  We present results of the prisoner's dilemma game on complex networks that
have population change. We introduce a death process with minimum requirements
and show that it induces a highly cooperative society. We also study the
effects of immigration on the society. We show that the acceptable number of
immigrants of the society is determined by the population of the society, the
ratio of cooperator among immigrants, and the immigration interval. In
addition, if immigrants have a preferential attachment link, the acceptable
number of immigrants increases.
