Deception refers to the act—big or small, cruel or kind—of causing someone to believe something that is untrue. Even the most honest people practice deception, with various studies showing that the average person lies several times a day. Some of those lies are big (“I’ve never cheated on you!”) but more often, they are little white lies (“That dress looks fine,”) that are deployed to avoid uncomfortable situations or spare someone's feelings.
Deception isn’t always an outward-facing act. There are also the lies people tell themselves, for reasons ranging from healthy maintenance of self-esteem to serious delusions beyond their control. While lying to oneself is generally perceived as harmful, some experts argue that there are certain kinds of self-deception—like believing one can accomplish a difficult goal even if evidence exists to the contrary—that can have a positive effect on overall wellbeing.
Researchers have long searched for ways to definitively detect when someone is lying. One of the most well-known, the polygraph test, has long been controversial, and evidence suggests that those with certain psychiatric disorders like Antisocial Personality Disorder cannot be accurately measured by polygraphs or other commonly-used lie detection methods.