People often infer that those who actively switch away from a default option have stronger preferences than those who passively accept it (termed asymmetric preference inferences). We test whether this classic effect reflects rational causal inference about how defaults provide alternative explanations for others’ mental states. This account predicts that asymmetric inferences should occur only when accepting the default provides an alternative explanation for choice (e.g., following a recommendation), and that asymmetry should diminish or disappear when it does not (e.g., a default licensing indulgence in a preferable option). In a pre-registered study (N=120), participants showed this effect: They made asymmetric inferences only when the default provided an alternative explanation for preference, and made symmetrical inferences when it did not. These findings suggest this classic effect reflects rational causal inference, providing a framework for predicting when people make asymmetric preference inferences from defaults.