Dr. van der Wee of the Leiden University Medical Center, previously at the University Medical Center in Utrecht, says the finding provides evidence of a long-suspected biological cause for social phobia also known as generalized social anxiety disorder.

The study, published in the Journal of Nuclear Medicine, used single-photon emission computed tomography to compare the brain activity of 12 people diagnosed with social anxiety disorder -- an extreme, psychopathological shyness -- compared to a control group of 12 healthy people matched by gender and age.

The study shows differences between those with and those without the disorder in the densities of the substances responsible for transferring signals from one neuron to another in the brain -- the neurotransmitters serotonin and dopamine.

"Our study provides direct evidence for the involvement of the brain's dopaminergic system in social anxiety disorder in patients who had no prior exposure to medication," van der Wee says in a statement. "It demonstrates that social anxiety has a physical, brain dependent component."