Justifying Conditionalization: Conditionalization Maximizes Expected Epistemic Utility

According to Bayesian epistemology, the epistemically rational agent updates her beliefs by conditionalization: that is, her posterior subjective probability after taking account of evidence X, p new, is to be set equal to her prior conditional probability p old (|X). Bayesians can be challenged to provide a justification for their claim that conditionalization is recommended by rationality-whence the normative force of the injunction to conditionalize?