Lie detection

Lie detection is the ability to tell when a person's lying based on an idea of the principle of the lie. There are many different theories for how to detect lies that date back to at least ancient China and probably before. The most common way is the polygraph, which looks for changes in pulse and other bodily signals. Recently, involuntary facial expressions have become a favored method of lie detection for law enforcement agencies. Various attempts to create a "truth serum" have also been explored by governments over the years; historically, sodium amytal and pentothal were used. It is alleged these drugs merely increase the reporting of any information, whether true or false.

Other "ways" to detect liars

 * Their pupils dilate
 * They stop salivating
 * Their breathing becomes irregular
 * They look to the right
 * They look to the left
 * They become fidgety
 * They become unpleasant
 * They fail to make eye contact
 * Their voice rises in pitch
 * They pause before speaking
 * Intuition

The problem with most of these is that they are based on body reactions to fear or uncertainty, which is then inferred to mean that the person is uncomfortable with telling a lie. A liar who is confident in their lie, who has learned how to control their reactions, or just has absolutely no qualms with deceiving people would not demonstrate these signs; conversely, they may be seen in someone who is telling the truth (or who thinks they're telling the truth), but is not confident in their story or is just spooked by the interrogation process. For example, if you ask pressing questions and a person gets nervous because they don't like you getting in their face, it might look like they're lying when they just want to be left alone.

A few of these behaviors (like fidgeting and not making eye contact) are totally normal for people with disabilities like autism or ADHD.

Ways which might actually work

 * If they are inconsistent
 * If they lack detail, and object to being asked for more
 * If they have more detail than they should remember or recall given the circumstances (a person being able to remember exactly what they did during a crime, down to how many steps they took during their alibi)
 * If they say "I don't remember" or "I don't recall" about details they should remember (and they don't have a memory-related condition that explains it)

Microexpressions
Microexpressions are involuntary facial expressions that occur when someone is trying to hide how they are feeling, or is not aware of their own emotions. These expressions can last as little as 1/25th of a second. Charles Darwin wrote that facial expressions of emotion are universal, not learned differently in each culture; that they are biologically determined. The basic emotions appear to be amusement, contempt, embarrassment, anxiety, guilt, pride, relief, contentment, pleasure, and shame. While Paul Ekman's work on these basic emotions has delivered robust evidence for microexpressions, the practical application of this research has been as of yet unsuccessful. Even professionally trained subjects do not fare much better than chance in detecting liars through the use of microexpressions. Regardless of this, programs based on reading microexpressions have recently been implemented in airport security. Only 1% of referrals resulting from this program have resulted in actual arrests, and these arrests are usually connected to criminal activity unrelated to terrorism.

According to Paul Ekman, microexpression training can also be used medically to develop emotional facial detection for schizophrenic people.

Certain drugs can affect emotional facial perception; MDMA, for example, makes it more difficult to detect fear.

In science fiction
The use of microexpressions for law enforcement is explored in the novel 1984, where facial expressions are constantly monitored through video screens, to determine possible thought criminals through their face crimes.