Letter

Nature 449, 713-716 (11 October 2007) | doi:10.1038/nature06137; Received 20 March 2007; Accepted 27 July 2007

Quantifying the evolutionary dynamics of language

Erez Lieberman1,2,3,5, Jean-Baptiste Michel1,4,5, Joe Jackson1, Tina Tang1 & Martin A. Nowak1

  1. Program for Evolutionary Dynamics, Department of Organismic and Evolutionary Biology, Department of Mathematics,
  2. Department of Applied Mathematics, Harvard University, Cambridge, Massachusetts 02138, USA
  3. Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
  4. Department of Systems Biology, Harvard Medical School, Boston, Massachusetts 02115, USA
  5. These authors contributed equally to this work

Correspondence to: Erez Lieberman1,2,3,5 Correspondence and requests for materials should be addressed to E.L. (Email: erez@erez.com).

Human language is based on grammatical rules1, 2, 3, 4. Cultural evolution allows these rules to change over time5. Rules compete with each other: as new rules rise to prominence, old ones die away. To quantify the dynamics of language evolution, we studied the regularization of English verbs over the past 1,200 years. Although an elaborate system of productive conjugations existed in English's proto-Germanic ancestor, Modern English uses the dental suffix, '-ed', to signify past tense6. Here we describe the emergence of this linguistic rule amidst the evolutionary decay of its exceptions, known to us as irregular verbs. We have generated a data set of verbs whose conjugations have been evolving for more than a millennium, tracking inflectional changes to 177 Old-English irregular verbs. Of these irregular verbs, 145 remained irregular in Middle English and 98 are still irregular today. We study how the rate of regularization depends on the frequency of word usage. The half-life of an irregular verb scales as the square root of its usage frequency: a verb that is 100 times less frequent regularizes 10 times as fast. Our study provides a quantitative analysis of the regularization process by which ancestral forms gradually yield to an emerging linguistic rule.

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