The idea of the self-categorization theory is that people conform to the norms of certain social groups whenever they have a personal desire to feel like they belong.

It is irrelevant whether a norm reflects people's preference, as long as the behavior is associated with the group.

We follow arbitrary norms that offer no rational reason for us to conform to them. The norms can snowball when we are influenced by people's earlier decisions.

For instance, if we see a packed restaurant next to an empty one, we assume the packed restaurant must be better. It is possible that an initial arbitrary decision by some early restaurant-goers cascaded into one restaurant being popular and the other one not.

Increasingly, social norms are being used to encourage pro-social behavior. They have been successfully used to encourage:

• Healthy eating
• Increase attendance at doctor appointments
• To reduce tax evasion
• To increase towel reuse at hotels
• To decrease long-term energy use
• To increase organ donor registrations.