Use of hand-held technologies, such as mobile phones, GameBoys and computers, has caused a physical mutation in the under-25s, according to new research.

The study, carried out in nine cities around the world, shows that the thumbs of the younger generation have overtaken their fingers as the hand's most muscled and dexterous digit.

The change affects those who have grown up with hand-held devices capable of text messaging, emailing and accessing internet services. Experts claim it proves technology is causing physical alterations that previously happened over generations.

'The relationship between technology and the users of technology is mutual: we are changing each other,' said Dr Sadie Plant, author of the study and founder of the Cybernetic Culture Research Unit at Warwick University. 'The fact that our thumbs operate differently from our fingers is one of the main things that defines us as humans. Discovering that the younger generation has taken to using thumbs in a completely different way and are instinctively using it where the rest of us use our index fingers is particularly interesting.'

Plant, who has written three books on the social impact of technology, spent six months collecting data on hundreds of mobile phone users in the world's largest cities, including London, Beijing, Chicago and Tokyo.

She noted how, while those less accustomed to mobile phones used one or several fingers to access the keypad, younger people used both thumbs ambidextrously, barely looking at the keys as they made rapid entries.

'They used the absolute minimal movement,' she said. 'Simply exerting pressure with the thumb rather than tapping at the phone.

'There are many ways to input information into these devices, but for some reason kids under 25 most often choose to use their thumbs over any other digit. There is no question that choice is having a clear effect on their physicality: thumbs are the new fingers.'

In Japan, the trend was particularly marked. Plant even found the under-25s referred to themselves as oya yubi sedai - the thumb generation, or thumb tribe.

As their thumbs become stronger and more dexterous, Plant found that the thumb tribe is using its favourite digit for other tasks that are traditionally the finger's job, such as pointing at things or ringing doorbells.

'The mobile is fast becoming an essential prop in the social life of 20-year-olds,' she said. 'It has even become part of their mating display, with young men trying to impress women with the advanced technology of their phones.'