There is research indicating that social anxiety disorder is often correlated with bipolar disorder. Some researchers believe they share an underlying cyclothymic-anxious-sensitive disposition. In addition, studies show that more socially phobic patients treated with anti-depressant medication develop hypomania than non-phobic controls. This can be seen as the medication creating a new problem, and also has this adverse effect in a proportion of those without social phobia.

When prevalence estimates were based on the examination of psychiatric clinic samples, social anxiety disorder was thought to be a relatively rare disorder. The opposite was instead true; social anxiety was common but many were afraid to seek psychiatric help, leading to an understatement of the problem. Prevalence rates vary widely because of its vague diagnostic criteria and its overlapping symptoms with other disorders. There has been some debate on how the studies are conducted and whether the illness truly impairs the respondents as laid out in the official criteria. Psychologist Dr. Ray Crozier argues, “it is difficult to ascertain whether the person being interviewed adheres to the DSM-III-R criteria or whether they are merely exhibiting poor social skills or shyness.”

Literary descriptions of shyness can be traced back to the days of Hippocrates around 400 B.C. Hippocrates described someone who ‘through bashfulness, suspicion, and timorousness, will not be seen abroad; loves darkness as life and cannot endure the light or to sit in lightsome places; his hat still in his eyes, he will neither see, nor be seen by his good will. He dare not come in company for fear he should be misused, disgraced, overshoot himself in gesture or speeches, or be sick; he thinks every man observes him’.

Mental disorders have been found to be relatively common, with more than one in three people in most countries reporting sufficient criteria for at least one diagnosis at some point in their life up to the time they were assessed. A new WHO global survey currently underway indicates that anxiety disorders are the most common in all but 1 country, followed by mood disorders in all but 2 countries, while substance disorders and impulse-control disorders were consistently less prevalent. Rates varied by region. Such statistics are widely believed to be underestimates, due to poor diagnosis (especially in countries without affordable access to mental health services) and low reporting rates, in part because of the predominant use of self-report data rather than semi-structured instruments.[citation needed] Actual lifetime prevalence rates for mental disorders are estimated to be between 65% and 85%.