Success in school and the labour market relies on more than high intelligence. Associations between "non-cognitive" skills in childhood, such as attention, self-regulation, and perseverance, and later outcomes have been widely investigated. In a systematic review of this literature, we screened 9553 publications, reviewed 554 eligible publications, and interpreted results from 222 better quality publications. Better quality publications comprised randomised experimental and quasi-experimental studies (EQIs), and observational studies that made reasonable attempts to control confounding. For academic achievement outcomes there were 26 EQI publications but only 14 were available for meta-analysis with effects ranging from 0.16 to 0.37SD. However, within sub-domains effects were heterogeneous. The 95% prediction interval for literacy was consistent with negative, null and positive effects (-0.13 to 0.79). Similarly heterogeneous findings were observed for psychosocial, cognitive and language, and health outcomes. Funnel plots of EQIs and observational studies showed asymmetric distributions and potential for small study bias. There is some evidence that non-cognitive skills associate with improved outcomes. However, there is potential for small study and publication bias that may over-estimate true effects, and heterogeneity of effect estimates spanned negative, null and positive effects. The quality of evidence from EQIs under-pinning this field is lower than optimal and more than a third of observational studies made little or no attempt to control confounding. Interventions designed to develop children's non-cognitive skills could potentially improve outcomes. The inter-disciplinary researchers interested in these skills should take a more strategic and rigorous approach to determine which interventions are most effective.