This study employed synthetic and meta-analytic techniques to review the literature on the Cognition Hypothesis, which predicts that increasing task complexity influences the quality of second language production. Based on 8 inclusion criteria, 17 published studies were synthesized according to key features. A subset of these studies (k = 9) was also meta-analyzed to investigate the overall effects of raising resource-directing task demands on learner output during monologic tasks. The synthesis of 17 primary studies revealed an assortment of treatments and measures. Among the 9 comparable studies, the meta-analysis uncovered small positive effects for accuracy and small negative effects for fluency. This lends support to the Cognition Hypothesis; however, the present study also disconfirms predictions regarding syntactic complexity. Implications for research and pedagogy are discussed.