The bottom quintile earned 2.2% of all earned income in 2013, but after adjusting for taxes and transfer payments, its share of spendable income rose to 12.9%—six times its proportion of earnings. The second quintile’s share more than doubled, rising from 7% of earned income to 13.9% of spendable income. For the third quintile, middle-income Americans, the increase was much smaller, from 12.6% to 15.4%.To put it another way, the effective marginal tax rate when a person moves from the bottom to the middle quintile is 1 - (15.4-12.9)/(12.6-2.2), or 76 percent.
Update: A reader points out that this calculation does not take into account various types of heterogeneity that might be correlated with income. True, but the graph in this old post shows that this probably does not matter much. Those in the bottom half of the income distribution face very high effective marginal tax rates.