Neurological imaging reveals people more inclined to begin smoking as teenagers tend to have reduced gray matter in two important brain regions, indicating they may play a significant role in inhibition and addiction.

"Smoking is perhaps the most common addictive behavior in the world, and a leading cause of adult mortality," says Cambridge University psychologist Trevor Robbins.

"The initiation of a smoking habit is most likely to occur during adolescence. Any way of detecting an increased chance of this, so we can target interventions, could help save millions of lives."

An international team led by Fudan University bioinformatician Tianye Jia and cognitive neuroscientist Shitong Xiang compared MRI brain scans from over 800 people, collected from the UK, Germany, France and Ireland across different time points. These volunteers also answered questionnaires on personality traits.

The researchers then compared those who began smoking by age 14 with non-smokers and repeated this again with the same patients at ages 19 and 23.

The images revealed those who took up smoking from the age of 14 had comparitively less gray matter in the left ventromedial prefrontal cortex; a part of the brain involved in emotional regulation, decision-making, and self-control.

Scans taken five years later revealed the opposite part of this same brain region (on the right) was also reduced in the smoker group compared to non-smokers. This side of the ventromedial prefrontal cortex has also been linked to pleasure.

"The ventromedial prefrontal cortex is a key region for dopamine, the brain's pleasure chemical," explains Cambridge psychiatrist Barbara Sahakian. "As well as a role in rewarding experiences, dopamine has long been believed to affect self-control."

Results from the questionnaires may suggest why.

"Both questionnaires examine the pursuit of thrilling experiences, but they measure distinct behaviors," explains Robbins. "The sensation-seeking scale focuses on pleasurable experiences, while the novelty-seeking questionnaire includes items on impulsiveness and rule breaking."

Participants whose answers revealed a greater tendency towards sensation seeking were more likely to have reduced brain matter on the right side of the ventromedial prefrontal cortex. While greater novelty seeking was associated with less gray matter on the left. This results in lower inhibitions and more risk taking, behaviors that increase the likelihood of smoking in teenagers.

"Smokers then experience excessive loss of gray matter in the right frontal lobes, which is linked to behaviors that reinforce substance use," says Jia – pressing those sensation-seeking pleasure button. "This may provide a causal account of how smoking is initiated in young people, and how it turns into dependence."

Another subset of the studied population were those who begin smoking by 19. They also had less gray matter in their left prefrontal cortex at 14 but their right side was the same as non-smokers until after they started smoking.

So reduced matter in the left frontal lobe may be an inheritable biomarker for people with tendencies toward addiction.

"Providing alternative non-drug rewards at the early stage of substance use may help prevent the transition to substance dependency," Jia and colleagues suggest.

They are keen to see if their findings also hold true for vaping.

One in five adult deaths in the US are attributed to smoking. Understanding who is most at risk before they even start could be life-changing.

This research was published in Nature Communications.