In this paper, genetic programming is employed to explore the signicance of speculative activities in economic theory. Unlike most previous studies, this paper explicitly take interaction of speculators into account. Through genetic programming, this interaction processes is modeled as a competitive process which applies the survival-of-the-ttest principle to the selection of trading strategies. There are two interesting ndings which make this paper distinctive. Firstly, while markets without speculators cannot be ecient, market with speculators can be even worse if appropriate trading restrictions are absent. Therefore, to induce speculators to play a stabilizing role, the design of trading restrictions is crucial. Secondly, it is also found that selection scheme can be relevant for the results achieved. However, from the viewpoint of a decentralized network economy, it is argued that tournament selection is preferred to proportionate selection. Key Words: Genetic Programming, Ec..