“In logic, an argument can be invalid even if its conclusion is true, and an argument can be valid even if its conclusion is false. It’s a confusing concept, and people are easily fooled when an argument’s validity and believability don’t match up, especially in the case of invalid arguments with conclusions that are believable. Psychological scientists call this phenomenon belief bias.”

– Edward J. N. Stupple, Linden J. Ball, Jonathan St. B. T. Evans, & Emily Kamal-, & Smith (2011).  Journal of Cognitive Psychology, 23 

Person 1:  People often react nervously when they purposely lie, or are purposely deceptive, when answering relevant questions.
Person 2:  You are reacting nervously when answering relevant questions. Conclusion:  Therefore, you must be lying.

But this is not a valid conclusion: Some people can lie without getting the least bit nervous while many truthful people will react nervously due to the mere insinuation that they are lying. – T.M. Cullen