Comparing rates of language change

We use comparisons of words from basic vocabulary between pairs of closely-related languages to identify instances of gain and loss of words. We identified patterns of word gain and loss by recording instances where a cognate form within a given semantic category was present in one language in a sister pair but not found in its sister language (Bromham et al., 2015a). A cognate class is a set of words identified as derived from a common ancestor, and therefore the presence of a cognate class in one language of a pair, and in other languages within the family, implies the presence of that cognate class in the common ancestral language of the pair. This method differs from approaches where the net dissimilarity between lists of terms is compared (Wichmann and Holman, 2009). Instead we use only those words that show a pattern of occurrence that is informative for determining differences in rates of gain and loss of words (Bromham et al., 2015a).

If a word form found in one sister language has a cognate in other languages in the language family, then it is likely to have been inherited from the common ancestor. This implies that the absence of that cognate form in the other sister language must be due to its loss after divergence from the common ancestor of the pair (Figure ​(Figure2).2). If one of the sister languages has a unique word form that has no recognized cognates in any other language in the family, then it presumably represents a gain of a new word since it split from its sister language. Therefore we can identify instances of word gain and loss in both members of a related pair of languages. Any such changes that have occurred in one sister pair of languages can be considered to have happened independently from changes in other sister pair of languages, so these comparisons can be treated as statistically independent data points (Bromham et al., 2015a).

Method for determining word gains and losses. If a cognate form is found in one member of a sister pair and in another language in the family, it must have been lost from the other sister language. A lexeme that has no cognates in any other language in the family, including its sister language, is considered to have been gained since they split from their shared common ancestor.

Our analysis only includes cognate classes showing rates-informative patterns that allow us to localize a word gain or loss to only one member of a sister pair (Figure ​(Figure2).2). There are two rates-informative patterns. Presence of a cognate class in one member of the pair but not the other indicates a loss of the shared ancestral cognate form from one sister language after divergence from the common ancestor. Presence of a novel form in one member of the pair that has no known cognates in any other member of the language family indicates the gain of a new word in one sister language after divergence from the common ancestor. We did not consider cognate forms that are present in both members of a sister pair because they have both inherited those forms from their common ancestor, and neither has lost that cognate, so those cognates are non-informative for rates of gain and loss. Similarly, we did not count any cognate class that is absent from both members of a sister pair, on the assumption that it was not present in their common ancestor.

We do not include any identified loan words in the analysis, so any cognate terms shared by two languages should be present in the language due to inheritance from a common ancestor, rather than borrowing (horizontal transfer) from another language. The addition of a new word does not necessarily involve the loss of an existing word as languages can have multiple lexemes for one category, therefore each recorded gain, or loss of a lexeme was counted as a separate event, regardless of semantic category. Any lexemes that were recorded as “doubtful” or “exclude” in the databases were excluded from our analysis. Any semantic categories that did not contain entries for both languages in the pair were also excluded as we are unable to ascertain if this absence is a true absence or simply missing data.

This counting procedure will in some cases count semantic shifts as a change (e.g., Danish træ “tree” is cognate with proto-Indo-European ^*dóru but has shifted to also mean “wood”). Due to the nature of these datasets (cognate classes coded within a limited number of semantic categories), we cannot quantify semantic shift, which may include gain, or loss of meaning from unrecorded semantic categories. Cognates that change meaning and undergo semantic shifts into a new category in the word list might appear as the gain of a new cognate into the recipient semantic category. If there is a subsequent change of meaning away from the original semantic category, then we would count this as loss of a cognate from the original semantic category. While this represents a somewhat different kind of change from the origin, replacement and loss of lexical items, it is still indicative of language change. In this way, we may include changes in both form and meaning. One of the ways that the population size hypothesis might affect language change is through altering semantics.

The total number of gains, losses, and non-informative results were counted for all available semantic categories for each pair of languages. The raw counts were standardized by the total number of comparisons made between the pairs (gains + losses + non informative + excluded) to allow for comparisons to be made between languages. We have developed a Python package, RateCounter (https://github.com/SimonGreenhill/RateCounter), to extract this rate information from common phylogenetic file formats.