An important application of this result is in the theory of portfolio decisions. In that
case, the demand bundle is the consumer’s contingent consumption over L states of the
world; it is standard to assume that the consumer has a von Neumann-Morgenstern utility
function u
α(x) = Pl
i=1 πiv
α(xi), where πi
is the subjective probability of state i and
v
α : R++ → R is the Bernoulli utility function. Suppose the coefficient of relative risk aversion,
−yvα 00(y)/vα 0
(y), does not vary by more than four on the domain of v
α. Then the
consumer’s demand for contingent consumption at different state prices will obey the law
of demand; this in turn implies that the law of demand holds for the consumer’s demand
for securities, whether or not the market is complete