" In Tom, it can be said, there is a tendency to appraise certain situations in such a way that a behavioural system is activated that results in his attacking his little sister and biting her. Further, the conditions that lead to this appraisal and so activate the system are specifiable, at least roughly. They comprise, perhaps, a combination on the one hand of a situation of mother attending to little sister and not to Tom and, on the other, of certain organismic states of Tom, themselves brought about by specifiable conditions, such, for example, as a rebuff from father, or fatigue, or hunger. Whenever certain combinations of these conditions obtain, it is predicted, a certain appraisal will be made, a certain behavioural system will be activated, and Tom will bite. "

― John Bowlby , Attachment