A model is presented in which assertion and aggression are seen as arising from two entirely different biopsychological systems in the human infant. Assertion derives from the universal tendency to be active, to seek stimuli, to generate plans, and to carry them out. It is a self-activating system and is generally associated with the positive affects of joy, interest, and excitement. Aggression derives from the equally universal self-protective system. This system, however, is reactive in the face of a perceived threat and is associated with the dysphoric affects of anger, fear, and distress. During the infant and toddler period, the child's assertions may be treated punitively by the parents as if they were aggressive acts. The child in turn feels threatened and becomes self-protective. The result is a contamination between assertion and aggression and the creation of a new self-sustaining system in which aggression appears to be spontaneous and self-activating. If assertions are frequently blocked, the contamination may be extensive, and it may appear as if an aggressive instinctual process is present. However, a systemic explanation provides a much better fit with the empirical observations than does an instinctual one. Four examples of different family styles are presented. The model permits one to understand the subtle qualitative differences in style reflecting different individual combinations of assertion and aggression and their associated affects.