When a model simply doesn't match reality

Karl Smith relates a story from Megan McArdle:

A woman called me out of the blue last week and told me her self-sufficiency counselor had suggested she get in touch with me. She had moved from a $25,000 a year job to a $35,000 a year job, and suddenly she couldn't make ends meet any more. I told her I didn't know what I could do for her, but agreed to meet with her. She showed me all her pay stubs etc. She really did come out behind by several hundred dollars a month. She lost free health insurance and instead had to pay $230 a month for her employer-provided health insurance. Her rent associated with her section 8 voucher went up by 30% of the income gain (which is the rule). She lost the ($280 a month) subsidized child care voucher she had for after-school care for her child. She lost around $1600 a year of the EITC. She paid payroll tax on the additional income. Finally, the new job was in Boston, and she lived in a suburb. So now she has $300 a month of additional gas and parking charges. She asked me if she should go back to earning $25,000. I told her that she should first try to find a $35k job closer to home. Also, she apparently can't fully reverse her decision to take the higher paying job because she can't get the child care voucher back (the waiting list is several years long she thinks). She is really stuck. She tried taking an additional weekend job, but the combination of losing 30 percent in increased rent and paying for someone to take care of her child meant it didn't help much either.

Ms, McArdle tries to make a supply side argument here, where she points out that we are failing to create policies to incentive work among the poor (who can suffer a marginal tax rate of > 100% in many circumstances). It is a really interesting question why we focus on the top marginal tax rate and not the marginal tax rate for people in lower income brackets (where there is less of a competition effect). However, Karl Smith notices the really interesting behavioral issue here:

She faced a marginal tax rate in excess of 100%. This meant as her earned income went up she got poorer. What did she do? She tried to earn even more income. It was only we she failed at the attempt to make ends meet by supplying ever more labor to the free market that she try to go back to making less money.

So, not only do we have evidence from Matt Yglesias and Felix Salmon that top income earners don't necessarily even know their marginal rate, but we see low income people (facing a > 100% marginal rate of taxes) desperately trying to get more income and not less. It is not the case that the woman in this heartbreaking story decides that she would prefer to spend more time in leisure (so we can't intice her into working more). It is that working actually costs her money.

And her response is to get a second job!

Is it really too late to put supply side economics into the "special circumstances only" bin and leave it there? It may influence the odd movie producer, consultant, or freelancer (who have the ability to take on work in discrete projects and who have income sufficiency already). But this conceptual model seems to be absolutely dreadful at making predictions about how real people will act in most employment situations.