Comparative Method (Historical Linguistics)

They share a name but not a logic. The general comparative method in the social sciences compares cases or systems to identify causes through similarity and difference. The historical-linguistic comparative method compares specific linguistic forms — cognates — and exploits the regularity of sound change to reconstruct an ancestral language and prove genetic descent. Its evidence is recurrent sound correspondences, not the covariation of variables across cases, which is why this entry is kept distinct from the generic comparative method.

Because sound change is regular, a genuine inherited relationship produces the same correspondence again and again across many unrelated cognate sets — for instance Latin p matching English f in word after word. Borrowing tends to be sporadic and to import sounds directly, and chance resemblances do not recur systematically. A web of recurrent, mutually consistent correspondences is therefore extraordinarily unlikely to arise without common ancestry, which is what makes the comparative method probative rather than merely suggestive.

The comparative method is primary: it establishes which words are true cognates and how the languages are related, through reconstruction and sound laws. Lexicostatistics and phylogenetic methods take cognacy judgments — ideally grounded in the comparative method — and quantify or model them to measure relatedness, build trees, or estimate dates. The quantitative methods are powerful for scaling and statistics but cannot replace the comparative method's ability to prove relationship and recover actual proto-forms.