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.