In “Earned Income Tax Credit (EITC) Punishes Marriage” I wrote:
If you and your partner are not married but have two children, you have some flexibility. Each of you can earn $17,100 and still collect the maximum $3,169 for a total of $6,338.
A married couple with two children earning the same $34,200 would only collect $2,734. The formula penalizes them $3,604, or 10.5% of their salaries, simply because they are legally married.
Here is a calculator showing that discrepancy: