Languages evolve just as species do, and just as with organisms, the rate of evolution is hardly uniform. Some words evolve rapidly, with a result that there are many different word forms, what linguists call cognates, for meanings across languages. “Bird,” for example, takes many disparate forms across other Indo-European languages: oiseau in French, vogel in German and so on.

But other words, like the word for the number after one, have hardly evolved at all: two, deux (French) and dos (Spanish) are very similar, derived from the same ancestral sound.

“If you study evolution, you immediately ask why is that the case?” said Mark Pagel, a professor at the University of Reading in England. Now he and colleagues Quentin D. Atkinson and Andrew Meade have come up with a mechanism to answer that question. Put simply, the more a word is used, the less it evolves.

In their research, described in Nature, they first looked at 200 word meanings across 87 Indo-European languages and determined how many cognates each had. That enabled them to develop estimates of how rapidly the words were evolving, Dr. Pagel said.

Then they analyzed spoken- and written-word databases in four of those languages: English, Spanish, Russian and Greek. The English database, for example, has 100 million words of spoken English. They looked to see how frequently the words from the first part of their research were used.

“What we found, to our great delight, is that at least for those four very disparate languages, correlation between the frequencies is very high,” Dr. Pagel said. Words that were frequently used had few cognates across the Indo-European family, while words that were used rarely had many.

As to how frequency of word use would affect evolution, Dr. Pagel said a possibility is that if errors are made in speaking common words, they may tend to be corrected, precisely because they are so common and so important for communication.

Rarely used words may not necessarily be corrected, however, because they are infrequently heard. “That allows you to develop a stronger linkage to the mutant form, and you are likely to express it again,” he said.

And as with living things, Dr. Pagel added, that kind of variation “is the raw stuff that evolution acts on.”