Studies link frequency of word use to how fast words evolve.

What did you say? Infrequently-used words have a habit of changing. Credit: Getty

The less often a word is said, the faster it will change over time, whereas more-often uttered words are resistant to change. In this week?s Nature, two groups publish analyses of this trend, which quantify it and compare it with biological evolution.

The idea that aspects of culture might ?evolve? in in a similar way as biological organisms dates back to Darwin himself. The notion was given a big push forwards in 1976, when Richard Dawkins introduced the concept of ?memes? ? aspects of culture or fashion that ?propagate themselves ? by leaping from brain to brain?. That idea proved to be a successful meme in itself, becoming often-referenced in the literature. But, as Tecumseh Fitch, a psychologist at the University of St Andrews in Scotland, points out ?not much has been done? to quantify the effect.

?There is this general idea that culture evolves, but it is more of a metaphor than something that has teeth ? that obeys precise mathematical rules,? says Erez Lieberman, a specialist in evolutionary maths at Harvard University. ?I wondered: can you make this metaphor be real? Can you show a process like natural selection that is affecting ideas or language??

I getted it

Lieberman was struck by this idea when he learned that the ten most common verbs in English (be, have, do, go, say, can, will, see, take, get) are all irregular. Instead of their past tenses ending in ?-ed?, as do 97% of English verbs, they take the peculiar forms of was, had, did, went, said, could, would, saw, took and got.

Researchers suppose that this is because often-used irregulars are easy to remember and get right. Seldom-used irregulars, on the other hand, are more likely to be forgotten, so speakers often mistakenly apply the ?-ed? rule. The most commonly used word that they found this happened to was the verb ?to help? ? the past tense used to be ?holp?, but is now ?helped?.

This could be seen as analogous to the way that crucially needed genes tend to stay the same throughout biological evolution, whereas those for less-often-used or specialist traits have more freedom to evolve.

Lieberman wondered whether he could quantify the effect. He and his team looked at 177 verbs with varying frequencies of use that were irregular in Old English, and examined how many had been regularized into the ?-ed? past tense by the eras of Middle and Modern English. They found that an irregular verb used 100 times less frequently is regularized 10 times as fast. For the more mathematically inclined, this can be expressed as: 'The half-life of irregular verbs is proportional to the square root of their frequency.'1

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At the same time, a different group led by evolutionary bio-informaticist Mark Pagel of the University of Reading, UK, looked at 200 basic words across different Indo-European languages, to estimate their rates of evolution.

Some words sound very similar, or can be traced back to a common origin by linguists, throughout many of the languages, which suggests these words were resistant to evolution. The English word 'water', for example, is similar to the now-extinct European Gothic word 'wato', along with words in many other modern European languages. Other words were markedly different across languages, suggesting they had evolved separately and more quickly (see ).

When Pagel?s team plotted frequency of word use against rate of evolution for four languages (Greek, Spanish, English and Russian), they too found that less-frequently-used words evolved faster, and that frequency can explain half of the rate of evolution.2

?The easiest way to think about it is that changes to the higher-frequency words are less likely to be accepted,? says Pagel. ?If I say there are two guys coming over the hill to kill us, and in fact there are 20, we might end up dead.?

The meaning of it all

Both papers were written by teams with evolutionary biology backgrounds, and both call attention to the similarities between language change and the evolution of species. Leiberman even refers to early English as a ?primordial soup? of verb forms in his paper.

Brian Joseph, a historical linguist at Ohio State University in Columbus and the editor of the journal Language, says that?s going too far. Early English, he says, was ?just as regular and rule-based? as modern language ? it just had more complicated rules.

?The analogy with darwinian evolution is crude, although not useless,? says Stephen Pinker, a language and cognition specialist at Harvard. ?Some portion of the variance can be accounted for by a quantitative variable such as frequency, but much more remains unexplained. You can?t understand language change without looking at the cognitive psychology of the human brains that learn and transmit the words.?

References 1 Lieberman, E. et al. Nature 449 713-716 (2007). 2 Pagel, M. et al. Nature 449 717-720 (2007). Download references

Authors Emma Marris View author publications You can also search for this author in PubMed Google Scholar

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