Forging convictions: The effects of active participation in a clinical trial

► Contextual effects influence recruitment, retention and outcome in randomised clinical trials. ► A new theory of active research participation explains these processes. ► Trial participants actively seek information and cues to support participation and form judgements about treatment outcome. ► Methods used to increase trial recruitment and retention enhance context effects and influence outcomes. ► All aspects of participant information and contact should be regarded as components of trial intervention.