The Secret Languages of Twins

It has long been observed that some young twins appear to speak to each other in a language that no one else understands. Researchers call this "cryptophasia", and for most of the twentieth century it was surrounded by myth. Popular magazines described twins who had invented complete languages with their own grammar and thousands of words. More careful research suggests that the reality is less dramatic, but no less interesting.

In 2011, a team led by the linguist Helena Straub at the Marlowe Institute began recording the speech of 64 pairs of twins aged between eighteen months and four years. The families were recruited through nurseries in three cities, and each pair was filmed at home for two hours a month over a period of two years. To allow comparison, the team also recorded 40 pairs of ordinary brothers and sisters who were close in age.

The researchers found that around half of the twin pairs used at least some words or expressions that their parents could not understand. However, in almost every case, these "private" words turned out to be mispronounced versions of words from the family's own language. One pair of twins who called milk "mimbo", for example, had simply combined the word with a sound from a favourite song. Only two pairs produced anything resembling an independent system, and even these used private words for a few dozen objects at most.

Why do twins develop such words more often than other children? Straub's explanation is that twins spend much of the day with a partner whose language is as immature as their own. An ordinary child talking with a parent is constantly corrected, whereas a twin's mistakes are often copied and reinforced by the other twin. The team noted that twins who attended nursery full-time used fewer private words than those who stayed at home, which supports this view.

Some of the siblings in the comparison group also used words their parents did not recognise, but these rarely lasted more than a few weeks. Among the twins, by contrast, some private words remained in use for over a year.

Parents are sometimes worried that a private language will delay their children's development. Straub's data offer some reassurance: by the age of five, every twin in the study was speaking the family language at a level considered normal for their age, although the twins had, on average, begun speaking in full sentences around three months later than the siblings. She advises parents not to discourage private words, but to make sure that each twin also has regular one-to-one conversations with an adult.

The study did not include identical and non-identical twins in equal numbers, and Straub acknowledges that it cannot say whether genetic similarity plays any role.