The expected value of utility from entering a risky prospect. Assume the risky prospect has n potential outcomes. Let the probability of outcome i occurring be pi and the pay-off if i occurs be Xi. If the utility derived from pay-off X is U(X), then expected utility is
The expected utility theorem states that if a consumer satisfies a set of axioms describing rationality they should act in risky situations as if they maximize expected utility. If utility is a linear function of X, then E[U] = U[E(Xi)], that is, expected utility is the utility of the expected value of the pay-offs. If utility is a concave function of pay-off, that is, d2U/dX2 < 0, then the expected utility of a risky prospect is less than the utility of the expected pay-off, E[U] < U[E(Xi)], so the consumer prefers a sure pay-off to a risky prospect of the same expected value. See also Allais paradox.