, we find consistent differences in behavior between the normal and extensive forms. In particular, we observe attempts to cooperate, and in some treatments we observe the achievement of cooperation, occurring more frequently in the extensive form. Cooperation in this context requires reciprocity, which is more difficult to achieve by means of intentionality detection in the normal as opposed to the extensive form games we study.
Our objective is to study behavior in the normal form representation of one of two extensive form bargaining games previously examined by using a variety of matching protocols (1). In the current study we limit the matching protocols to single play and to repeat play with the same pairs, and we study comparisons of the normal and extensive forms.Comparisons of behavior in the normal and extensive forms of various games have been made most notably in refs. 2 and 3. The latter's emphasis is on the rationality principles of iterated dominance and backward induction as factors in individual behavior in addition to whether behavior in a game is invariant to the form of its representation-all fundamental principles in game theory (4). Rapoport (2) and Schotter et al. (3) strongly rejected the invariance principle, but explication in terms of game theory was illusive: ''where we expected our rationality principles would predict behavioral differences across game forms, either no such differences appeared or they were not what we expected.' ' (ref. 3, pp. 446-447).Rapoport (2) provides transparent examples of two extensive form versions of the ''Battle-of-the Sexes'' game and the same game in matrix normal (strategic) form. His examples make clear how order-of-play information provides a principle that can better coordinate player strategies in the extensive forms. That principle, as we would describe it, derives from the human capacity to read another person's thoughts or intentions by placing themselves in the position and information state of the other person. Because of the example's transparency and special character, his experiments are conducted by using a three-person resource dilemma, a public good, and a pure coordination game.In our games we try to predict certain core features of the variation in behavior with the game form, but only by reaching outside of the traditional rationality principles of game theory to include concepts of reciprocity and ''mindreading,'' from evolutionary psychology. Our ultimate goal is to provide an empirical foundation for modeling this behavior in terms of distributions of types of player-some of whom have a disposition toward noncooperative behavior, whereas others are disposed toward cooperation.Our wellsprings are hardly new. The essence of our conceptual approach was stated 38 years ago: ''A normative theory must produce strategies that are at least as good as what people can do without them. More, it must not deny or expunge details of the game that can demonstrably benefit two or more players and that the players, consequently, should not expunge...