A roulette game played while undergoing a brain scan has shown that people with bipolar disorder are wired for risk.
Sufferers of the serious mental illness appear to be dominated by the brain's "pleasure centre" that drives us to seek out and pursue rewards.
The ancient brain area, called the nucleus accumbens, was more strongly activated in people with bipolar disorder than healthy volunteers.
Bipolar disorder, once known as manic depression, is marked by unpredictable and extreme mood swings.
Periods of deep depression alternate with manic episodes of intense excitement, irritability and hyperactivity which can lead to highly risky behaviour.
The condition is difficult to treat and associated with reduced life expectancy, a high risk of suicide, and a destructive impact on work and personal relationships.
For the study, scientists invited participants to make safe or risky gambles as they spin the wheel in a game of roulette.
At the same time, brain activity was measured using a functional magnetic resonance imaging (fMRI) scan that monitors blood flow in the brain.
Professor Wael El-Deredy, from the University of Manchester, said: "The greater buzz that people with bipolar disorder get from reward is a double-edged sword.
"On the one hand, it helps people strive towards their goals and ambitions, which may contribute to the success enjoyed by many people with this diagnosis.
"However, it comes at a cost: these same people may be swayed more by immediate rewards when making decisions and less by the long-term consequences of these actions."
Another key difference seen in people with bipolar disorder was found in the pre-frontal cortex, a recently evolved brain region linked to conscious thought.
Like an orchestra conductor, it co-ordinates various drives and impulses, allowing people to make sensible decisions that are less immediately rewarding but better in the long run.
In healthy volunteers, the pre-frontal cortex guided them towards safer gambles when playing roulette. But in people with bipolar disorder the balance swung the other way.
The findings are published in the latest edition of the journal Brain.
A number of celebrities such as Stephen Fry and Catherine Zeta-Jones have spoken openly about their diagnosis of bipolar disorder, helping to raise awareness of the condition.
But social stigma, stereotyping and prejudice still make life difficult for sufferers, said the researchers.
Co-author Professor Richard Bentall, from the University of Liverpool, said: "This study shows how we can use the new tools of neuroscience to better understand the psychological mechanisms that lead to a psychiatric disorder which, until now, has been very difficult to understand."
A total of 20 patients with bipolar disorder aged 18 to 45 took part in the study along with 20 matched healthy "controls".
Participants did not stand at a physical roulette table. Instead they played a computerised modified version of the game while in the MRI scanner, making bets on which of four colours would come up after each spin of the wheel.