Breadwinners by class I have a hypothesis that single-income families greatly increase as you get into the mid to upper middle class bracket. And that 2+ income sources are common in lower classes and also the extremely rich.

I can't verify this though because I can't find any research that looks into how many single income households there are by class. Instead, I see mostly gender connections. 

Does anyone know of any research that could be helpful here?
According to the third row in this table from the American Enterprise Institute (they credit the Census Bureau but I could not find the same indicators in their data)



your hypothesis is incorrect: the Mean Number of Earners per Household is increasing in income for each Quintile. 

I am surprised by the average number of earners in the highest quintile, but I think it is not very surprising that the poorest people are poor because they have trouble finding stable employment.