What you learned in school
Criminal justice and psychology classes taught that polygraph machines were scientific instruments that could accurately detect lies by measuring physiological responses. Students learned that lie detectors were nearly foolproof tools used by law enforcement and that passing or failing a polygraph test was strong evidence of truthfulness or deception.
What we know now
Criminal justice and psychology classes taught that polygraph machines could accurately detect lies by measuring physiological responses. Students learned that lie detectors were scientific instruments that could determine truth from deception. Polygraphs measure stress responses, not lies specifically. They can be fooled by people who remain calm while lying or triggered by people who are nervous but truthful. They have high rates of false positives and false negatives. Most courts don't accept polygraph evidence because of reliability issues. The machines detect arousal and stress, which can have many causes besides deception.