Cass Sunstein

people are most likely to need nudges when decisions require scarce attention, when decisions are difficult, when people do not get prompt feedback, and when they have trouble translating aspects of the situation into terms that they can easily understand.

Design is not just what it looks like and feels like. Design is how it works.
—Steve Jobs

many people will take whatever option requires the least effort, or the path of least resistance

Choice architects (such as Carolyn the cafeteria director) have many opportunities to choose defaults