Abstract:Final expression of social behaviour selection depends on the mutual action and balance between two social forces: the internal social driven force and the external social driven force. The orientation and size of the mutual action and balance determine the mode of actual social behaviour selection. Generally, the individual behaviour selection appears unlimited microscopic appearances, always tends to break through the formed common regulations, but the population behaviour selection appears macroscopic mean value of the numerous individual behaviour. This paper points out that the internal driven force always leads the selection of individual behaviour to form complex disorder, while the external driven force always leads the selection of population behaviour to form social order. So, the organization soundness of actual social appearance depends on the action and negotiation of two driven forces (internal and external). One conclusion of the research is that the two forces restrict each other, compromise and finally achieve a balance to determine the ideal social behaviour selection schema. The paper draws the other conclusion:the probability of the internal and external forces is identically equal to 1. For the statistically sense, the Poisson Distribution can be used as a mathematical tool to judge the gap between ideal and real social behaviour selection. The standard of the ideal social behaviour selection is just the result of the equal 0.5 of two driven forces selection probability.