Insurance with undiversifiable risk: Contract structure and organizational form of insurance firms

Previous explanations of the contract choice and organizational form of insurance firms do not explain, by themselves, the recent proliferation of mutuals and new contract designs. We first present risk-bearing arguments to address these phenomena. We present two forms of insurance. The first is a conventional transfer of risk whereas the second decomposes risk between idiosyncratic and nonidiosyncratic. We show that the latter form leads to more active trade in insurance markets with correlated exposures. Moreover, the decomposed form dominates the simple transfer. These results qualify and extend the work of Borch (1962) and Marshall (1974). Market responses to the recent liability insurance crisis are compatible with these predictions.This research was conducted while G. Dionne was visiting the University of Pennsylvania under grants from Huebner Foundation and CRSH Canada. N. Doherty wishes to acknowledge with thanks support from the National Science Foundation under grant #88-09299. We wish to thank Patricia Danzon, Beverly Dahlby, Claude Fluet, James Garven, Scott Harrington, Howard Kunreuther, Clifford Smith Jr., George Theall, Kip Viscusi, and a referee for helpful comments.