Your own group feels diverse. Every other group looks the same

People readily see the range of personalities within their own circle, but perceive outsiders as a uniform block — even when the outsiders were only just randomly assigned to a group. The effect appears within minutes of a group forming and helps explain why stereotypes about 'them' feel so obviously true from inside 'us'.

— George Quattrone and Edward Jones, The Perception of Variability Within In-Groups and Out-Groups — Journal of Personality and Social Psychology, 1980