Well, one thing they glossed over is the cost of tuition at the colleges in question varies. The poorest families are more likely to send their children to cheaper colleges. You might get an S curve if it was for colleges with $50K tuition, or whatever.
I agree but I would say it's not the poorest families that choose cheap colleges but lower middle-class families. People like teachers, office workers, et. al. who can't afford every college or who have to take out loans to send their kids to college are going to be price-sensitive. The poorest people generally make so little that they qualify for great financial aid without having to do academic scholarships.
