The “sharing economy” has attracted a great deal of attention in recent months. Platforms such as Airbnb and Uber are experiencing explosive growth, which, in turn, has led to regulatory and political battles. Boosters claim the new technologies will yield utopian outcomes—empowerment of ordinary people, efficiency, and even lower carbon footprints. Critics denounce them for being about economic self-interest rather than sharing, and for being predatory and exploitative. Not surprisingly, the reality is more...