 You don't generally build something at first to go and sell it to low-income occupants, but filtering is not a theory. It's just something that you can go and check in the basic property tax records when you just see when was that thing built and realize that that's how people with low income get places, is they get places that were not originally built for them, and then they filter down to other people in this very natural process. Brian Kaplan, Build Baby Build.