Groupthink happens when a group of people want to believe the same thing, and that stops them from making smart decisions on their own. For example, someone who is allergic to dairy may join their friends in eating ice cream all because they want to have the same opinion as everyone else, even when they know it is a bad thing.

Researcher Irving Janis proposed eight symptoms of groupthink:^[1]

Type I: Thinking the group is too strong or good

• Falsely believing you're that strong, causing you to take too many risks
• Ignoring the consequences of your actions

Type II: Closed-mindedness

• Trying to ignore warnings that you don't agree with
• Thinking people you don't agree with are bad

Type III: Pressures toward uniformity

• Self-censorship of ideas that deviate from the apparent group consensus.
• Illusions of unanimity among group members, silence is viewed as agreement.
• Direct pressure to conform placed on any member who questions the group, couched in terms of "disloyalty".
• Mindguards— self-appointed members who shield the group from dissenting information.