[bunderbunder]

It might vary by region. It's fairly standard among my circle of friends. For most of us, doing the math revealed that, with both parents working, child care expenses and the like would eat up all or most of what the lesser-earning parent was bringing home after taxes.

Some of that is about how the tax system works in the US, too - the way that "married filing jointly" works was designed under the assumption that married women are homemakers, and so treating two-income families fairly wasn't a priority.