The globe holds plenty of reasons to take to the streets, yet would-be protesters seldom act on their grievances. What stirs people out of their passivity and into action? After revisiting 182 previous studies of some 15,000 people, social psychologist Martijn van Zomeren and his colleagues find that people’s social identities—that is, their attachment and commitment to their groups—are what drive them to protest.

“If you can build the sense among people that they are part of a group, then they will be more likely to act on behalf of that group,” summarizes van Zomeren, an assistant professor of social psychology at VU University Amsterdam.

For their study, the authors use a statistical method called meta-analysis, which combines many previous findings into a single powerful study. The researchers find that the more people identify with a group, the more likely they are to perceive injustice against that group, to believe their group can remedy the injustice, and to support acts of protest such as signing petitions and attending demonstrations.

Van Zomeren admits that his team is making a “very basic point: psychology matters,” he says. “But other approaches don’t always assume this.” For example, economists, sociologists, and other social scientists argue that objective factors—a society’s level of income inequality or nonprofits’ access to resources, for instance—inspire social action. (For example, see "Brother Spared a Dime” in the fall 2005 issue of the Stanford Social Innovation Review.)

Yet these objective factors are not enough to explain when people actually rally. To predict protests, “you need to know how people perceive the situation, how they perceive their group’s efficacy, and, most important, how they perceive themselves” in relation to their group, van Zomeren says.

In social movements, nongovernmental organizations (NGOs) are the touchstone groups with which people identify. So to propel a movement forward, NGOs must strengthen their members’ emotional ties to themselves and to the cause. Some movements do this better than others, notes van Zomeren. In the environmental movement, for example, “people know the issues, but they don’t yet identify with any specific groups,” he says. “For the environmental movement to get off the ground, people need to identify with specific organizations.”

Van Zomeren suggests that organizations tell members how many people are ready to act. “You have to make movements about people,” he says.

Martijn van Zomeren, Tom Postmes, and Russell Spears, “Toward an Integrative Social Identity Model of Collective Action: A Quantitative Research Synthesis of Three Socio-Psychological Perspectives,” Psychological Bulletin, 134, 2008.

Read more stories by Alana Conner.