Welfare economics says that public decisions should be based on people’s preferences. For that reason we put so much effort in measuring these preferences. They are assumed stable and independent of the context in which they are measured. Behavioural economics shows that these assumptions do not hold. People deviate from rational models and preferences are context-dependent. For example, we saw that their valuations of safety improvements depend on the starting value and whether we use willingness to pay or willingness to accept. Can valuations that are so unstable be used in public policy? Behavioural economists made several proposals for how to use their insights in policy. The most influential are nudges.