As multi-hospital kidney exchange has grown, the set of players has grown from patients and surgeons to include hospitals. Hospitals can choose to enroll only their hard-to-match patient-donor pairs, while conducting easily-arranged exchanges internally. This behavior has already been observed....

Because no marketplace operates in isolation from the larger world, marketplace designs may need to adapt to changes in the larger environments. I discuss such changes in connection with the labor markets for new doctors, new Ph.D. economists, and for kidney exchange transplants. But while...