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Causes of language change 
- Economy: Speakers tend to make their utterances as efficient and effective as possible to reach communicative goals. Purposeful speaking therefore involves a trade-off of costs and benefits.
- the principle of least effort: Speakers especially use economy in their articulation, which 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 a standard. For instance: going to [ˈɡoʊ.ɪŋ.tʊ] → gonna [ˈɡɔnə] or [ˈɡʌnə], with examples of both vowel reduction [ʊ] → [ə] and elision [nt] → [n], [oʊ.ɪ] → [ʌ].
- Analogy: reducing word forms by likening different forms of the word to the root.
- Language contact: borrowing of words and constructions from foreign languages.
- The medium of communication.
- Cultural environment: Groups of speakers will reflect new places, situations, and objects in their language, whether they encounter different people there or not.
- Migration/Movement: Speakers will change and create languages, such as pidgins and creoles.
Types of language change 
All languages change constantly, and do so in many and varied ways.
Lexical changes 
The ongoing influx of new words in the English language (for example) helps make it a rich field for investigation into language change, despite the difficulty of defining precisely and accurately the vocabulary available to speakers of English. Throughout its history English has not only borrowed words from other languages but has re-combined and recycled them to create new meanings, whilst losing some old words.
Dictionary-writers try to keep track of the changes in languages by recording (and, ideally, dating) the appearance in a language of new words, or of new usages for existing words. By the same token, they may tag some words as "archaic" or "obsolete".
Phonetic and phonological changes 
The sociolinguist William Labov recorded the change in pronunciation in a relatively short period in the American resort of Martha's Vineyard and showed how this resulted from social tensions and processes. Even in the relatively short time that broadcast media have recorded their work, one can observe the difference between the pronunciation of the newsreaders of the 1940s and the 1950s and the pronunciation of today. The greater acceptance and fashionability of regional accents in media may[original research?] also reflect a more democratic, less formal society — compare the widespread adoption of language policies.
The mapping and recording of small-scale phonological changes poses difficulties, especially as the practical technology of sound recording dates only from the 19th century. Written texts provide the main (indirect) evidence of how language sounds have changed over the centuries . But note Ferdinand de Saussure's work on postulating the existence and disappearance of laryngeals in Proto-Indo-European as an example of other methods of detecting/reconstructing sound-changes within historical linguistics.
Spelling changes 
Standardisation of spelling originated relatively recently. Differences in spelling often catch the eye of a reader of a text from a previous century. The pre-print era had fewer literate people: languages lacked fixed systems of orthography, and the handwritten manuscripts that survive often show words spelled according to regional pronunciation and to personal preference.
Semantic changes 
Semantic changes are shifts in meaning of the existing words. They include:
- pejoration, in which a term acquires a negative association
- amelioration, in which a term acquires a positive association
- widening, in which a term acquires a broader meaning
- narrowing, in which a term acquires a narrower meaning
Sociolinguistics and language change 
The sociolinguist Jennifer Coates, following William Labov, describes linguistic change as occurring in the context of linguistic heterogeneity. She explains that “[l]inguistic change can be said to have taken place when a new linguistic form, used by some sub-group within a speech community, is adopted by other members of that community and accepted as the norm.”
Can and Patton (2010) provide a quantitative analysis of twentieth century Turkish literature using forty novels of forty authors. Using weighted least squares regression and a sliding window approach, they show that, as time passes, words, in terms of both tokens (in text) and types (in vocabulary), have become longer. They indicate that the increase in word lengths with time can be attributed to the government-initiated language “reform” of the 20th century. This reform aimed at replacing foreign words used in Turkish, especially Arabic- and Persian-based words (since they were in majority when the reform was initiated in early 1930s), with newly coined pure Turkish neologisms created by adding suffixes to Turkish word stems (Lewis, 1999).
Can and Patton (2010), based on their observations of the change of a specific word use (more specifically in newer works the preference of ama over fakat, both borrowed from Arabic and meaning 'but', and their inverse usage correlation is statistically significant), also speculate that the word length increase can influence the common word choice preferences of authors.
Quantifying language change 
Altintas, Can, and Patton (2007) introduce a systematic approach to language change quantification by studying unconsciously-used language features in time-separated parallel translations. For this purpose, they use objective style markers such as vocabulary richness and lengths of words, word stems and suffixes, and employ statistical methods to measure their changes over time.
Languages perceived to be "higher status" stabilise or spread at the expense of other languages perceived by their own speakers to be "lower-status".
Historical examples are the early Welsh and Lutheran bible translations, leading to the liturgical languages Welsh and High German thriving today, unlike other Celtic or German variants.
For prehistory, Forster and Renfrew (2011)  argue that in some cases there is a correlation of language change with intrusive male Y chromosomes but not with female mtDNA. They then speculate that technological innovation (transition from hunting-gathering to agriculture, or from stone to metal tools) or military prowess (as in the abduction of British women by Vikings to Iceland) causes immigration of at least some males, and perceived status change. Then, in mixed-language marriages with these males, prehistoric women would often have chosen to transmit the "higher-status" spouse's language to their children, yielding the language/Y-chromosome correlation seen today.
See also 
- Wave model (linguistics)
- Phono-semantic matching
- Origin of language
- Cohen, Marcel (1975) . Language: its structure and evolution. Translated by Leonard Muller. London: Souvenir Press (Educational & Academic). pp. 74–98. ISBN 0-285-64779-2. "[...] the shifting movements of languages in light of whatever knowledge is available of the history of humanity."
- Cohen, Marcel (1975) . Language: its structure and evolution. Translated by Leonard Muller. London: Souvenir Press (Educational & Academic). pp. 98–141. ISBN 0-285-64779-2. "Internal evolution [...] is the passing from one system to another. [...] Internal evolution proceeds progressively, by modification and substitution of details. It is the sum of these details which, at the end of a certain period of time, constitutes a total change."
- William Labov, 1963. "The social motivation of a sound change." Word 19.273-309
- Coates, 1992: 169
- Barker, Christopher (1588). The Bible in Welsh. London.
- Forster P, Renfrew C; Renfrew (2011). "Mother tongue and Y chromosomes". Science 333 (6048): 1390–1391. Bibcode:2011Sci...333.1390F. doi:10.1126/science.1205331. PMID 21903800.
- Altintas, K.; Can, F.; Patton, J. M. (2007). "Language Change Quantification Using Time-separated Parallel Translations". Literary and Linguistic Computing 22 (4): 375–393. doi:10.1093/llc/fqm026.
- Can, F.; Patton, J. M. (2010). "Change of Word Characteristics in 20th Century Turkish Literature: A Statistical Analysis". Journal of Quantitative Linguistics 17 (3): 167–190. doi:10.1080/09296174.2010.485444.
- Coates, Jennifer (1993). Women, men, and language: a sociolinguistic account of gender differences in language. Studies in language and linguistics (2 ed.). Longman. p. 228. ISBN 978-0-582-07492-7. Retrieved 2010-03-30.
- Labov, William (1994, 2001), Principles of Linguistic Change (vol.I Internal Factors, 1994; vol.II Social Factors, 2001), Blackwell.
- Lewis, G. (1999). The Turkish Language Reform: A Catastrophic Success. Oxford : Oxford University Press.
- Wardhaugh, R. (1986), An Introduction to Sociolinguistics, Oxford/ New York.
- Wittmann, H. (1983), "Les réactions en chaîne en morphologie diachronique". Actes du Colloque de la Société internationale de linguistique fonctionnelle 10.285–92.
- Sounds Familiar? The British Library website provides audio examples of changing accents and dialects from across the UK.