Finally, Nordgren's team tested the idea that "restraint bias" could explain why drug addicts are so prone to relapse. They recruited 55 participants through a smoking-cessation programme, all of whom had been smoke free for at least three weeks. Those who said they had more impulse control also tended to say they wouldn't be trying so hard to avoid temptation, such as the company of other smokers. Four months' later, those with the inflated sense of impulse control were more likely to have relapsed.

"The restraint bias suggests that people are willing to experiment with addictive drugs simply because they believe they can overcome the addiction," the researchers said.