Results when Googling "behavioral activation meta-analysis" (as of Nov 2018):

David Ekers (Jun 2014) Behavioural Activation for Depression; An Update of Meta-Analysis of Effectiveness and Sub Group Analysis

• 26 randomized trials, with trials having poor quality, such as short follow-up time periods.
• Effect size compared to placebo was 0.74 (large), and 0.42 (medium) compared to medication
• Pim Cuijpers was co-author

Anna Huguet (Aug 2018) A systematic review and meta-analysis on the efficacy of Internet-delivered behavioral activation.

• 9 RCTs, but low underlying quality of evidence
• Anna is a research scientist at IWK Health Centre, a hospital in Halifax, Nova Scotia

Pim Cuijpers (Apr 2007) Behavioral activation treatments of depression: A meta-analysis

• 16 studies included
• Large effect size (0.87) compared to controls