ERIC POSNER is the Kirkland and Ellis Distinguished Service Professor at the University of Chicago Law School. Posner has written many books and hundreds of articles, and he also maintains a blog, www.ericposner.com; a Twitter feed; and a regular column in Slate, where “A Moral Market” first appeared. Posner first co-wrote an academic study— “Altruism Exchanges and the Kidney Shortage”—with fellow law professors Stephen J. Choi and G. Mitu Gulati. (This article can be found online through a Google search; the Slate article appears in Chapter 7.)