Question: Why would Jack instantly believe the weed wasn't Denny's? Isn't he supposed to be a human lie detector?
MikeH
10th Apr 2020
Meet the Parents (2000)
Answer: There are certain physical tells for when someone may be lying as well. This is what Jack is doing at the end of the movie when he's interrogating Greg at the airport: checking his pulse, monitoring his breathing, and looking at his eyes.
Answer: Confirmation bias - he believes what he wants to believe is true, since he wouldn't want to believe his own son is a drug user, and wants to believe Greg is.