Behaviour change policies have been gaining momentum since the 2000s, especially in the energy conservation field. At the same time, studies about energy consumption behaviours and practices have been increasing. They are usually categorised into two approaches, namely behavioural and socio-anthropological, which have both gained legitimacy and become institutionalised. However, despite the availability of the two perspectives, energy conservation polices mobilise behavioural sciences much more than socio-anthropological approaches. While this issue has already been discussed by scholars, this paper sheds some light on it by drawing upon an empirical study on the rise of behaviour change in Japan’s energy conservation policy. Indeed, the challenges faced by Japan, that is drastically reducing its energy consumption while supporting economic growth, are no different to those of most OECD countries. Furthermore, Japan’s strategy has similarly long been dominated by techno-economic approaches. Thus, the Japanese case provides a valuable insight into how energy conservation policies change and utilise the behavioural sciences. Drawing upon literature in public policy analysis and policy transfer, I argue that the success of the behavioural sciences can be explained by the coherence of their discourse – and the “mismatch” of socio-anthropological ones – with the culture and expectations of energy conservation policymakers. Three interrelated factors explain this coherence. First, the translation of the behavioural sciences into economics increased their legitimacy and made them appear more scientific in the eyes of policymakers. Second, recent transformations in their methodology enhanced the accuracy and reliability of their results, thus providing practical tools to policymakers. Third, behavioural approaches deliver concrete levers of actions which are compatible with the pursuit of economic growth. For these reasons, and in contrast with socio-anthropological approaches, the behavioural sciences are considered as “practical knowledge” by policymakers.