Parents of twins often notice that their children develop strange habits of speech. When one child starts a sentence, the other will finish it. Or the twins may invent private words and phrases that no one understands but themselves. Such behaviour occurs with about 40 per cent of all twins, but normally stops around the age of three. In exceptional cases, however, this secret communion between twins may last longer and go much further.
The most famous example in recent times was the case of Grace and Virginia Kennedy, identical twins born in Georgia, USA, in 1970. As infants they were left in the care of a German-speaking grandmother and saw little of their English-speaking parents or of other children. By the time they were two, the twins had developed the habit of chattering to one another in what appeared to be complete gobbledegook. They had special names for each other - Grace was 'Poto' and Virginia became 'Cabenga' - and produced sentences like 'Snap aduk, Cabenga, chase die-dipana', which apparently was an invitation to play with their doll's house. Of English they could speak not a word.
Until they reached school age, Grace and Virginia were considered mentally retarded. But then their speech was identified as an extraordinary example of 'idioglossia', the development of a private language between two individuals. A speech therapist found that their idioglossia was a mixture of distorted elements of English and German with a few inventions of their own.
The main barrier to understanding the private language was that the twins spoke extremely fast. With speech therapy, the Kennedy girls soon began to speak English and were found to be of quite normal intelligence.