Governing by « Nudge » — Using Cognitive bias Instead of Legislation ? By focusing on the unconscious and emotional part of our minds, the affective and behavioural sciences have revealed a way of orienting our actions without resorting to coercion. It is therefore tempting for governments to exploit the results of such research to steer our conduct using psychology instead of legislation. By skilful “nudges”, the government becomes the “choice architect” in charge of creating an environment inciting obedience. In other words, nudging is the creation of a behavioural environment (choice architecture) that provides a context conducive to the adoption of a determined behaviour without apparent constraint. Such a mechanism assists in implementing laws and public policies in a more flexible way. In fact, persuasion must be preferred to coercion if the former proves to be sufficiently effective. Yet, the creation of conducive behavioural environments is potentially very intrusive and likely to bypass our free will. It is therefore up to the authorities to retain only the environments that are not manipulative in nature. Provided with a legal basis, targeting a public interest, proportionate, consonant with good faith, transparent and non-discriminatory, they have their full place in a democratic constitutional state.