In linguistics, the technique of glottochronology is used to estimate the time of divergence of two related languages. It is analogous to the use of C14 dating of organic materials in that a "lexical half-life" is estimated and used to extrapolate to the time the two languages being compared diverged. The method presumes that the basic vocabulary may be used as a sort of clock, on the assumption that basic vocabulary changes at a more-or-less constant rate through time. Morris Swadesh compiled a list of concepts for a basic vocabulary, the Swadesh list. The method is highly controversial and many linguists argue that there is no evidence that language change occurs at a steady rate. Glottochronological results are considered by many linguists to be invalid.
Lexicostatistics involves measuring the percentage of cognates (that is, similar words with similar meanings in two languages where the similarity is attributable to descent from a common ancestral form in an ancestral language) in "basic word lists". The larger the percentage of cognates, the more recently the two languages being compared are presumed to have separated.
