The behavioral reactions to unfamiliar events are basic phenomena in all vertebrates. Four-month-old infants who show a low threshold to become distressed and motorically aroused to unfamiliar stimuli are more likely than others to become fearful and subdued during early childhood, whereas infants who show a high arousal threshold are more likely to become bold and sociable. After presenting some developmental correlates and trajectories of these 2 temperamental biases, I consider their implications for psychopathology and the relation between propositions containing psychological and biological concepts.