The wilful rejection of psychological and behavioural interventions

Why do people turn down interventions designed to help them? New research from Cambridge and LSE offers an answer.

Psychological and behavioural interventions, such as nudges, misinformation inoculation, and mental health programmes, are designed to help society tackle big challenges. While these approaches often show strong efficacy in controlled lab settings, their real-world impact is far less certain. A key reason? Many people consciously reject them, even when they understand their purpose and potential benefits. This deliberate decision is what researchers call wilful rejection.

So why do people ignore interventions that could objectively help them? That’s the question Jon Roozenbeek and David J. Young from the University of Cambridge, together with Jens Koed Madsen from LSE, set out to answer in a recent paper published in Current Opinion in Psychology.

Building on previous work from LSE by Alexandra Johnson and Jens Koed Madsen that shows that lack of trust in the developer of the inoculation strategies influences people’s willingness to engage with inoculation, the authors introduce a Bayesian model to explain this wilful rejection. At its core, the model treats decision-making as a process of belief updating: people start with prior beliefs about whether an intervention is relevant, effective, and if the source of the intervention is trustworthy. When they encounter a message encouraging participation, they weigh this new information against their existing beliefs. The model uses statistical tools to capture how strongly people hold their beliefs and how confident they are in them. For example, someone who already doubts the intervention and feels highly confident in that doubt is unlikely to change their mind and may even downgrade their trust in the source after exposure, creating a backfire effect. This is because, in their minds, if a person proposes an intervention they strongly believe is wrong, it is a plausible explanation that the source is unreliable. Conversely, individuals with moderate confidence and higher trust in the source are more likely to engage. This approach provides a structured way to predict when interventions succeed or fail, and why trust in the messenger matters as much as the message itself.

The researchers’ simulations show that uptake rises when people trust the source and believe the intervention matters. Conversely, strong prior doubts can trigger a backfire effect, reducing trust in the source.

So what does this mean for practitioners who want to design effective real-world interventions to address key challenges? Firstly, make sure that people genuinely believe that the problem you’re trying to address is important. Secondly, use sources that they really trust. Failure to consider these factors may actually lead to worse outcomes and reduced trust in the sources going forward.

Jens Koed Madsen said: "The relationship between interventions and perceived trust and efficacy is critical to designing and conveying interventions. We cannot assume that we are necessarily trusted or that our ideas are seen as relevant a priori – if we are not, this raises completely different communication challenges of community outreach and trust building instead of ‘just’ conveying more data or information."

This article provides a summary of research findings from the following papers:

Johnson, A., & Madsen, J. K. (2024). Inoculation hesitancy: an exploration of challenges in scaling inoculation theory. Royal Society Open Science, 11(6), 231711. https://doi.org/10.1098/rsos.231711

Roozenbeek, J., Young, D. J., & Madsen, J. K. (2025). The wilful rejection of psychological and behavioural interventions. Current Opinion in Psychology, 66, 102138. https://doi.org/10.1016/j.copsyc.2025.102138