herd instinct 
noun
An inclination in people or animals to behave or think like the majority: What makes them go? In my view, it’s the herd instinct...
- He gives solid reasons for denying that the sense of moral obligation could arise from a herd instinct, from social convention, or from a Freudian superego.
- The origin of these summer traditions is a primal herd instinct, the urge to join with others in a festive act.
- Unfortunately, thanks to the herd instinct in our current media culture, anyone who publicly raises this question is immediately labeled a conspiracy theorist.