Glottochronology


Glottochronology is the part of lexicostatistics dealing with the chronological relationship between languages.
The idea was developed by Morris Swadesh under two assumptions: there indeed exists a relatively stable basic vocabulary in all languages of the world; and, any replacements happen in a way analogous to radioactive decay in a constant percentage per time elapsed.
Over time many different extensions of the Swadesh method evolved; however, Swadesh's original method is so well known that 'glottochronology' is usually associated with him.

Methodology

Word list

The original method presumed that the core vocabulary of a language is replaced at a constant rate across all languages and cultures and so can be used to measure the passage of time. The process makes use of a list of lexical terms. Lists were compiled by Morris Swadesh and assumed to be resistant against borrowing. The core vocabulary was designed to encompass concepts common to every human language, eliminating concepts that are specific to a particular culture or time. It has been found that the ideal is really impossible and that the meaning set may need to be tailored to the languages being compared. Many alternative word lists have been compiled by other linguists and often use fewer meaning slots.
The percentage of cognates in the word lists is then measured. The larger the percentage of cognates, the more recently the two languages being compared are presumed to have separated.

Glottochronologic constant

obtained a value for the "glottochronological constant" of words by considering the known changes in 13 pairs of languages using the 200 word list. He obtained a value of 0.805 ± 0.0176 with 90% confidence. For his 100-word list Swadesh obtained a value of 0.86, the higher value reflecting the elimination of semantically unstable words. The constant is related to the retention rate of words by the following formula:
L is the rate of replacement, ln represents the natural logarithm and r is the glottochronological constant.

Divergence time

The basic formula of glottochronology in its shortest form is this:
t = a given period of time from one stage of the language to another, c = proportion of wordlist items retained at the end of that period and L = rate of replacement for that word list.
One can also therefore formulate:
By testing historically verifiable cases in which t is known by nonlinguistic data, Swadesh arrived at the empirical value of approximately 0.14 for L, which means that the rate of replacement constitutes around 14 words from the 100-wordlist per millennium.

Results

Glottochronology was found to work in the case of Indo-European, accounting for 87% of the variance. It is also postulated to work for Afro-Asiatic, Chinese and Amerind. For Amerind, correlations have been obtained with radiocarbon dating and blood groups as well as archaeology.
The approach of Gray and Atkinson, as they state, has nothing to do with "glottochronology".

Discussion

The concept of language change is old, and its history is reviewed in Hymes and Wells. Glottochronology itself dates back to the mid-20th century. An introduction to the subject is given in Embleton and in McMahon and McMahon.
Glottochronology has been controversial ever since, partly because of issues of accuracy but also because of the question of whether its basis is sound. The concerns have been addressed by Dobson et al., Dyen and Kruskal, Dyen and Black. The assumption of a single-word replacement rate can distort the divergence-time estimate when borrowed words are included. Chrétien purported to disprove the mathematics of the Swadesh-model. At a conference at Yale in 1971, his criticisms were shown to be invalid. See the published proceedings under Dyen The same conference saw the application of the theory to Creole language.
An overview of recent arguments can be obtained from the papers of a conference held at the McDonald Institute in 2000. The presentations vary from "Why linguists don't do dates" to the one by Starostin discussed above.
Since its original inception, glottochronology has been rejected by many linguists, mostly Indo-Europeanists of the school of the traditional comparative method. Criticisms have been answered in particular around three points of discussion:
Somewhere in between the original concept of Swadesh and the rejection of glottochronology in its entirety lies the idea that glottochronology as a formal method of linguistic analysis becomes valid with the help of several important modifications. Thus, inhomogeneities in the replacement rate were dealt with by Van der Merwe by splitting the word list into classes each with their own rate, while Dyen, James and Cole allowed each meaning to have its own rate. Simultaneous estimation of divergence time and replacement rate was studied by Kruskal, Dyen and Black.
Brainard allowed for chance cognation, and drift effects were introduced by Gleason. Sankoff suggested introducing a borrowing parameter and allowed synonyms.
A combination of the various improvements is given in Sankoff's "Fully Parameterised Lexicostatistics". In 1972, Sankoff in a biological context developed a model of genetic divergence of populations. Embleton derives a simplified version of that in a linguistic context. She carries out a number of simulations using this which are shown to give good results.
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. As such, they no longer require the assumption of a constant rate of change.

Starostin's method

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. On the other hand, it shows that glottochronology can really be used only as a serious scientific tool on language families whose historical phonology has been meticulously elaborated.

Time-depth estimation

The McDonald Institute hosted a conference on the issue of time-depth estimation in 2000. The published papers give an idea of the views on glottochronology at that time. They vary from "Why linguists don't do dates" to the one by Starostin discussed above. Note that in the referenced Gray and Atkinson paper, they hold that their methods cannot be called "glottochronology" by confining this term to its original method.