People often make simplifying assumptions in order to understand complex systems. This fact can explain the ubiquity of overprecision. (Overprecision is the tendency for people to be too certain they are right.) The reason is that using simplified models that ignore some variation-inducing complexity underestimates uncertainty and causes judgments to be overprecise. We introduce a simple model of overprecision and then provide empirical evidence, both from the experimental laboratory and from the field, that tests the predictions of the theory.