Is this proof smoking lowers your IQ? Study suggests those on 20 a day are less intelligent

Smoking has long been known to damage lungs and cause heart disease. But it could also lower your IQ, research has found.

Young people who smoke regularly are likely to have markedly lower intelligence levels than those who do not smoke, and, according to the study of 20,000 young adults, the heavier the smoker, the lower the IQ.  

Those who smoke a pack or more of cigarettes a day averaged an IQ seven and a half points lower than that of those who do not smoke.

A typical 18 to 21-year-old smoker was found to have an IQ of 94, while non-smokers of the same age averaged 101. 

Those who smoked more than a pack a day had particularly low IQs of around 90. An average intelligence IQ score ranges from 84 to 116 points.

Crucially, brothers scored differently depending on whether or not they smoked. 

Despite similar environmental conditions, non-smoking siblings achieved higher IQs than their smoking brothers.

The results come from a study of 20,000 young men conducted by the Sheba Medical Center at the Tel Hashomer Hospital in Israel.

Dr. Mark Weiser, who led the research, said it is unclear whether smoking causes IQ levels to drop or whether less intelligent people are simply more inclined to smoke.

'It was really quite a straightforward study,' he said. 

'We looked at cross-sectional data on IQ and smoking cigarettes, and looked at people's smoking status and their IQs.

'IQ scores are lower in male adolescents who smoke compared to non-smokers and in brothers who smoke compared to their non-smoking brothers. The IQs of adolescents who began smoking between ages 18 to 21 are lower than those of non-smokers.

'It's very clear that people with low IQs are the ones who choose to smoke. It's not just a matter of socioeconomic status - if they are poor or have less education,' he said.

Dr Weiser suggested the results could confirm a previously held conviction that those with lower IQs tend to make poorer decisions regarding their health – that they are more likely to take drugs, eat unhealthy food and exercise less.

The study could also be used to prevent smoking in young people by targeting those with lower IQs, Dr Weiser said.

Researchers found that 28per cent of the teenagers polled smoked one or more cigarettes a day, three per cent admitted to having smoked in the past, while 68 per cent of the young men had never smoked.

In 2004, researchers from the University of Aberdeen first found a possible link between smoking and reduced mental function.

Hundreds of volunteers who had taken part in the Scottish Mental Survey in 1947 aged 11, retook tests 53 years later.

Smokers performed worse than ex-smokers and those who had never smoked. 

Scientists cannot yet conclusively explain the link between impaired lung function and cognitive ageing but it has been suggested that smoking could put the brain under oxidative stress, which causes DNA damage.