social Trends' new research on the role of women in the household is that 40 percent of homes with children rely on a mother's income. That's substantial, and probably unexpected. But the data behind that trend — and the response to it — show how employment and gender roles in the United States continue to shift away from the Leave It to Beaver model. Murphy Brown is winning.

The 40 percent figure tracks the number of households with children under the age of 18 in which women are the primary earner. That chart can be seen at right. That the trend has now passed the 40 percent mark is not particularly remarkable; there's been no sudden spike in either the data for single mothers or households in which the mother earns the higher or only salary. It's been consistently increasing since 1960 (three years before Leave It to Beaver went off the air). Working mothers are so much the norm at this point that to even point that out seems unnecessary and incongruous.

Despite this ongoing shift, people still embrace the traditional view of a family — that it is "better" for the mother to stay home with the children. A majority hold that to be the case. Three-quarters of those polled feel that a family does just as well if the father goes to work.

Meanwhile, 64 percent of Americans view the increasing number of children born to unwed mothers as a big problem. But note the trend in the data when broken down by demographic. Younger people are far less likely to hold that view. Those who are most concerned about the trend are older, whiter, more Republican — in other words, the profile of a more traditionalist American.

But Pew also notes that the 64 percent figure is a significant decline since 2007. That year, 71 percent of Americans considered the issue to be a big problem. For unwed mothers — who comprise a quarter of households with children! — that's a significant change.

Younger people are also less likely to consider the effects of more women working outside the home to be detrimental. Seventy-four percent of Americans think that more women working will make it harder to raise children — but only 60 percent of those under 30 do.

Part of the generational difference is likely due to the firsthand experience of younger people growing up in families that have already stretched beyond traditional boundaries. (It's worth noting that the research is tremendously heteronormative, no doubt in part because Pew's research relies heavily on analysis of Census data.) The data demonstrates that the roles of men and women have steadily evolved over the past 50 years. In most households, the man and woman have the same level of education. Where they don't, it's more likely that the woman is better educated, a trend that changed in 2000.

More striking is the shift in pay: In nearly 23 percent of households with children, the mother earns more, compared to 3.8 percent in 1960. When the woman earns more, the household does, too. Median income for a family where the father earns more is $78,000. In a two-parent home where the mother earns more, it's $79,800. Sixty-three percent of Americans see no issue with the woman earning more.

Despite those shifts, one aspect of motherhood seems to be stubbornly persistent. Households run by women who've never been married are more commonly run by women who are younger and of color. The majority of single mothers are non-white; two-thirds of those who've never been married are as well.

Those households, Pew notes, have a median family income of only $17,400.

Beyond gender composition, what an American household looks like is starkly different than what it did half a century ago. And the Americans growing up in those households don't see much wrong with that.

Photo: An American family from Indiana. (AP)