Health care consumers often make choices that are imperfectly informed and inconsistent with their expressed preferences. Past research suggests that these shortcomings become more pronounced as choices become more complex, through either additional options or more performance metrics. But it is unclear why this is true: Consumer choice remains a "black box" that research has scarcely illuminated. In this article, we identify four pathways through which complexity may impair consumer choice. We examine these pathways using data from an experiment in which consumers (hypothetically) selected a primary care physician. Some of the loss of decision quality accompanying more complex choice sets can be explained by consumers’ skills and decision-making style, but even after accounting for these factors, complexity undermines the quality of decision making in ways that cannot be fully explained. We conclude by discussing implications for report designers, sponsors, and policy makers aspiring to promote consumer empowerment and health care quality.