A lie is an assertion that is believed to be false, typically used with the purpose of deceiving someone. The practice of communicating lies is called lying. A person who communicates a lie may be termed a liar. Lies may serve a variety of instrumental, interpersonal, or psychological functions for the individuals who use them.
Generally, the term "lie" carries a negative connotation, and depending on the context a person who communicates a lie may be subject to social, legal, religious, or criminal sanctions.
The potential consequences of lying are manifold; some in particular are worth considering. Typically lies aim to deceive, when deception is successful, the hearer ends up acquiring a false belief (or at least something that the speaker believes to be false). When deception is unsuccessful, a lie may be discovered. The discovery of a lie may discredit other statements by the same speaker, staining his reputation. In some circumstances, it may also negatively affect the social or legal standing of the speaker. Lying in a court of law, for instance, is a criminal offense (perjury).
Hannah Arendt spoke about extraordinary cases in which an entire society is being lied to consistently. She said that the consequences of such lying are "not that you believe the lies, but rather that nobody believes anything any longer. This is because lies, by their very nature, have to be changed, and a lying government has constantly to rewrite its own history. On the receiving end you get not only one lie – a lie which you could go on for the rest of your days – but you get a great number of lies, depending on how the political wind blows."
Polygraph "lie detector" machines measure the physiological stress a subject endures in a number of measures while giving statements or answering questions. Spikes in stress indicators are purported to reveal lying. The accuracy of this method is widely disputed. In several well-known cases, application of the technique was proven to have been deceived. Nonetheless, it remains in use in many areas, primarily as a method for eliciting confessions or employment screening. The unreliability of polygraph results are the basis of such evaluations not being admissible as court evidence and, generally, the technique is perceived to be pseudoscience.
A recent study found that composing a lie takes longer than telling the truth and thus, the time taken to answer a question may be used as a method of lie detection, however, it also has been shown that instant answers with a lie may be proof of a prepared lie. A recommendation provided to resolve that contradiction is to try to surprise the subject and find a midway answer, not too quick, nor too long.
Utilitarian philosophers have supported lies that achieve good outcomes – white lies. In his 2008 book, , Iain King suggested a credible rule on lying was possible, and he defined it as: "Deceive only if you can change behaviour in a way worth more than the trust you would lose, were the deception discovered (whether the deception actually is exposed or not)."
Aristotle believed no general rule on lying was possible, because anyone who advocated lying could never be believed, he said. Although the philosophers, St. Augustine, St. Thomas Aquinas, and Immanuel Kant, condemned all lying, Thomas Aquinas did advance an argument for lying, however. According to all three, there are no circumstances in which, ethically, one may lie. Even if the only way to protect oneself is to lie, it is never ethically permissible to lie even in the face of murder, torture, or any other hardship. Each of these philosophers gave several arguments for the ethical basis against lying, all compatible with each other. Among the more important arguments are:
In Lying, neuroscientist Sam Harris argues that lying is negative for the liar and the person who's being lied to. To say lies is to deny others access to reality, and often we cannot anticipate how harmful lies can be. The ones we lie to may fail to solve problems they could have solved only on a basis of good information. To lie also harms oneself, makes the liar distrust the person who's being lied to. Liars generally feel badly about their lies and sense a loss of sincerity, authenticity, and integrity. Harris asserts that honesty allows one to have deeper relationships and to bring all dysfunction in one's life to the surface.
In Human, All Too Human, philosopher Friedrich Nietzsche suggested that those who refrain from lying may do so only because of the difficulty involved in maintaining lies. This is consistent with his general philosophy that divides (or ranks) people according to strength and ability; thus, some people tell the truth only out of weakness.
Possession of the capacity to lie among non-humans has been asserted during language studies with great apes. In one instance, the gorilla Koko, when asked who tore a sink from the wall, pointed to one of her handlers and then laughed.
Deceptive body language, such as feints that mislead as to the intended direction of attack or flight, is observed in many species. A mother bird deceives when she pretends to have a broken wing to divert the attention of a perceived predator – including unwitting humans – from the eggs in her nest, instead to her, as she draws the predator away from the location of the nest, most notably a trait of the killdeer.
It is asserted that the capacity to lie is a talent human beings possess universally.
The evolutionary theory proposed by Darwin states that only the fittest will survive and by lying, we aim to improve other's perception of our social image and status, capability, and desirability in general. Studies have shown that humans begin lying at a mere age of six months, through crying and laughing, to gain attention.
Scientific studies have shown differences in forms of lying across gender. Although men and women lie at equal frequencies, men are more likely to lie in order to please themselves while women are more likely to lie to please others. The presumption is that humans are individuals living in a world of competition and strict social norms, where they are able to use lies and deception to enhance chances of survival and reproduction.
Stereotypically speaking, Smith asserts that men like to exaggerate about their sexual expertise, but shy away from topics that degrade them while women understate their sexual expertise to make themselves more respectable and loyal in the eyes of men and avoid being labelled as a ‘scarlet woman’.
Those with Parkinson's disease show difficulties in deceiving others, difficulties that link to prefrontal hypometabolism. This suggests a link between the capacity for dishonesty and integrity of prefrontal functioning.
Pseudologia fantastica is a term applied by psychiatrists to the behavior of habitual or compulsive lying. Mythomania is the condition where there is an excessive or abnormal propensity for lying and exaggerating.
Some people believe that they are convincing liars, however in many cases, they are not.
The Old Testament and New Testament of the Bible both contain statements that God cannot lie and that lying is immoral (Num. 23:19, Hab. 2:3, Heb. 6:13–18). Nevertheless, there are examples of God deliberately causing enemies to become disorientated and confused, in order to provide victory (2 Thess. 2:11; 1 Kings 22:23; Ezek. 14:9).
Various passages of the Bible feature exchanges that assert lying is immoral and wrong (Prov. 6:16–19; Ps. 5:6), (Lev. 19:11; Prov. 14:5; Prov. 30:6; Zeph. 3:13), (Isa. 28:15; Dan. 11:27), most famously, in the Ten Commandments: "Thou shalt not bear false witness" (Ex. ; Deut. ); Ex. 23:1; Matt. 19:18; Mark 10:19; Luke 18:20 a specific reference to perjury.
Other passages feature descriptive (not prescriptive) exchanges where lying was committed in extreme circumstances involving life and death, however, most Christian philosophers would argue that lying is never acceptable, but that even those who are righteous in God's eyes sin sometimes. Old Testament accounts of lying include:
In the New Testament, Jesus refers to the Devil as the father of lies (John 8:44) and Paul commands Christians "Do not lie to one another" (Col. 3:9; cf. Lev. 19:11). In the Day of Judgement, unrepentant liars will be punished in the lake of fire. (Rev. 21:8; 21:27).
Augustine of Hippo wrote two books about lying: On Lying (De Mendacio) and Against Lying (Contra Mendacio). He describes each book in his later work, Retractions. Based on the location of De Mendacio in Retractions, it appears to have been written about AD 395. The first work, On Lying, begins: "Magna quæstio est de Mendacio" ("There is a great question about Lying"). From his text, it can be derived that St. Augustine divided lies into eight categories, listed in order of descending severity:
Despite distinguishing between lies according to their external severity, Augustine maintains in both treatises that all lies, defined precisely as the external communication of what one does not hold to be internally true, are categorically sinful and therefore, ethically impermissible.
Augustine wrote that lies told in jest, or by someone who believes or opines the lie to be true are not, in fact, lies.
The fourth of the five Buddhist precepts involves falsehood spoken or committed to by action. Avoiding other forms of wrong speech are also considered part of this precept, consisting of malicious speech, harsh speech, and gossip. A breach of the precept is considered more serious if the falsehood is motivated by an ulterior motive  (rather than, for example, "a small white lie"). The accompanying virtue is being honest and dependable, and involves honesty in work, truthfulness to others, loyalty to superiors, and gratitude to benefactors. In Buddhist texts, this precept is considered most important next to the first precept, because a lying person is regarded to have no shame, and therefore capable of many wrongs. Lying is not only to be avoided because it harms others, but also because it goes against the Buddhist ideal of finding the truth.
The fourth precept includes avoidance of lying and harmful speech. Some modern Buddhist teachers such as Thich Nhat Hanh interpret this to include avoiding spreading false news and uncertain information. Work that involves data manipulation, false advertising, or online scams can also be regarded as violations. Anthropologist Barend Terwiel reports that among Thai Buddhists, the fourth precept also is seen to be broken when people insinuate, exaggerate, or speak abusively or deceitfully.
Zoroaster teaches that there are two powers in the universe; Asha, which is truth, order, and that which is real, and Druj, which is "the Lie". Later on, the Lie became personified as Angra Mainyu, a figure similar to the Christian Devil, who was portrayed as the eternal opponent of Ahura Mazda (God).
Herodotus, in his mid-fifth-century BC account of Persian residents of the Pontus, reports that Persian youths, from their fifth year to their twentieth year, were instructed in three things – "to ride a horse, to draw a bow, and to speak the Truth". He further notes that: "The most disgraceful thing in the world [the Persians] think, is to tell a lie; the next worst, to owe a debt: because, among other reasons, the debtor is obliged to tell lies."
In Achaemenid Persia, the lie, drauga (in Avestan: druj), is considered to be a cardinal sin and it was punishable by death in some extreme cases. Tablets discovered by archaeologists in the 1930s  at the site of Persepolis give us adequate evidence about the love and veneration for the culture of truth during the Achaemenian period. These tablets contain the names of ordinary Persians, mainly traders and warehouse-keepers. According to Stanley Insler of Yale University, as many as 72 names of officials and petty clerks found on these tablets contain the word truth. Thus, says Insler, we have Artapana, protector of truth, Artakama, lover of truth, Artamanah, truth-minded, Artafarnah, possessing splendour of truth, Artazusta, delighting in truth, Artastuna, pillar of truth, Artafrida, prospering the truth, and Artahunara, having nobility of truth.
It was Darius the Great who laid down the "ordinance of good regulations" during his reign. Darius' testimony about his constant battle against the Lie is found in the Behistun Inscription. He testifies: "I was not a lie-follower, I was not a doer of wrong ... According to righteousness I conducted myself. Neither to the weak or to the powerful did I do wrong. The man who cooperated with my house, him I rewarded well; who so did injury, him I punished well."
Darius had his hands full dealing with large-scale rebellion which broke out throughout the empire. After fighting successfully with nine traitors in a year, Darius records his battles against them for posterity and tells us how it was the Lie that made them rebel against the empire. At the Behistun inscription, Darius says: "I smote them and took prisoner nine kings. One was Gaumata by name, a Magian; he lied; thus he said: I am Smerdis, the son of Cyrus ... One, Acina by name, an Elamite; he lied; thus he said: I am king in Elam ... One, Nidintu-Bel by name, a Babylonian; he lied; thus he said: I am Nebuchadnezzar, the son of Nabonidus. ... The Lie made them rebellious, so that these men deceived the people." Then advice to his son Xerxes, who is to succeed him as the great king: "Thou who shalt be king hereafter, protect yourself vigorously from the Lie; the man who shall be a lie-follower, him do thou punish well, if thus thou shall think. May my country be secure!"