How to protect eyewitness memory against the misinformation effect: A meta-analysis of post-warning studies

Four decades of research and hundreds of studies speak to the power of post-event misinformation to bias eyewitness accounts of events (see e.g., Loftus’ summary, 2005). A subset of this research has explored if the adverse influence of misinformation on remembering can be undone or at least reduced through a later warning about its presence. We meta-analyzed 25 such post-warning studies (including 155 effect sizes) to determine the effectiveness of different types of warnings and to explore moderator effects. Key findings were that (1) post-warnings are surprisingly effective, reducing the misinformation effect to less than half of its size on average. (2) Some types of post-warning (following a theoretical classification) seem to be more effective than others, particularly studies using an enlightenment procedure (Blank, 1998). (3) The post-warning reduction in the misinformation effect reflects a specific increase in misled performance (relative to no warning), at negligible cost for control performance. We conclude with a discussion of theoretical and practical implications.