Biology
The biological foundation of the social fear/anxiety component of shyness is found in the action of the amygdala and hippocampus. The amygdala appears to be implicated in the association of specific stimuli with fear. The more general, pervasive conditioning of background factors related to the conditioning stimuli is known as contextual conditioning. This diffuse contextual conditioning occurs more slowly and lasts longer than most traditional CS-US classical conditioning. It is experienced as anxiety and general apprehension in situations that become associated with fear cues, such as classrooms and parties, for shy people. Contextual conditioning involves the hippocampus, crucial in spatial learning and memory, as well as the amygdala. The bed nucleus of the striate terminalis (BNST) is also involved in emotional-behavioral arousal and extends to the hypothalamus and the brain stem. Both the hypothalamus and the brain stem relay anxiety to the rest of the body. The hypothalamus triggers the sympathetic nervous system and the physiological symptoms of shyness, among them, trembling, increased heart rate, and muscle tension.