Middle class struggle
Elizabeth Warren suggests that the American middle class is in decline. Over the last 40 years the only real income gain for families has come through women entering the workforce (i.e., two income families). As you would expect childcare costs have mushroomed during that time. But also home and medical costs have grown much faster than salaries (I would add education costs to the mix).
Clifford Thies also recently examined marginal tax rates when you include the phase out of various tax credits and other assistance programs. It turns out that for income between $20K and $40K the marginal tax rate hovers around 100%. This shows how difficult it is for the working poor to get ahead. For someone supporting their family on $20K/yr. They could potentially work really hard and double their salary and their family would be no better off (ouch!).
The US Census Bureau states the 2008 median household income is around $50K. This means well over a third of households are probably in this dead zone.