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The idea that someone is guilty unless they can come up with another person who is either guilty or a strong suspect. The idea is usually based on an imprecise process of elimination.
Parent to child: Did you do it?
Child: No.
Parent: Well if you didn't do it, then who did?
Nearby observer: Ah ha, she's using the guilty until alternate suspect principle.
by BillyBuggy March 02, 2011