Enemies of the Poor:

our patchwork, uncoordinated system of antipoverty programs does have the effect of penalizing efforts by lower-income households to improve their position: the more they earn, the fewer benefits they can collect. In effect, these households face very high marginal tax rates. A large fraction, in some cases 80 cents or more, of each additional dollar they earn is clawed back by the government.The question is what we could do to reduce these high effective tax rates.

That’s Friedman’s argument for a negative income tax.