Default effects are pervasive, but the reason they arise is often unclear. We study optimal policy when it is ambiguous whether an observed default effect reflects a welfare-relevant preference or a mistake by decision-makers. Within a broad class of models, determining optimal policy
is impossible without resolving this normative ambiguity. Depending on the resolution, optimal policy tends in opposite directions: either minimizing the number of non-default choices or promoting active choices. We illustrate our results using data on pension contribution defaults.
When selecting a non-default option reduces employee welfare by less than $160, the optimal policy promotes active choices.