This means social information can be a powerful force for social change, because people look to each other when deciding how to express themselves. For example, Chris Crandall and colleagues surveyed people to find out how socially acceptable they thought it was to be prejudiced against a variety of groups, ranging from child molesters to librarians. They also asked respondents to report their personal attitudes towards these groups. The two ratings were almost perfectly correlated: people reported as much prejudice as they believed was socially acceptable.

As social norms shift, individuals shift with them: adopting popular opinions and behaviors, and dropping ones that fall out of style. Norms are especially powerful as they gain steam. In one set of studies, Gregg Sparkman and Greg Walton presented diners in a cafeteria with evidence that 30% of the U.S. population was vegetarian, or that 30% of the population was newly vegetarian. People who learned about this second, “dynamic social norm” were twice as likely to order a meatless lunch themselves. They saw not just where the crowd was, but where it was going, and didn’t want to be left behind.

Conformity can seem spineless, but in fact it reflects an ancient yearning to be part of something greater than ourselves—a smart yearning, given the many social advantages of coordinating and cooperating with others. It goes deeper than words, sometimes changing what we see, what we value, and how we behave, even privately. And no matter what we think of this, we are and will always be a herd species, more prone to move together than alone. Social norms will continue to change, and we will change with them.