The evidence taken from the observation of the behavior of apes and children suggests that there are three clearly separable groups of simple causes for the outbreak of fighting and the exhibition of aggressiveness by individuals.

One of the most common causes of fighting among both children and apes was over the possession of external objects. The disputed ownership of any desired object—food, clothes, toys, females, and the affection of others—was sufficient ground for an appeal to force. On Monkey Hill disputes over females were responsible for the death of thirty out of thirty-three females. Two points are of particular interest to notice about these fights for possession.

In the first place they are often carried to such an extreme that they end in the complete destruction of the objects of common desire. Toys are torn to pieces. Females are literally torn limb from limb. So overriding is the aggression once it has begun that it not only overflows all reasonable boundaries of selfishness but utterly destroys the object for which the struggle began and even the self for whose advantage the struggle was undertaken.

In the second place it is observable, at least in children, that the object for whose possession aggression is started may sometimes be desired by one person only or merely because it is desired by someone else. There were many cases observed by Dr Isaacs where toys and other objects which had been discarded as useless were violently defended by their owners when they became the object of some other child’s desire. The grounds of possessiveness may, therefore, be irrational in the sense that they are derived from inconsistent judgments of value. Whether sensible or irrational, contests over possession are commonly the occasion for the most ruthless(残忍的) use of force among children and apes.

One of the commonest kinds of object arousing possessive desire is the notice, good will, affection, and service of other members of the group. Among children one of the commonest causes of quarreling was "jealousy"—the desire for the exclusive possession of the interest and affection of someone else, particularly the adults in charge of the children. This form of behavior is sometimes classified as a separate cause of conflict under the name of "rivalry" or a "jealousy". But, in point of fact, it seems to us that it is only one variety of possessiveness. The object of desire is not a material object—that is the only difference. The object is the interest and affection of other persons. What is wanted, however, is the exclusive right to that interest and affection—a property in emotions instead of in things. As subjective emotions and as causes of conflict, jealousy and rivalry are fundamentally similar to the desire for the uninterrupted possession of toys or food. Indeed, very often the persons, property which is desired, are the sources of toys and food.

Possessiveness is, then, in all its forms a common cause of fighting. If we are to look behind the mere facts of behavior for an explanation of this phenomenon, a teleological ( 目的论的 ) cause is not far to seek. The exclusive right to objects of desire is a clear and simple advantage to the possessor of it. It carries with it the certainty and continuity of satisfaction. Where there is only one claimant to a good, frustration and the possibility of loss is reduced to a minimum. It is, therefore, obvious that, if the ends of the self are the only recognized ends, the whole powers of the agent, including the fullest use of his available force, will be used to establish and defend exclusive rights to possession.

Another cause of aggression closely allied to possessiveness is the tendency for children and apes greatly to resent the intrusion of a stranger into their group. A new child in the class may be laughed at, isolated, and disliked and even set upon and pinched and bullied. A new monkey may be poked and bitten to death. It is interesting to note that it is only strangeness within a similarity of species that is resented. Monkeys do not mind being joined by a goat or a rat. Children do not object when animals are introduced to the group. Indeed, such novelties are often welcomed. But when monkeys meet a new monkey or children a strange child, aggression often occurs. This suggests strongly that the reason for the aggression is fundamentally possessiveness. The competition of the newcomers is feared. The present members of the group feel that there will be more rivals for the food or the attention of the adults.