Closed-form solutions for options in incomplete markets
This paper reconsiders the predictions of the standard option pricing models in the context of incomplete markets. We relax the completeness assumption of the Black-Scholes (1973) model and as an immediate consequence we can no longer construct a replicating portfolio to price the option. Instead, we use the good-deal bounds technique to arrive at closed-form solutions for the option price. We determine an upper and a lower bound for this price and find that, contrary to Black-Scholes (1973) options theory, increasing the volatility of the underlying asset does not necessarily increase the option value. In fact, the lower bound prices are always a decreasing function of the volatility of the underlying asset, which cannot be explained by a Black-Scholes (1973) type of argument. In contrast, this is consistent with the presence of unhedgeable risk in the incomplete market. Furthermore, in an incomplete market where the underlying asset of an option is either infrequently traded or non-traded, early exercise of an American call option becomes possible at the lower bound, because the economic agent wants to lock in value before it disappears as a result of increased unhedgeable risk.