[2] He developed the idea under two assumptions: there indeed exists a relatively stable basic vocabulary (referred to as Swadesh lists) in all languages of the world; and, any replacements happen in a way analogous to radioactive decay in a constant percentage per time elapsed.
His methods aimed to aid linguistic anthropologists by giving them a definitive way to determine a separation date between two languages.
The formula provides an approximate number of centuries since two languages were supposed to have separated from a singular common ancestor.
The core vocabulary was designed to encompass concepts common to every human language such as personal pronouns, body parts, heavenly bodies and living beings, verbs of basic actions, numerals, basic adjectives, kin terms, and natural occurrences and events.
This leads to a critique of the glottochronologic formula because some linguists argue that the morpheme decay rate is not guaranteed to stay the same throughout history.
[15] For Amerind, correlations have been obtained with radiocarbon dating and blood groups[dubious – discuss] as well as archaeology.
[21] The assumption of a single-word replacement rate can distort the divergence-time estimate when borrowed words are included (Thomason and Kaufman 1988).
Simultaneous estimation of divergence time and replacement rate was studied by Kruskal, Dyen and Black.
Improvements in statistical methodology related to a completely different branch of science, phylogenetics; the study of changes in DNA over time sparked a recent renewed interest.
The new methods are more robust than the earlier ones because they calibrate points on the tree with known historical events and smooth the rates of change across them.
Another attempt to introduce such modifications was performed by the Russian linguist Sergei Starostin, who had proposed the following: The resulting formula, taking into account both the time dependence and the individual stability quotients, looks as follows: In that formula, −Lc reflects the gradual slowing down of the replacement process because of different individual rates since the least stable elements are the first and the quickest to be replaced, and the square root represents the reverse trend, the acceleration of replacement as items in the original wordlist "age" and become more prone to shifting their meaning.
This formula is obviously more complicated than Swadesh's original one, but, it yields, as shown by Starostin, more credible results than the former and more or less agrees with all the cases of language separation that can be confirmed by historical knowledge.