Proof of principle randomised controlled trial of theory-based messages to increase colorectal cancer screening in never-testers, non-responders, and prior-testers

Background: Early detection of colorectal cancer can save lives, yet screening uptake is suboptimal. Several determinants increase uptake, including modifiable theoretical determinants (i.e., self-efficacy), and non-modifiable determinants (i.e., past behaviour). Purpose: To develop theory-based public health messages and test their effects on target mechanisms of action (MoAs, self-efficacy and intentions) in an experimental Proof-of-Principle Randomised Controlled Trial (RCT).Methods: Messages were developed systematically linking determinants, MoAs, and behaviour change techniques and four were selected for testing. Participants were randomly allocated to receive an online message (targeting self-efficacy, emotion, automatic influences, and self-image) or no-message. Participants (N=404) were over 50 years old (M=59.85, SD=7.31), female (N=248, 61.7%), white (N=384, 95.3%). Participants reported past behaviour, negative beliefs, and the outcomes self-efficacy and intentions. Results: There was a 3-way interaction between past behaviour, negative beliefs, and messages on self-efficacy F(14, 370)=2.94, p