Over a century ago, the French polymath Gustave Le Bon explored the hidden forces at work when people gather in crowds. He argued that being part of a crowd leads to an “enlargement of the ego” (a sense of power), the release of impulses, a contagious group dynamic, and increased suggestibility. According to Le Bon, this happens because individuals lose their personal identity and adopt that of the crowd. Crowd leaders, he believed, use simple ideas to inspire action. These ideas are repeated, creating a cycle: the idea sparks an emotion, which then leads to action. People in the crowd mimic the leader, and this behavior quickly spreads to everyone present. Le Bon’s insights on crowd behavior offer lessons for change practitioners, especially in building commitment and momentum for change.

Mark Granovetter’s early research at Harvard, summarized in “The Strength of Weak Ties” (1973), showed that people with weak connections (those you don’t know well) are better at spreading new ideas than those with strong connections (close relationships). He theorized that weak ties help ideas spread faster because they reach new networks, while strong ties form an “echo chamber” where ideas bounce around but don’t spread widely. Granovetter later introduced the “Threshold Model” (1983), which suggested that people decide to adopt a new behavior based on how many others are already doing it.

Damon Centola extended Granovetter’s work in his book “How Behavior Spreads: The Science of Complex Contagions” (2018). Centola argued that strong ties can sometimes be more effective than weak ties in spreading ideas, depending on the situation. He distinguished between simple contagions (like catching a disease) and complex contagions (which involve higher risks, such as financial, psychological, or reputational risks). Simple contagions need only exposure and are best spread through weak ties, as they can reach more people quickly. However, complex contagions require stronger ties because people need repeated contact and trust before taking on these higher-risk behaviors.

To make complex change contagious, Centola suggests two strategies. First, identify “key brokers” (trusted individuals who connect different groups) and “wide bridges” (multiple connection points between a few people in different groups) to help spread innovation across teams. In organizations, balanced networks help test and filter new ideas. Once an idea gains traction in overlapping groups, strategies can be developed to spread it more broadly. Use simple network analysis to find key brokers and wide bridges, and include them in your communications.

Second, use redundancy effectively. This means having multiple sources reinforce the desired behaviors, which is crucial for both local adoption (within small groups) and widespread change. Local commitment also helps sustain challenging new behaviors over time.