Human beings are by nature social animals, but for some, social scrutiny is a source of extreme anguish. Those with social phobia, for example, suffer excessive and often disabling concern about potential and real social-evaluative threat. As new and effective therapies for this condition are pursued, there is a simultaneous movement to extend the understanding of this disorder's etiology. In psychiatry, as in the rest of medicine, this development of new treatments often occurs in parallel with increasing sophistication about causes of illness. Advances in one area typically inform and predictably lead to advances in the other. Social phobia is recognized as a relatively common and significantly impairing anxiety disorder. As with other psychiatric disorders, emerging models of the etiology of social phobia are derived from converging evidence of interacting biological and environmental contributions. Current theories regarding the evolution of social phobia will be addressed, including biological preparedness to fear scrutiny by others, genetically transmitted predisposition to fear acquisition, nongenetic familial and environmental factors, as well as other possible causes and antecedents. Additionally, we describe recent work on behavioral inhibition in infancy as an identifiable early marker of proneness to the development of anxiety disorders, including social phobia.