Abstract: In important contexts, people prefer option A to option B when they evaluate the two separately, but prefer option B to option A when they evaluate the two jointly. In consumer behavior, politics, and law, such preference reversals are often a product of the pervasive problem of "evaluability." Some important characteristics of options are difficult or impossible to assess in separate evaluation, and hence choosers disregard or downplay them; those characteristics are much easier to assess in joint evaluation, where they might be decisive. But the empirical findings do not resolve central questions: Is either mode of evaluation reliable? Which mode of evaluation is better? Some people insist that joint evaluation is more reliable than separate evaluation, because it offers more information. But that conclusion is far too simple. In joint evaluation, certain characteristics of options may receive excessive weight, because they do not much affect people's actual experience or because the particular contrast between joint options distorts people’s judgments. In joint as well as separate evaluation, people are subject to manipulation, though for different reasons. It follows that neither mode of evaluation is reliable. The appropriate approach will vary depending on the goal of the task – increasing consumer welfare, preventing discrimination, achieving optimal deterrence, or something else. Under appropriate circumstances, global evaluation would be much better, but it is often not feasible. These conclusions bear on preference reversals in law and policy, where joint evaluation is often better, but where separate evaluation might ensure that certain characteristics or features of situations do not receive excessive weight.