According to some theoretical research in social dilemmas (SD), “the trigger strategy”, in which an actor cooperates as long as all other members cooperate in an iterated SD, is very effective in solving the free-rider problem. When all members adopt“the trigger strategy”, cooperation is reasonable, because as soon as someone defects, all other members immediatly defect. Thus, no one can be a free-rider. The purpose of this research was to test this theoretical hypothesis experimentally. In short, we tested whether members came to adopt strategies like“the trigger strategy”in an iterated SD. The results of this experiment supported the hypothesis. The most basic strategy of all submitted strategies was“GENERALIZED TIT-FOR-TAT”, in which an actor decides between cooperation or defection based on the number of cooperators in the past trial. As the experiment continued, this“GENERALIZED TIT-FOR-TAT”strategies came to be like“the trigger strategy”. This transformation may be explained by the transformation of“elementary cooperation”into“instrumental cooperation” (Yamagishi, 1986).