Language change is variation over time in a language's features. It is studied in several subfields of linguistics: historical linguistics, sociolinguistics, and evolutionary linguistics. Traditional theories of historical linguistics identify three main types of change: systematic change in the pronunciation of phonemes, or sound change; borrowing, in which features of a language or dialect are altered as a result of influence from another language or dialect; and analogical change, in which the shape or grammatical behavior of a word is altered to more closely resemble that of another word. All living languages are continually undergoing change. Some commentators use derogatory labels such as "corruption" to suggest that language change constitutes a degradation in the quality of a language, especially when the change originates from human error or is a prescriptively discouraged usage. Modern linguistics rejects this concept, since from a scientific point of view such innovations cannot be judged in terms of good or bad. John Lyons notes that "any standard of evaluation applied to language-change must be based upon a recognition of the various functions a language 'is called upon' to fulfil in the society which uses it". Causes Economy: Speech communities tend to change their utterances to be as efficient and effective (with as little effort) as possible, while still reaching communicative goals. Purposeful speaking therefore involves a trade-off of costs and benefits. The principle of least effort tends to result in phonetic reduction of speech forms. See vowel reduction, cluster reduction, lenition, and elision. After some time a change may become widely accepted (it becomes a regular sound change) and may end up treated as standard. For instance: going to → gonna or , with examples of both vowel reduction and elision , . Expressiveness: Common or overused language tends to lose its emotional or rhetorical intensity over time; therefore, new words and constructions are continuously employed to revive that intensity Analogy: Over time, speech communities unconsciously apply patterns of rules in certain words, sounds, etc. to unrelated other words, sounds, etc. Language contact: Words and constructions are borrowed from one language into another. Cultural environment: As a culture evolves, new places, situations, and objects inevitably enter its language, whether or not the culture encounters different people. Migration/Movement: Speech communities, moving into a region with a new or more complex linguistic situation, will influence, and be influenced by, language change; they sometimes even end up with entirely