Tackling policy challenges often requires government employees to steer behaviour. In making sense of behaviour and how to influence it, employees are likely to rely on implicit theories regarding behaviour, that is their personal mental models about how behaviour works. This preregistered study investigates how such implicit theories, specifically concerning cooperation, behavioural malleability, and self-control, shape policy preferences among Dutch government employees (N = 1,015). Combining both a correlational survey design and a quasi-experimental approach, we find that employees assuming high levels of cooperative behaviour and behavioural malleability favour more government intervention. In addition, informing participants about real-world behavioural data prompted employees to update their implicit theories towards greater accuracy. However, implicit theories regarding self-control showed no significant effect on policy preferences. Our findings suggest that implicit theories are embedded within broader sociopolitical worldviews. We discuss the role of implicit theories in public administration, and benefits of attending to behavioural assumptions in policy.