Nudge theory (or Nudge) is a concept in behavioural science, political theory and economics which argues that positive reinforcement and indirect suggestions to try to achieve non-forced compliance can influence the motives, incentives and decision making of groups and individuals, at least as effectively – if not more effectively – than direct instruction, legislation, or enforcement.
Nudge theory's most celebrated influences include the formation of a British Behavioural Insights Team, often called the ‘Nudge Unit’, at the British Cabinet Office, headed by Dr David Halpern and US President Barack Obama's appointment of Cass R. Sunstein as administrator of the Office of Information and Regulatory Affairs. In Australia, the government of New South Wales established a Behavioural Insights community of practice.
The "Nudge" idea has been criticised. Dr Tammy Boyce, from public health foundation The King's Fund, has said:
We need to move away from short-term, politically motivated initiatives such as the 'nudging people' idea, which are not based on any good evidence and don't help people make long-term behaviour changes.
Other scholars have echoed similar concerns, particularly with regard to the need to better understand the psychological factors that predict long-term behavioral changes. 
Definition of a nudge
At the heart of nudge theory is the concept of nudge. This was originally defined by Richard Thaler and Cass Sunstein as:
“A nudge, as we will use the term, is any aspect of the choice architecture that alters people’s behavior in a predictable way without forbidding any options or significantly changing their economic incentives. To count as a mere nudge, the intervention must be easy and cheap to avoid. Nudges are not mandates. Putting fruit at eye level counts as a nudge. Banning junk food does not.”
One of nudges’ most frequently cited examples is the etching of the image of a housefly into the men’s room urinals at Amsterdam’s Schiphol Airport, which is intended to ‘improve the aim’.
“Nudge,” as it is often referred, is usually credited to Richard Thaler a prominent professor of Behavioural Science and Economics at the University of Chicago Booth School of Business. Thaler's role in developing the “Nudge Theory” is usually discussed in parallel with Daniel Kahneman, an American psychologist.
Nudge theory rose to global prominence in 2008 with the release of the book Nudge: Improving Decisions About Health, Wealth, and Happiness, by Thaler and legal scholar Cass R. Sunstein. The volume not only brought the discourse on Nudge theory to the wider public, but secured a significant following among contemporary US and UK political personalities as well as the private sector involved with public health and related fields. Nudge theory and similar policy frameworks have been criticized by some psychologists for failing to take into account the psychological determinants of the behaviours that they are trying to change, despite the ethical implications.
Most recently, the political machinery of both President Barack Obama in the United States and Prime Minister David Cameron in the UK have sought to employ Nudge Theory to advance their respective domestic policy goals. In both the UK  and the Australian state of NSW  there is a Behavioural Insights Team in the government.
Nudge theory has also found its way into the business management and corporate culture. Health Safety and Environment (HSE) and Human Resources are two areas that have applied the theory to internal safety or management culture. Regarding its application to HSE, one of the primary goals of nudge is to achieve a "zero accident culture".
- Andrew Sparrow (2008-08-22). "Speak 'Nudge': The 10 key phrases from David Cameron's favourite book". London: The Guardian. Retrieved 2009-09-09.
- Carol Lewis (2009-07-22). "Why Barack Obama and David Cameron are keen to 'nudge' you". London: The Times. Retrieved 2009-09-09.
- James Forsyth (2009-07-16). "Nudge, nudge: meet the Cameroons’ new guru". The Spectator. Retrieved 2009-09-09.
- Lakhani, Nina (December 7, 2008). "Unhealthy lifestyles here to stay, in spite of costly campaigns". The Independent (London). Retrieved April 28, 2010.
- van der Linden, S. (2013). "A Response to Dolan. In A. Oliver (Ed.)" (PDF). pp. 209–2015.
- R. Thaler and C. Sunstein. (2008). Nudge. Penguin Books.
- See: Dr. Jennifer Lunt and Malcolm Staves
- van der Linden, Sander (2013). "A response to Dolan". In Oliver, Adam. Behavioural Public Policy. Cambridge University Press. pp. 209–215. ISBN 9781107617377.
- Fischer, Mira; Lotz, Sebastian (2014). "Is Soft Paternalism Ethically Legitimate? - The Relevance of Psychological Processes for the Assessment of Nudge-Based Policies". Cologne Graduate School Working Paper Series (05-02). Retrieved 2014-05-30.
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