Glottochronology (Lexical Dating)

Glottochronology is Morris Swadesh's method for estimating the time depth at which two related languages separated, derived from the proportion of basic-vocabulary cognates they still share. Building directly on lexicostatistics, it adds a crucial extra assumption — a 'glottoclock' — that basic vocabulary is lost at an approximately constant rate over time, analogous to radioactive decay. Plugging the observed cognate proportion into a logarithmic decay formula yields an estimated separation date in years. The method is historically important but has been heavily criticized, and most historical linguists today treat its dates with great caution.