Working memory capacity, our ability to manage incoming information for processing purposes, predicts achievement on a wide range of intellectual abilities. Three randomized experiments (N = 310) tested the effectiveness of a brief psychological intervention designed to boost working memory efficiency (i.e., state working memory capacity) by alleviating concerns about incompetence subtly generated by demanding tasks. Sixth graders either received or did not receive a prior 10-min intervention designed to reframe metacognitive interpretation of difficulty as indicative of learning rather than of self-limitation. The intervention improved children's working memory span and reading comprehension and also reduced the accessibility of self-related thoughts of incompetence. These findings demonstrate that constructing a psychologically safe environment through reframing metacognitive interpretation of subjective difficulty can allow children to express their full cognitive potential. (Contains 3 figures and 5 footnotes.)