I just want to know what is a group think fallacy

Someone supports a mandate or policy position based solely on the fact that it is the group's position.

Quote:

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A reasoner commits the group think fallacy if he or she substitutes pride of membership in the group for reasons to support the group's policy. If that's what our group thinks, then that's good enough for me. It's what I think, too. “Blind” patriotism is a rather nasty version of the fallacy.

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Source: Internet Encyclopedia of Philosophy