Current research funding models for health psychologists tend to be biased toward support for large-scale ‘definitive’ behavioural trials. This approach emphasizes rigorous tests of one or more key questions, but, unintentionally, may lead to the funding of interventions that are based on myriad untested assumptions. We propose that future funding models should provide support for ‘iterative’ research that tests assumptions at each stage of the intervention development process, including design, deployment, efficacy, implementation, and sustainability. More funding should be allocated to these developmental stages with funding allocated to testing the efficacy of definitive trials only when it is appropriately supported by research that indicates that key assumptions have been met. This shift should foster more robust behavioural interventions that have appropriate efficacy and effectiveness, and ‘work’ in the ‘real world’ contexts. Funders should support assumption testing using a diversity of methods (e.g., qualitative, quantitative, expert consensus), and encourage behavioural researchers to adjust their assumptions according the data produced. We contend that time is now to shift funding models to support assumption-testing research and ensure that funding applications for research testing ‘definitive’ behavioural trials has clear evidence supporting underlying assumptions.