Hatzigeorgiadis et al. 2011, Self-talk and sports performance, a meta-analysis

Meta-analysis of 32 studies (62 effect sizes) of self-talk interventions in sport. Self-talk produced a positive moderate overall effect on performance (ES about 0.48), larger for fine and novel tasks and when instructional rather than purely motivational. The effect is real but heterogeneous and moderated by task type. Moderate.