Opportunity Costs

Poor people are people who make decisions. They are not a combination of circumstances that can be tweaked to make them stop acting like poor people. They like babies and sleeping in for the same reasons you do. And they are generally asked to give up those things in return for much less reward than the middle class people who cluck at them for their bad decisions.

Megan McArdle, "What Do Low-Income Communities Need?"

Sample Selection Bias for Laypeople

Now, economists hear this sort of argument all the time. "That's ridiculous! I would never start working fewer hours because my taxes went up!" This ignores the fact that you may not be the marginal case. The marginal case may be some consultant who just can't justify sacrificing valuable leisure for a new project when he's only making 60 cents on the dollar. The result will nonetheless be the same: less economic activity.

Jane Galt (Megan McArdle), "A really, really, really long post about gay marriage that does not, in the end, support one side or the other"