Abstract
Now, man makes a very complicated society. Bur when we see the history of evolution in Primates, we can see that this complicated society originated in a simple one like what prosimian (primitive monkey) forms. Nocturnal prosimians, which are most primitive Primates, live solitarily. Diurnal prosimians form groups, and many advanced monkeys make more complicated society with order system. I think it very important to know the mechanism of this social evolution in Primate in order to make progress in the study of sociological basic problems such as the origin of order or the origin of norms. In this paper I examine the ultimate factors of group formation in Primates for the first step of this approach.
As the result of several ESS (evolutionarily stable strategy) models, it was revealed that the factor of avoiding predators is important in many cases to make group strategy advantageous. Concerning resources such as food or females (for males), group strategy is usually disadvantageous because it makes a competition serious. But when the population density is high or resources distribute unequally, it was revealed that the strategy of defending resources in union can be advantageous if an adequate distribution system of resources and a joint struggle system exist.
