Harvard psychology professor Jerome Kagan has proved that readily measurable traits like the tendency to be startled by novel stimuli can be shown most readily in those infants who go on to develop into shy children and adults. (Pert, Candace, PhD. Molecules of Emotion. p 132. NY: Scribner, 1997.)

Norepinephrine lowers the threshold of reaction in the amygdalae. Children with higher levels of norepinephrine have greater sympathetic reactivity. There appears to be an association between norepinephrine levels and the high-arousal characteristic of introversion, which may be linked to shyness as well. (Ornstein, Robert, PhD. The Roots of the Self. p 40-41. NY: HarperCollins, 1995.)

A temperament pattern known as behavioral inhibition (e.g., fearful, shy responses to people and events) is characteristic of about 20% of healthy 4-month-olds. This pattern tends to persist into adulthood. (Restak, Richard, MD. The Secret Life of the Brain. p 32. The DANA Press and Joseph Henry Press. Washington D.C.: The Dana Press and Joseph Henry Press, 2001.)