People need to receive reinforcement (or “social proof”) from multiple adopters to be convinced—and for the new behavior to propagate.

people are sensitive to issues of legitimacy, coordination, or social proof—are often the ones people are most committed to once they finally adopt them.

The hallmark of a well-designed team is that it preserves intellectual diversity while enabling coordination.

Simple contagions benefit from reach in the social network.

legitimacy barrier. • Excitement: Some innovations and behaviors are appealing only when people are emotionally energized by one another.

When people are scared, they hold on to what they know.

Rapid economic growth and increased social mobility expanded the audience for Broadway shows. At the same time, the loss of many of Broadway’s talented artists during the Second World War created a vacuum, bringing an influx of new blood into the New York City creative scene.

Each of these barriers to adoption can be overcome by social reinforcement.

Kitchener’s campaign promised that people who enlisted together would also fight together. This strategy targeted residential neighborhoods and professional communities that had previously enforced the strong social norms against professional men enlisting. Kitchener transformed these strong ties into the opposite: the sources of social proof that