Divergent effects of economic and behavioural policy coupling on electric vehicle adoption at individual and system levels

Policy mixes are key to accelerate low-carbon transitions, yet, how combined policies impact electric vehicle (EV) adoption decisions and scale to system-level change remains unclear. We bridge these perspectives by integrating choice experiments, attention process tracing, and agent-based modelling of technology diffusion to examine how coupling a carbon tax with an information intervention affects attention, EV adoption, their diffusion, and public support for EV policies. Across four countries (Mexico, South Africa, USA, UK; N=1,589), the policies competed for attention but had additive effects on stated adoption choices. When embedded in diffusion simulations, the behavioural responses translated into super-additive outcomes, increasing EV adoption by up to nine percentage points compared with single policies. Synergies peaked when technology was improving but not yet self-sustaining. Policy coupling also increased public support for policies alongside growing diffusion. Our findings show how integrating actual decision preferences into diffusion models can reveal nonlinear pathways through which policies shape low-carbon transitions.