Those who take on many tasks at once often wrongly believe they are good at multi-tasking - and have trouble focusing on one task at a time, the study by Utah University in Salt Lake City found.

The US professors found the motorists most likely to talk on their mobile phones while driving tend to be 'impulsive, sensation-seeking' individuals who have a heightened risk of having a crash, the 'Daily Mail' reported.

Some people like doing two or more things at once because it is stimulating, interesting, challenging and more exciting even if it hurts their overall performance, the findings suggested.

"What's alarming is people who talk on cell phones while driving tend to be the people least able to multi-task well," researcher David Sanbonmatsu said.

"People talking on cell phones while driving are people who probably shouldn't. We showed people who multi-task the most are those who appear to be the least capable of multi-tasking effectively," Sanbonmatsu said.

He gave 310 undergraduate psychology students tests and questionnaires to measure their actual multi-tasking ability, perceived multi-tasking ability and mobile phone use while driving.

They also had to look at their use of a wide array of electronic media and personality traits such as impulsivity and sensation-seeking.

"The people who are most likely to multi-task harbour the illusion they are better than average at it, when in fact they are no better than average - and often worse," study co-author Professor David Strayer said.

Those who scored highest on actual multi-tasking ability tended not to engage in it because they were more able to focus attention on the job at hand.

Their perceived multi-tasking ability was found to be significantly inflated, the study said. In fact, 70 per cent of participants thought they were above average at

multi-tasking.

People with high levels of impulsivity and sensation-seeking reported more multi-tasking.

Those who engaged in multi-tasking often do so because they are less able to block out distractions and focus on a singular task - rather than because they have the ability, the study found.

Sanbonmatsu said that the 25 per cent who performed best on multi-tasking ability are the people who are least likely to multi-task and are most likely to do one thing at a time.