More than 26 million people have changed their Facebook picture to a rainbow flag. Here’s why that matters.

In the wake of a landmark Supreme Court decision that made same-sex marriage a right nationwide, a whopping 26 million people slapped a rainbow flag over their Facebook photos to “celebrate pride.” The movement, fomented by a photo-editing tool that Facebook launched June 26, is a response to major news events that we’ve seen before: Profile picture change campaigns have become as common as cat videos on certain social networks. There were green filters for Iranian protesters in 2009, yellow ribbons for Hong Kong in 2014, black dots to oppose sexual violence in India, Arabic “Ns” to support Iraqi Christians. Profile pictures, arguably, are a very particular and effective type of message. They don’t dictate how you should or must behave, as laws and PSAs typically do; instead, they simply tell you how your peers are behaving. In other words, they support marriage equality; why don’t you?

“When people try to change behavior, they often focus on … telling people what they should do,” the social psychologist Melanie Tannenbaum explained in 2013. “We often underestimate just how strongly we respond to what other people actually do.” Case in point? Facebook itself has found that people tend to change their profile pictures in response to their friends’ picture changes: In a paper on the HRC’s 2013 campaign, published in February, Facebook data scientists found that users were more likely to adopt the equal-sign icon if they saw multiple friends doing so. The more friends they saw, up to a point, the more likely they were to change. That social influence was more of a factor, in fact, than even religion, politics or age.