We constantly compare our actions and beliefs to those of our peers, and then alter them to fit in. This means that if our social group believes something, we are more likely to follow the herd.
This effect of social influence on behaviour was nicely demonstrated back in 1961 by the street corner experiment. The experiment was simple (and fun) enough for you to replicate. Just pick a busy street corner and stare at the sky for 60 seconds.
Most likely very few folks will stop and check what you are looking at – in this situation Milgram found that about 4% of the passersby joined in. Now get some friends to join you with your lofty observations. As the group grows, more and more strangers will stop and stare aloft. By the time the group has grown to 15 sky gazers, about 40% of the by-passers will have stopped and craned their necks along with you. You have almost certainly seen the same effect in action at markets where you find yourself drawn to the stand with the crowd around it.
The principle applies just as powerfully to ideas. If more people believe a piece of information, then we are more likely to accept it as true. And so if, via our social group, we are overly exposed to a particular idea then it becomes embedded in our world view. In short social proof is a much more effective persuasion technique than purely evidence-based proof, which is of course why this sort of proof is so popular in advertising (“80% of mums agree”).
Mark Lorch writing in Business Insider