One provocative explanation for the continued persistence of minority achievement gaps involves the performance-dampening anxiety thought to be experienced by minority students in highly evaluative settings (i.e., "stereotype threat"). Recent field-experimental studies suggest that modest, low-cost "buffering" interventions informed by this phenomenon may be highly effective at reducing minority achievement gaps. This field-experimental study evaluates such an intervention in which students complete a self-directed "self affirmation" exercise that encourages them to identify and reflect upon their core personal values. This within-classroom randomized trial was conducted among 2,500 7th and 8th graders from six Philadelphia-area middle schools during the 2008-09 and 2009-10 academic years. Although this study failed to replicate the earlier findings indicating that the affirmation generated large increases in the academic performance of minority students, this treatment did lead to statistically significant improvements in the performance of the minority students in more supportive classroom environments. However, the treatment contrast also "reduced" the performance of female students in those settings.