The eighth Commandment forbids lying (false witness). People use the term “white lie,” but there is no such thing in moral theology or ethics.
There are major lies and minor lies, but all lies are sinful—some venial, others mortal. Lying under oath in court (perjury) is a mortal sin. Lying about someone in print (for instance, in the newspaper) is called libel in civil law, and if only verbal, then it is called slander or calumny. Both are sinful and immoral, since the person telling the lie knows that what he or she is saying is not true and that these lies can ruin someone’s reputation.
Lying is the telling of a falsehood with the direct intention of deceiving another (Catechism #2482). Subjective lies are statements which contradict what is known or believed in the mind (intellect). It is not speaking what you know to be the truth. Objective lies are statements which are not true in fact (reality). In both cases, however, there must be an intent to deceive. Error of fact or inaccurate information is not lying if one innocently has the wrong information.
Can you lie about your age? No. Lying would be the intentional deception of your age, as in the case of filling out an official financial, government, or business form, or when asked in court. When someone at a social gathering asks how old you are, and you reply “Thirty-nine” when you are actually forty-seven, there is no presumption of accuracy and no intention of deception, since social convention is that anyone who says they’re thirty-nine is merely replying that they do not wish to answer your question.
There is no expectation, either, that the answer is truthful when the reply is “Thirty-nine” and the person obviously looks much older. While some might call this a white lie, it is not lying since there is no intention of deception, rather, someone is merely withholding information that someone else is not entitled to know. Since the questioner, upon hearing the answer, realizes that the person answering is being evasive, there is no deception and therefore no lie. The phrase white lie is a misnomer since it implies that some lies, albeit minor ones, are not immoral. All true lies are immoral; some are more grave than others. Withholding the truth is not lying when there is no moral obligation to divulge it.
Likewise, telling bedtime stories and fairy tales to children is not lying since there is no intent to deceive, merely to amuse, as in the case of telling a joke or when actors perform on stage or on film. The intent is not to deceive someone who is expecting the truth or who would mistake the deception for the truth. Magicians and illusionists and special effects artists deceive, but there is no expectation that they are showing reality when the purpose is to amuse or entertain.
Lying is withholding the truth when one is morally obligated to tell it. When someone refuses to violate physician-patient confidentiality or attorney-client privilege, that is not lying—it is keeping a secret that they are legally obligated to keep.
Priests are forbidden to reveal anything they hear in confession; this is called the seal of confession.
Giving false or inaccurate information is lying when the intent is to deceive and the recipient is morally entitled to the information. Government spies, secret agents, undercover cops, and the like, are not lying but using subterfuge and mental reservation to protect the safety and welfare of innocent persons who would be at risk from an enemy knowing certain facts and information. Officials in government, business, or even individuals in church authority who deceive the public to cover up crimes, avoid embarrassment, or escape punishment are lying. Maintaining serious confidentiality, avoiding unnecessary scandal, or protecting innocent victims from possible exploitation would not be lying if there is no culpability involved, that is, the person making the statement is not at fault.
Gossip is the sin of telling statements which may or may not be true about someone with the intention of tarnishing that person’s reputation or of making the person look inferior or foolish.
Detraction is the sin of telling the truth with the intention not of preventing imminent harm to another, but instead with the intention of ruining his reputation. You see Fred come home drunk. You tell your neighbor so he thinks less of Fred; that is detraction. If you tell Fred’s wife so she can get him help, that is not detraction.
Withholding the truth is sinful when it is morally obligatory to tell it, for example, when parents do not tell their children it is sinful for a boyfriend and girlfriend to live together outside of or before marriage, or when loved ones do not tell an immediate family member that he or she has a serious drinking, drug, or gambling problem.