Comparative linguistics

Comparative linguistics (originally comparative philology) is a branch of historical linguistics which is concerned with comparing languages in order to establish their historical relatedness.

Relatedness implies a common origin or proto-language, and comparative linguistics aims to reconstruct proto-languages and specify the changes which have resulted in the documented languages. In order to maintain a clear distinction between attested and reconstructed forms, comparative linguists prefix an asterisk to any form which is not found in surviving texts.

The fundamental technique of comparative linguistics is the comparative method, which aims to compare phonological systems, morphological systems, syntax and the lexicon. In principle, every difference between two related languages should be explicable to a high degree of plausibility, and systematic changes, for example in phonological or morphological systems, are expected to be highly regular. Although the proto-languages reconstructed by the comparative methods are hypothetical, a reconstruction may have predictive power. The most notable example of this is Saussure's proposal that the Indo-European consonant system contained laryngeals, a type of consonant attested in no Indo-European language known at the time. The hypothesis was vindicated with the discovery of Hittite, which proved to have exactly the consonants Saussure had hypothesized in the environments he had predicted.

Where languages are derived from a very distant ancestor, and are thus more distantly related, the comparative method becomes impracticable. In particular, attempting to relate two reconstructed proto-languages by the comparative method has not generally produced results that have met with wide acceptance. A number of methods based on statistical analysis of vocabulary have been developed to overcome this limitation. The theoretical basis of such methods is that vocabulary items can be matched without a detailed reconstruction and that comparing enough vocabulary items will negate individual inaccuracies.

The earliest method of this type was glottochronology, which proposed a mathematical formula for establishing the date when two languages separated, based on percentage of a core vocabulary of 100 (later 200) items which are cognate in the languages being compared. Glottochronology has met with continued scepticism, and is seldom applied today. Even more controversial is mass lexical comparison which disavows any ability to date developments, aiming simply to show which languages are more and less close to each other, in a method similar to those used in cladistics in evolutionary biology. However, since mass comparison eschews the use of reconstruction and other traditional tools, it is flaty rejected by the majority of historical linguists.

Such vocabulary-based methods are able solely to establish degrees of relatedness and cannot be used to derive the features of a proto-language, apart from the fact of the shared items of compared vocabulary. These approaches have been challenged for their methodological problems - without a reconstruction or at least a detailed list of phonological correspondences there can be no demonstration that two words in different languages are cognate. However, lexical methods can be validated statistically and by their consistency with independent findings of history, archaeology and population genetics.

There are other branches of linguistics which involve comparing languages, which are not, however, part of comparative linguistics:
 * Linguistic typology compares languages in order to classify them by their features. Its ultimate aim is understand the universals which govern language, and the range of types found in the world's language is respect of any particular feature (word order or vowel system, for example). Typological similarity does not imply a historical relationship. However, typological arguments can be used in comparative linguistics: one reconstruction may be preferred to another as typologically more plausible.
 * Contact linguistics examines the linguistic results of contact between the speakers of different languages, particular as evidenced in loan words. Any empirical study of loans is by definition historical in focus and therefore forms part of the subject matter of historical linguistics. One of the goals of etymology is to establish which items in a language's vocabulary result from linguistic contact. This is also an important issue both for the comparative method and for the lexical comparison methods, since failure to recognize a loan may distort the findings.
 * Contrastive linguistics compares languages usually with the aim of assisting language learning by identifying important differences between the learner's native and target languages. Contrastive linguistics deals solely with present-day languages.