I introduce and test for preference for simplicity in choice under risk. I characterize the theory axiomatically, and derive its properties and unique predictions relative to canonical models. By designing and running theoretically motivated experiments, I document that people value simplicity in ways not fully captured by existing models that study risk premia in financial markets. Participants' risk premia increase as complexity increases, holding moments fixed; their dominance violations increase in complexity; their behavior is predicted by simplicity's characterizing axiom; and their complexity aversion is heterogeneous in cognitive ability. None of expected utility theory, cumulative prospect theory, prospect theory, rational inattention, sparsity, salience, or probability weighting that differs by number of outcomes fully capture the experimental findings. I generalize the underlying theory to additionally capture broader measures of complexity, including obfuscation, computation, and language effects.

We document a new anomaly that we prove standard preference models are unable to capture, regardless of functional form or parametric specification used. Analyzing trading behavior in the binary option market for retail investors, we find that market participants purchase binary options although strictly dominant bull spreads are available at lower prices: 15% of S&P index, 19% of gold, and 25% of silver trades violate no-dominance conditions consistently across three different asset classes. Buyers of dominated binaries lose on average 34% of the contract price by forgoing the dominating product. We prove that neither prospect theory nor ambiguity aversion nor other popular theoretical justifications for retail anomalies such as rational inattention and salience, can capture these results. We also test for, and reject, standard financial explanations including trading costs, liquidity, exchange fixed effects, and noise trading. We show that our results are consistent with retail investors valuing simple, easy-to-understand binary bets. Our work provides a theoretically-grounded empirical impetus for research in behavioral finance which goes beyond historically pervasive utility frameworks.