In the United States, the number of people who need a kidney transplant outweighs the number of kidneys available. One way to motivate more living people to donate kidneys might be to have a carefully regulated, national system that provides fixed payments to kidney donors. However, some worry that offering payment for kidneys could create problems. First, people might donate kidneys without fully considering their own health. Second, payment might push poor people into donating kidneys. Third, people might be less likely to donate a kidney just to help another person. However, because we do not have such a payment system, we do not know whether some of these unwanted results would really happen.