More research has been published linking smoking to health risks - with a study suggesting the habit affects IQ

Researchers from the Universities of Aberdeen and Edinburgh looked at how the cognitive abilities of smokers and non-smokers changed over time.

They found smokers performed significantly worse in five separate tests.

The research, part of the Scottish Mental Health Survey, is published in New Scientist magazine.

Around 465 people were tested on their mental abilities in 1947 when they were aged 11.

They were then tested a second time between 2002 and 2002, when they reached the age of 64.

On this occasion they underwent tests to evaluate their non-verbal reasoning, memory and learning, how quickly they processed information, decisions about how to act in particular circumstances and construction tasks.

Current or former smokers were found to perform less well in the tests even after factors such as childhood IQ, education, occupation and alcohol consumption were taken into account.

The effect appeared to be stronger in current smokers according to the study, which was also published in the journal Addictive Behaviors.

Intellectual development

The researchers suggest a "small but significant" negative effect of 4% linked to the combined effects of smoking and impaired lung function - itself linked to smoking.

It has been suggested in previous studies that there could be a link between impaired lung function and a negative effect on the thinking processes, but it is not clear what the mechanism for that might be.

Dr Lawrence Whalley of the University of Aberdeen, who led the research, said the explanation could be that smoking causes oxidative stress - cumulative damage caused by molecules called "free radicals" - to organs including the brain.

"Ageing neurons are very sensitive to oxidative damage."

Writing in Addictive Behaviors, Dr Whalley added: "The harmful effects of smoking on lung function are well established."

But he said the detrimental effect on cognition could be due to be attributable to poor heart and lung function affecting the brain, or directly harmful effects of smoking on brain - as well as lung - tissue.

Amanda Sandford, of the group Action on Smoking and Health said: "It would appear that the well-worn cliché that 'smoking stunts your growth' may be true when it comes to intellectual development.

"Contrary to what many people commonly believe - that smoking may help brain function, it is in fact more likely to wreak havoc with brain cells and IQ.

"Any teenager tempted to smoke should heed the message that it really is dumb to take up this noxious habit."