It is what parents of teenagers who `revise` in front of the computer have long feared. Students who use Facebook while they study get significantly lower grades than those who do not, according to psychologists.

A study has found that the exam results of those who used the social networking site while working, even if it was on in the background, were 20 per cent lower than non-users.

Researchers say the findings undermine the theory that young people`s brains are better at multitasking on digital gadgets.

Study author Professor Paul Kirschner said: `The problem is that most people have Facebook or other social networking sites, their emails and maybe instant messaging constantly running in the background while they are carrying out other tasks.

His team studied 219 students aged between 19 and 54 at an American university.

The Facebook users among them had a typical grade point average - a score from zero up to four - of 3.06. Non-users had an average GPA of 3.82.

`Our study, and other previous work, suggests that while people may think constant task-switching allows them to get more done in less time, the reality is it extends the amount of time needed to carry out tasks and leads to more mistakes.`

BDST: 1535 HRS, 7 September 2010