In the past, remarks categorized as sarcasm included any bitter or biting commentary designed to cut or insult someone. More recently, sarcastic language has been more narrowly defined to include only those statements that rely on understatement or irony for their power. This use of language is sometimes identified as unplain speaking, in which what is said is different from what is meant. Learning to identify sarcasm can be difficult for some people, but it is important to comprehend this type of speech in order to be considered a fully functional speaker of a language.
Most fluent speakers of a language are able to use sarcasm, but not all may be able to identify the mechanism by which this type of speech functions. Generally, sarcasm works by stating a thing that is untrue in a specific tone of voice associated with this device in a given language. Simply making an untrue statement is usually not enough to identify the remark as sarcastic, and the tone of voice in which the remark is made helps others understand that the statement is not to be taken seriously.
Sarcastic expressions are typically said to work through irony, but it is important to differentiate between an ironic situation and a sarcastic remark. Speech that is sarcastic is dependent on the speaker, but ironic speech is usually unintentional on the part of the speaker, thereby creating an ironic situation. It can therefore be said that sarcastic language relies on irony for its humorous value, but that it does not create an ironic situation.
Some languages have special ways of identifying sarcastic remarks and other unreal phrases. Special punctuation for sarcastic text has also been proposed. There are many informal ways of indicating that text is sarcastic, which can be useful because written text cannot have any of the intonation that helps people identify sarcasm.
Children often learn sarcasm naturally and do not need to be taught to use this device, although in certain contexts they may misunderstand how the meaning is to be taken. In many studies, children as young as five years old are shown to be capable of perceiving sarcasm. Some people, however, never learn to identify sarcastic usages of language due to various problems interpreting social situations or language. Autism, for example, can make it very difficult for a person to understand when a person is not using language literally. An inability to identify sarcasm can also, in certain cases, point to brain lesions or brain damage.