The relationship between life stress and working memory capacity (WMC) has been documented in college students and older adults. It has been proposed that intrusive thoughts about life stress are the mechanism responsible for the impairments seen in WMC. To examine the mechanism responsible for these impairments the current study attempted to induce intrusive thoughts about personal events. The current study allowed for a test of predictions made by two theories of mind wandering regarding the impact of these intrusive thoughts on WMC task performance. One hundred fifty undergraduates were assigned to a control group, positive event group, or negative event group. Participants in the positive and negative event groups completed a short emotional disclosure about an imagined future positive or negative event, respectively, to induce positive or negative intrusive thoughts. WMC measures were completed prior to and following the emotional writing. Results indicated a significant relationship between WMC and mind wandering, however the writing manipulation did not result in any consistent changes in intrusive thoughts or WMC. The results suggest a causal relationship between WMC and mind wandering. The emotional valence of the intrusive thought altered the impact on WMC. No relationship was seen between the measures of stress …