We assessed whether students’ reasoning about historical causality is biased by accessibility and previous knowledge. Undergraduates in Canada provided explanations for historical events: the attacks on Pearl Harbor and September 11, 2001. Participants were then given several explanations for these events, including conventional historical and alternative conspiracist explanations, and reported how satisfying they found them. Overall, historical explanations were more satisfying than alternative explanations. Historical explanations were more satisfying for Pearl Harbor than 9/11, and alternative explanations were more satisfying for 9/11 than Pearl Harbor. Similarly, ease of generating an explanation was associated with satisfaction with historical explanations for Pearl Harbor, and with satisfaction with alternative explanations for 9/11. Participants’ general conspiracist beliefs were also associated with acceptance of alternative and historical explanations. Students learn about Pearl Harbor in history class, whereas information about 9/11 comes primarily from the media, leaving the door open to a multiplicity of ahistorical explanations.