The disjunction effect violates Savage’s sure-thing principle: that is, if a is preferred over b regardless of whether relevant outcome x occurs, then a should always be preferred over b [L.J. Savage, The Foundations of Statistics, New York, Wiley, 1954]. We tested “reason-based” and “reluctance-to-think” accounts of the disjunction effect. According to the former account, the disjunction effect occurs when different reasons underlie the preference for a under x versus the preference for a under not x. According to the latter account, the disjunction effect is due to the failure to consider preferences when x is unknown. We tested these accounts by varying the number of reasons underlying choices in the x and not x conditions. Consistent with the reason-based account, when only one reason was available, the disjunction effect was reduced. In addition, we propose a new method of measuring the disjunction effect under different conditions based on the logic proposed by Lambdin and Burdsal (2007) [C. Lambdin, C. Burdsal, The disjunction effect reexamined: relevant methodological issues and the fallacy of unspecified percentage comparisons, Organizational Behavior and Human Decision Processes 103 (2007) 268–276].