How many people does it take to start a revolution?

Amber Minogue (0:02)

Bababam How many people does it take to start a revolution?

Thanks for asking.

Picture a packed football stadium. One fan stands up hoping to start a Mexican wave. Nothing happens. Ten people join in. Still nothing. Twenty? No.

But once around thirty people rise together, the wave spreads through the crowd. Physicists have studied this phenomenon in stadiums around the world and found a surprisingly consistent pattern. It takes about thirty people to get a wave going. That's known as a critical mass, the minimum number of people needed for collective behaviour to take hold. Could the same idea apply to something far more consequential, like a protest movement or even a revolution? Sociologists think so. They call it complex contagion. In a landmark 1978 paper, sociologist Mark Gravenutter argued that people don't adopt risky behaviours because of a single influence. They need reinforcement from several people around them. Joining a protest, defying a social norm or making a major lifestyle change can all carry a cost. Most of us won't take that step because of one viral post from a stranger. So where's the tipping point?

That's what University of Pennsylvania sociologist Damon Centola set out to measure. In a 2018 study published in Science, volunteers first agreed on a name for a face. Once that shared norm was established, Centola introduced a group of participants who consistently insisted on a different name. He gradually increased their numbers. When the dissenters made up less than 25% of the group, nothing changed. But as soon as they reached roughly a quarter, the group rapidly abandoned the old name and adopted the new one. In some experiments, adding just one extra person was enough to trigger the switch. That suggests a committed minority, around 25%, can be enough to overturn an established consensus.

Is that figure fixed?

No, Centola stresses that the threshold depends on the context. During periods of economic hardship, political instability, or widespread injustice, people are often more receptive to change. The critical mass needed to mobilize can shrink, making large-scale movements easier to ignite. That may help explain why movements like Hashtag Me Too or the Arab Spring seem to erupt overnight. In reality, the conditions had been building for years. The threshold for collective action had been falling until one event finally pushed enough people to act. Which is why there's no universal answer to the question. There you have it.

Now you know how many people it takes to start a revolution. In under three minutes, we answer your questions and help you understand the true meaning behind the trends, concepts and acronyms that are making headlines. Listen along and you really will know for sure.