Researchers at the Wellcome Trust Centre for Neuroimaging at University College London think they may have found some explanation for impulsive behavior. Their study showed that increased levels of dopamine which is a brain chemical linked with reward, can make us cave in to instant gratification, even when we wanted to control our desires.
The new data re-confirms what we already knew, that increased levels of dopamine cause us to lean toward impulsive behavior. Perhaps more importantly, it may bring us more understanding as to how dopamine dysfunction is involved in some disorders like ADHD.
"The research may help explain why people affected by conditions such as Attention Deficit/Hyperactivity Disorder (ADHD), characterised by increased levels of dopamine in the brain, tend to show extremely impulsive behaviour. Similarly, it highlights why such behaviour can be a potential negative side-effect of L-dopa, a drug used to help alleviate the symptoms of Parkinson's disease."
http://www.sciencedaily.com/releases/2010/06/100629170922.htm