Objectives: In a randomised controlled trial, we found that informing women about overdetection changed their breast screening decisions. We now present a mediation analysis exploring the psychological pathways through which study participants who received the intervention processed information about overdetection and how this influenced their decision-making. We examined a series of potential mediators in the causal chain between exposure to overdetection information and women’s subsequently reported breast screening intentions. Design: Serial multiple mediation analysis within a randomised controlled trial. Setting: New South Wales, Australia. Participants: 811 women aged 48–50 years with no personal history of breast cancer. Interventions: Two versions of a decision aid giving women information about breast cancer deaths averted and false positives from mammography screening, either with (intervention) or without (control) information on overdetection. Main outcome: Intentions to undergo breast cancer screening in the next 2–3 years. Mediators: Knowledge about overdetection, worry about breast cancer, attitudes towards breast screening and anticipated regret. Results: The effect of information about overdetection on women’s breast screening intentions was mediated through multiple cognitive and affective processes. In particular, the information led to substantial improvements in women’s understanding of overdetection, and it influenced—both directly and indirectly via its effect on knowledge—their attitudes towards having screening. Mediation analysis showed that the mechanisms involving knowledge and attitudes were particularly important in determining women’s intentions about screening participation. Conclusions: Even in this emotive context, new information influenced women’s decision-making by changing their understanding of possible consequences of screening and their attitudes towards undergoing it. These findings emphasise the need to provide good-quality information on screening outcomes and to communicate this information effectively, so that women can make well-informed decisions. Trial registration number: This study was prospectively registered with the Australian New Zealand Clinical Trials Registry (ACTRN12613001035718) on 17 September 2013.This work was supported by the National Health and Medical Research Council of Australia through project grant number 1062389
