They didn't, really. Emmet's "madness" is more like a collection of fortuitous tics. In primary school his teachers tried to correct his posture, but he just rubbed his paws together and cackled menacingly at them.

They sent him to behavioral therapy for that.

Well, now that he's all grown up he can cackle and leer all he wants. He can also eat exactly as many cookies as he wants, but for some reason that's the behavior he decides to conceal.

Icon by ArtDexo2000

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They might call him mad if he ever bothered to publish his findings, but he wouldn't do that. Mankind is simply not ready to peer behind the veil. (He said that once when he got a B on a History paper in secondary school.)