The initial dispositions to approach or to avoid unfamiliar events are 2 temperamental characteristics of children--among the many that have been described--that appear to be moderately stable over time and associated with distinct, physiological profiles that may be under partial genetic control. The display of high versus low levels of both motor activity and crying to unfamiliar in 4-month-old infants predicts these 2 temperamental profiles in the 2nd year. This fact implies, but does not prove, that variation in the excitability of those brain areas that mediate motor activity and crying participates in the actualization of the temperamental categories called inhibited and uninhibited to the unfamiliar.