Today at Berkeley, Hilary Hoynes spoke about the impacts of the Earned Income Tax Credit on female labor supply. She and a coauthor find increases in working of about 6 percentage points among single mothers after the EITC expansion of the early 1990s, compared to single women without kids. Moms with more kids and thus more tax credits also worked (even) more.
This is a bread-and-butter labor supply issue, great for applied teaching. To paraphrase her words, the substitution effect dominates, and moms who see higher after-tax wages because of the EITC substitute away from leisure and toward working even though their incomes also rose.