Financial disincentives to live kidney donation: richer people donate at a higher rate

The paper "Population Income and Longitudinal Trends in Living Kidney Donation in the United States" by Jagbir Gill, Jianghu Dong and John Gill, in the Journal of the American Society of Nephrology, published online July 17, 2014 has this striking figure:

The difference in living donation rates per million population between the highest and lowest income quintile populations increased over time and was largest in 2010. 

Note that the recession caused a decrease in donation among the least wealthy. I think this is as clear a demonstration of the financial disincentives for donation as I've seen.