- 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 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 other languages.
- Geographic separation: when people move away from each other, their language will diverge, at least for the vocabulary, due to different experiences.
- 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.
- Imperfect learning: According to one view, children regularly learn the adult forms imperfectly, and the changed forms then turn into a new standard. Alternatively, imperfect learning occurs regularly in one part of society, such as an immigrant group, where the minority language forms a substratum, and the changed forms can ultimately influence majority usage.
- Social prestige: Language may not only change towards a prestigious accent, but also away from one with negative prestige, as in the case of rhoticity of Received Pronunciation. Such movements can go back and forward.
According to Guy Deutscher, the tricky question is "Why are changes not brought up short and stopped in their tracks? At first sight, there seem to be all the reasons in the world why society should never let the changes through." He sees the reason for tolerating change in the fact that we already are used to "synchronic variation", to the extent that we are hardly aware of it. For example, when we hear the word "wicked", we automatically interpret it as either "evil" or "wonderful", depending on whether it is uttered by an elderly lady or a teenager. Deutscher speculates that "[i]n a hundred years' time, when the original meaning of 'wicked' has all but been forgotten, people may wonder how it was ever possible for a word meaning 'evil' to change its sense to 'wonderful' so quickly."