This study examined two widely available light-touch, writing-based mindset interventions: one that targeted students' purpose for learning and one that aimed to increase students' growth mindset. In order to examine the potential mechanisms underlying previously reported effects of mindset interventions, we analyzed these interventions' effects on low-income, ethnic minority adolescents' academic outcomes, task persistence, task-relevant anxiety, critical motivation, and sense of belonging. Results indicated that the purpose for learning intervention had a small negative impact on students' self-reported grades the following year, and null results for the other outcomes. The growth mindset intervention was administered one year following the purpose for learning intervention and we found no evidence of treatment impacts on any outcomes. Analyses of treatment impact moderation suggested that certain student characteristics, such as student gender and race could play a role, but most of these tests also presented null results. The primarily null results of both interventions suggest that further study is needed to determine the effectiveness of one-time, self-administered mindset interventions across a variety of contexts and student populations. [Abstract provided by submitter. This paper was published in "Journal of Research on Educational Effectiveness."]