• Highlighting the decisions of others you consider influential. Reading “Most other guests staying at this hotel reuse towels,” may make you feel compelled to align your behavior with the majority.
• “Injunctive norms” focus on how one should act in a particular situation. “Reusing towels meets a high standard for environmental responsibility,” highlights self-imposed standards. It involves a belief about right and wrong that consider abstract concepts.

A way to nudge people involves changing the decision environment. 

For instance, a grocery store that is trying to encourage consumers to purchase ecologically responsible products will display the product repeatedly throughout the shop. It will re-trigger the internalized norm and will increase environmentally responsible purchases.

Nudges enhance or suppress virtuous behaviors. They cannot make people do something they don't want to do. They only encourage them to make a decision that may be hidden by other factors.