Abstract: A disjunction effect occurs when one will do A given event E occurs and will do A given event E does not occur, yet will not do A when the outcome of event E is unknown, which violates Savage’s “Sure-Thing Principle”. The present article reviewed the three assumptions about why the disjunction effect occurs, which are the reason-based account, the reluctance-to-think account and the equate-to-differentiate model. Whether the effect really exists or not, and what kind of experimental design would be fitting for investigating the effects should be further tested in the future research. Future research might also benefit from the insights of the work of Nobel Prize Laureate Aumann. Understanding the mechanism of the disjunction effect is conducive to making a “rational” decision

Key words: disjunction effect, Sure-thing principle, reasons-based account, reluctance-to-think account, equate-to -differentiate model