THE great virtue of economists is that they see a particular dimension of social reality with great precision. The great danger is that having seen a slice of human life, they imagine themselves to have seen the whole, and they prescribe reforms with a passion that befits those who believe that an irrational world is ignoring their perfectly rational models. In their new book on organ markets entitled The U.S. Organ Procurement System: A Prescription for Reform, David L. Kaserman and A.  H. Barnett display both the virtues and vices of economists, but more the vices than the virtues.