Professor DeLong,(posted 8752 days ago)I agree that the EITC is an example of good tax policy. (I teach a Tax Policy course in the graduate tax program of a Philadelphia area law school.) The second criticism you mention is that the EITC "penalizes working-class two-earner families." You may wish to address this criticism more directly in a follow-up piece. The heart of it is, I think, that the EITC statute requires that "in the case of an individual who is married ... [the credit is available] only if a joint return is filed ... ." See Internal Revenue Code section 32(d).
The EITC tables, together with a comprehensive explanation of the provision, are published each year in IRS Publication 596, available for reading or download at the IRS web site. The tables permit ready comparison of the credit for single parent and two-parent working families with one or two children. The incentives for marriage or divorce provided by the credit in its current, section 32(d) form then may be evaluated in comparison with some other evident alternative, such as elimination of the section 32(d) requirement.
One might evaluate the different policies and perhaps speculate on their constituencies.