In a trend accelerated by the recent recession and an increase in births to single mothers, nearly four in 10 families with children under the age of 18 are now headed by women who are the sole or primary breadwinners for their families, according to a report released Wednesday by the Pew Research Center. The report reveals a sweeping change in traditional gender roles and family life over a few short decades: The number of married mothers who out-earn their husbands has nearly quadrupled, from 4 percent in 1960 to 15 percent in 2011. Single mothers, who are sole providers...