Abstract This paper considers the influence of temperamental factors on the development of anxious symptoms in children and adolescents. About 20 percent of healthy children are born with a temperamental bias that predisposes them to be highly reactive to unfamiliar stimulation as infants and to be fearful of or avoidant to unfamiliar events and people as young children. Experiences act on this initial temperamental bias and, by adolescence, about one-third of this group is likely to show signs of serious social anxiety. These children are also likely to have one or more biological features, including a sympathetically more reactive cardiovascular system, asymmetry of cortical activation in EEG favoring a more active right frontal area, more power in the EEG in the higher frequency range, and a narrower facial skeleton. The data imply that this temperamental bias should be conceptualized as constraining the probability of developing a consistently fearless and spontaneous profile rather than as determining an anxious or introverted phenotype.