Self-Confirming Games: Unawareness, Discovery, and Equilibrium

Transcription

1 Self-Confirming Games: Unawareness, Discovery, and Equilibrium Burkhard C. Schipper Preliminary: April 6, 07 Abstract Equilibrium notions for games with unawareness in the literature cannot be interpreted as steady-states of a learning process because players may discover novel actions during play. In this sense, many games with unawareness are self-destroying as a player s representation of the game must change after playing it once. We define discovery processes where at each state there is an extensive-form game with unawareness that together with the players play determines the transition to possibly another extensive-form games with unawareness in which players are now aware of actions that they have previously discovered. A discovery process is rationalizable if players play extensive-form rationalizable strategies in each game with unawareness. We show that for any game with unawareness there is a rationalizable discovery process that leads to a self-confirming game that possesses an extensive-form rationalizable self-confirming equilibrium. his notion of equilibrium can be interpreted as steady-state of a learning and discovery process. Keywords: Self-confirming equilibrium, conjectural equilibrium, extensive-form rationalizability, unawareness, extensive-form games, equilibrium, learning, discovery. JEL-Classifications: C70, C7. I thank Aviad Heifetz and seminar participants at the University of oronto, the Barcelona workshop on Limited Reasoning and Cognition, SAE 05, and CSLI 06 for helpful discussions. Moreover, I am grateful for financial support through NSF SES Department of Economics, University of California, Davis. bcschipper@ucdavis.edu

2 Introduction How do players arrive at their conception(s) of a strategic situation? Game theory is mostly concerned with finding optimal behavior given a formal representation of the strategic situation. However, where do player s representations of the strategic situation come from? Shouldn t they be the result of discoveries in earlier strategic interaction? If this is the case, then the formal representation of the strategic situation should be a result of endogenous strategic interaction rather than an assumption. his is the main issue attacked in this paper. his view leads to further questions such as Need representations of the strategic situation be necessarily common among all players? or How to model discoveries of novel actions?. With regard to the former question, note that even in standard games of incomplete information the description of the strategic situation including all relevant uncertainties is shared by all players (and the analyst) and is thus common to all players. Players may have different information but all conceive of the same set of uncertainties, all players actions etc. With regard to the second question, game theory has been criticized previously as a formal apparatus that is incapable of modeling novelty and surprise (and consequently the lack of discoveries). For instance, Shackle (97, p. 6) wrote he heory of Games thus supposes the players to have knowledge of all the possibilities: surprise, the most powerful and incise element in the whole art of war, is eliminated by the theoretical frame itself; and novelty, the changing of what appeared to be the roles of the game, the continually threatening dissolution of the conditions and circumstances in which either player may suppose himself to be operating, is eliminated also, by the supposition that each player, like a chess player of super-human intellectual range, knows everything that can happen. We demonstrate that the language of game theory is sufficient rich to model novelty, surprise, transformative experiences, discoveries, shattering of player s views of the strategic situation etc. his paper takes a lot of inspiration from the literature on unawareness in games. In particular, our motivation is the quest for a natural notion of equilibrium to games with unawareness. Various frameworks for modeling dynamic games with unawareness have been recently introduced (Halpern and Rego, 04, Rego and Halpern, 0, Feinberg, 0, Li 008, Grant and Quiggin, 03, Heifetz, Meier, and Schipper, 03; for a non-technical survey, see Schipper, 04). While all of those frameworks are capable of modeling strategic interaction under asymmetric unawareness at various degrees of generality and tractability, the solution concepts proposed for those frameworks and thus the implicit behavioral assumptions under unawareness differ. he solution concepts that have been proposed in the literature can roughly be divided into equilibrium notions (Halpern and Rego, 04, Rego and Halpern, 0, Feinberg, 0, Li 008, Grant and Quiggin, 03, Ozbay, 007, Meier and Schipper, 03) and rationalizability notions (Heifetz, Meier, and Schipper, 03, 0, Meier and Schipper, 0). Authors proposing equilibrium notions to dynamic games with unawareness appear to be mainly guided

3 by extending the mathematical definitions of equilibrium in standard games to the more sophisticated frameworks with unawareness. Yet, I believe less attention has been paid to the interpretations of the behavioral assumptions embodied in these standard equilibrium concepts and whether or not such interpretations could meaningfully apply also to dynamic games with unawareness. In standard game theory, equilibrium is interpreted as an outcome in which each player plays optimally given the opponents play that could have emerged in a steady-state of some learning process. his interpretation cannot apply generally to game with unawareness. his is because players may be unaware of actions and may discover novel actions during play. he next time they play the game, they actually play a different game in which now they are aware of previously discovered actions. hat is, dynamic learning processes in games with unawareness must not only deal with learning about opponents play but also with discoveries that may lead to transformative changes in players representations of the game. Games with unawareness may be self-destroying representations of the strategic situation in the sense that (rational) play may destroy some player s representation of the strategic situation. Only when a representation of the strategic situation is self-confirming, i.e., (rational) play in such a game does not lead to (further) changes in the players representation of the game, an equilibrium notion as a steady-state of a learning process may be applied. Our paper makes this precise. We introduce a notion of self-confirming equilibrium to extensive-form games with unawareness. In self-confirming equilibrium players play in a way that nobody discovers that their own view of the game may be incomplete. Moreover, players play optimally given their beliefs and their beliefs are not falsified given the play. We show that such a self-confirming equilibrium may fail to exist in an extensive-form game with unawareness because rational play may lead to discoveries. We formalize the notion of discovered game: For any extensive-form game with unawareness and strategy profile, the discovered game is a game in which each player s awareness is updated given their discoveries but their information stays essentially the same (modulo awareness). his leads to a notion of a stochastic game in which states correspond to extensiveform games with unawareness and the transition probabilities model for each extensive-form game with unawareness and strategy profile the transition to the discovered game. Such a stochastic game and a Markov strategy that assigns to each extensive-form game with unawareness a mode of behavior we call a discovery process. We select among discovery processes by requiring the stochastic games Markov strategy to assign only rationalizable strategies to each extensive-form game with unawareness. We show that for every finite extensive-form game with unawareness, there exists an extensive-form rationalizable discovery process that leads to a extensive-form game with unawareness that is an absorbing state of the process. We consider it as a steady state of conceptions when players play with common (strong) belief in rationality and call it rationalizable self-confirming game. In such a game, it makes sense to look also for a steady state of behavior by focusing on self-confirming equilibrium involving only extensive- 3

4 form rationalizable strategies. We show that for every extensive-form game with unawareness there exists a rationalizable discovery process leading to a rationalizable self-confirming game that possesses a rationalizable self-confirming equilibrium. his is a notion of equilibrium both in terms of conceptions of the strategic situation as well as strategic behavior. he paper is organized as follow: In the next section, we illustrate our basic approach with a simple example. In Section 3 we outline the formal framework. Self-confirming equilibrium is defined in Section 4. Discovery processes are defined in Section 5. We introduce rationalizable discovery processes in Section 6. Section 7 contains our main result. Finally, Section 8 concludes with a discussion of related literature. Proofs are relegated to the appendix. Simple Illustrating Examples In this section, we introduce very simple examples in order to make the point that straightforward extensions of standard definitions of equilibrium to settings with unawareness may miss the essence of what is equilibrium. hey are hard to interpret as a steady-state of behavior. he examples will also serve as an illustration of our framework and the notion of equilibrium that we will propose for games with unawareness. Example here are two players, and. Player (e.g., the principal) moves first. She can either delegate to player (e.g., agent) or do the work by herself. In the latter case, the game ends and both players receive their payoffs. If player delegates to player, then player can take one out of three actions. So far, it sounds like a most basic two-stage principalagent problem. he non-standard but straightforward detail is that player is not aware of all of player s actions. She considers only two actions of player. his strategic situation is modeled in the game depicted in Figure. here are two trees. he tree at the bottom,, is a subtree of the tree at the top,, in the sense that action m of player is missing in. he information and awareness of both players is modeled with information sets. he solid-lined blue spheres and arrows belong to player, the dashed green spheres belong to player. here are two non-standard features of these information sets. First, the information set of a decision node in one tree may consist of decision nodes in a lower tree. For instance, player s information set at the beginning of the game in the upper tree is in the lower tree. his signifies the fact that initially player is unaware of player s action m and thus considers the strategic situation to be represented by the tree at the bottom,. Second, we added information sets at terminal nodes. he reason is that in order to discuss notions of equilibrium under unawareness, it will be useful to analyze also the players views at the end of the game. As usual, the information in extensive-form games is represented by information sets. Players receive a payoff at each terminal node. he 4

5 Figure : (Initial) Game of Example l r l r m, 7 0, 5 0, 0 5, 6 l r l r, 7 0, 5 5, 6 first component at each terminal node refers to player s payoff whereas the second component refers to player s payoff. What is equilibrium in this game? A basic requirement is that in equilibrium players should play rational. hat is, each player at each information set where (s)he is to move should play an action that maximizes her expected payoff subject to her belief over the opponent s behavior. At the beginning of the game, player thinks that she faces the situation depicted in tree. Clearly, with this mindset only action l is rational because no matter what she expects player to do, she obtains a higher expected payoff from playing l than from r. At the information set in the upper tree, player is aware of his action m. Since m strictly dominates any of his other actions, the only rational action for player at this information set is to choose m. hus, the path of play emerging from any rational play is (l, m ) with player obtaining zero payoff and player obtaining a payoff of 0. Yet, we strongly believe that no profile of rational strategies can be reasonably called an equilibrium in this setting because any profile of strategies in which player chooses l and player chooses m cannot be interpreted as a steady state of a learning process. After players choose rationally in the game, player s awareness changed. She discovered action m of player. his is symbolized by player s information set at the terminal node after m in the tree. hus, the next time players do not play the game of Figure but a discovered version of it in which player is aware of action m upfront. his discovered game is depicted in Figure. At the beginning of the 5

6 game, player s information set is now in the upper tree. Consequently she is aware of all actions of all players. She won t be surprised by any terminal node as her information sets at terminal nodes in the upper tree also lie in this tree. he lower tree becomes in some sense redundant as players are now commonly aware of the strategic situation modeled by. Yet, since they are aware, they can envision themselves also in a situation in which both players are unaware of m, which is what now represents although this counterfactual mindset is not behaviorally relevant. Figure : Game of Example after being played once l r m l r, 7 0, 5 0, 0 5, 6 l r l r, 7 0, 5 5, 6 In the discovered version shown in Figure, the only rational action for player at the beginning of the game is to choose r in. hus any steady state of a learning (and discovery) process must prescribe r for player in. o sum up, we note first that games with unawareness may not possess equilibria that can be interpreted as steady states of a learning process (see the game in Figure ). Second, an equilibrium notion capturing the idea of a steady-state of a learning (and discovery) process in games with unawareness must not only involve usual conditions on behavior of players but must also impose restrictions on representations of the strategic situation. hat is, their representations of the strategic situation must be consistent with their behavior and behavior must be consistent with their representations of the strategic situations. o emphasize this, we will use 6

7 the terminology of self-confirming games. he game of Figure is not a self-confirming game while the game of Figure is. When players play the game in Figure, no further changes of awareness are feasible. he representation of the game (together with rationality) induces the behavior and the behavior just confirms the representation. In contrast, when players play rationally in the game depicted in Figure then player discovers features of the game that she was previously unaware of. hat is, player s initial representation of the game is destroyed and a new version is discovered in which rational behavior differs from rational behavior in the initial version. Example he particular Example should not mislead the reader to believe that selfconfirming games must involve common awareness of the strategic situation and that rational discovery would justify restricting the focus to standard games like given by tree in Figure. One can easily extend the example to discuss a situation in which the self-confirming game involves understandings of the strategic situation that differ by players. For instance, Figure 3 depicts a slightly more complicated version of the prior example in which initially each player is aware of an action of which the other player is unaware. Note first that trees and together with their information sets are just as in Figure. rees and are similar but contain an additional action for player (indicated by red edges). Initially, player is aware of action but unaware of action m. his is indicated by the blue arrow that leads from the initial node in tree to the blue information set containing the initial node of tree. In contrast, player is initially unaware of action but aware of his action m. his is shown by green intermitted arrows from his nodes after history l and r in tree to the green intermitted information set containing the analogous node in tree. It is easy to see that for player, action is strictly dominated. hus, she will never use it in any kind of equilibrium. Consequently, player won t be able to discover it and will remain unaware of it. ogether with arguments about rational play in Example, it implies that after the game is rationally played once by both players, the representation must change to the one depicted in Figure 4. In this discovered version, player is aware of both actions and m (i.e., she lives in tree ). his is indicated by the blue information sets in the upmost tree, which are different from Figure 3. Player realizes that player remains unaware of and believes that player views the strategic situation as represented by and. Rational play is as in the game of Figure. hus, player won t become aware of and differences in players awareness persist despite the fact that the game of Figure 4 is self-confirming. Last situation is probably symptomatic for most strategic situations in reality. Players interact with different models of the game and settle in a steady-state of behavior that does his terminology may sound odd at the first glance as in standard game theory, the representation of the strategic situation is given and the behavior is endogenous. But the point is precisely that in our setting the representation of the strategic situation becomes endogenous too. 7

8 Figure 3: (Initial) Game of Example l r m 0, 5 0, 0 5, 6 * l r, 7 -, 8 0 l r 00 -, 8 l r * l r m, 7 l r, 7 0, 5 0, 0 5, 6 0, 5 5, 6 l r l r, 7 0, 5 5, 6 expoint fonts used in EMF. not allow them to learn (or more precisely discover) that they may use different models. he game in Figure 4 is not just a self-confirming game. It is a rationalizable self-confirming game because it is a discovered version of the game in Figure 3 after players played rationally. If players do not play rationally, they may discover other versions that are self-confirming. For instance, the game in Figure 5 is a discovered version of the game in Figure 3 after player played once. Note that is not rationalizable for player to play. hus, this discovered version is not rationalizable. It is also not self-confirming because when player learns to play rationally in the game of Figure 5, player gets to play and player would rationally chose m. Consequently, player would discover that player has action m. he discovered version is depicted in Figure 6. his game is also self-confirming since no further action could be discovered. But it is not a rationalizable self-confirming game because the discoveries required to evolve views from Figures 3 or 5 to 6 cannot be rationalized. 3 Extensive-Form Games with Unawareness In this section, we outline extensive-form game with unawareness as introduced by Heifetz, Meier, and Schipper (03) together with some crucial extensions required for our analysis. 8

9 Figure 4: Game of Example after being rationally played once l r m 0, 5 0, 0 5, 6 * l r, 7 -, 8 0 l r 00 -, 8 l r * l r m, 7 l r, 7 0, 5 0, 0 5, 6 0, 5 5, 6 l r l r, 7 0, 5 5, 6 expoint fonts used in EMF. o define a extensive-form game with unawareness Γ, consider first, as a building block, a finite game with perfect information and possibly simultaneous moves. his tree is there to outline all physical moves. here is a finite set of players I and possibly a special player nature with index 0. We denote by I 0 the set of players including nature. Further, there is a nonempty finite set of decision nodes D and a player correspondence P : D I0 \ { } that assigns to each node n D, a nonempty set of active players P (n) I 0. (hat is, we allow for simultaneous moves.) For every decision node n D and player i P (n) who moves at that decision node, there is a nonempty finite set of actions A i n. Moreover, there is a set of terminal nodes Z. Since we will also associate information sets with terminal nodes for each player, it will be useful to extent P to Z by P (z) = I and let A i z = for all i I, z Z. Finally, each terminal node z Z is associated with a vector of payoffs (u i (z)) i I. We require that nodes in N := D Z constitute a tree denoted by. hat is, nodes in N are partially ordered by a precedence relation with which ( N, ) forms an arborescence (that is, the predecessors of each node in N are totally ordered by ), there is a unique node in N with no predecessors (i.e., the root of the tree), for each decision node n D there is a bijection ψ n between the action profiles i P (n) Ai n at n and n s immediate successors, and any terminal node in Z has no successors. Note that so far we treat nature like any other player except that at terminal nodes we do 9

10 Figure 5: Non-rationalizable Discovered Version of the Game in Example l r m 0, 5 0, 0 5, 6 * l r, 7 -, 8 0 l r 00 -, 8 l r * l r m, 7 l r, 7 0, 5 0, 0 5, 6 0, 5 5, 6 l r l r, 7 0, 5 5, 6 expoint fonts used in EMF. not assign payoffs to nature. We do not need to require that nature moves first or that nature moves according a pre-specified probability distribution (although these assumptions can be imposed in our framework). Consider now a join-semilattice of subtrees of. 3 A subtree is defined by a subset of nodes N N for which (N, ) is also a tree (i.e., an arborescence in which a unique node has no predecessors). wo subtrees, are ordered, written if the nodes of constitute a subset of the nodes of. We require three properties:. All the terminal nodes in each tree are in Z. hat is, we don t create new terminal nodes.. For every tree, every node n, and every active player i P (n) there exists a nonempty subset of actions A i, n A i n such that ψ n maps the action profiles A n = i P (n) Ai, n bijectively onto n s successors in. Alternatively, we could assign at every terminal node the same payoff to nature. 3 A join semilattice is a partially order set in which each pair of elements has a join, i.e., a least upper bound. 0

11 Figure 6: Non-rationalizable Self-confirming game in Example l r m 0, 5 0, 0 5, 6 * l r, 7 -, 8 0 l r 00 -, 8 l r * l r m, 7 l r, 7 0, 5 0, 0 5, 6 0, 5 5, 6 l r l r, 7 0, 5 5, 6 expoint fonts used in EMF. 3. If for two decision nodes n, n with i P (n) P (n ) it is the case that A i n A i n, then A i n = A i n. Within the family of subtrees of, some nodes n appear in several trees. In what follows, we will need to designate explicitly appearances of such nodes n in different trees as distinct entities. o this effect, in each tree label by n the copy in of the node n N whenever the copy of n is part of the tree, with the requirement that if the profile of actions a n A n leads from n to n, then a n leads also from the copy n to the copy n. More generally, for any,, with such that n, n is the copy of n in the tree, n is the copy of n in the tree, and (n ) is the copy of n in the tree, we require that nodes commute, n = (n ). For any and any n, we let n := n. Denote by D the union of all decision nodes in all trees, by Z the union of terminal nodes in all trees, and by N = D Z. Copies n of a given node n in different subtrees are now treated distinct from one another, so that N is a disjoint union of sets of nodes. In what follows, when referring to a node in N we will typically avoid the subscript indicating the tree for which n when no confusion arises. For a node n N we denote by n the tree containing n. 4 4 Bold capital letters refer to sets of elements across trees.

12 Denote by N the set of nodes in the tree. Similarly, denote by Di the set of decision nodes in which player i is active in the tree. Finally, denote by Z the set of terminal nodes in the tree. Information sets model both information and awareness. At a node n of the tree n, the player may conceive the feasible paths to be described by a different (i.e., less expressive) tree. In such a case, her information set will be a subset of rather than of n and n will not be contained in the player s information set at n. In order to define a notion of self-confirming equilibrium we also need to consider the player s view at terminal nodes. hus we will also devise information sets of terminal nodes that model both the player s information and awareness at the ends of the game. his is different from Heifetz, Meier, and Schipper (03) but akin to signal, outcome, or feedback functions in some works on self-confirming equilibrium, see for instance Battigalli and Guaitoli (997) and Battigalli et al. (05). Formally, for each node n N (including terminal nodes in Z), define for each active player i P (n) a nonempty information set h i (n) with the following properties: 5 U0 Confined awareness: If n and i P (n), then h i (n) with. U Generalized reflexivity: If, n, h i (n) and contains a copy n of n, then n h i (n). I Introspection: If n h i (n), then h i (n ) = h i (n). I3 No divining of currently unimaginable paths, no expectation to forget currently conceivable paths: If n h i (n) (where is a tree) and there is a path n,..., n such that i P (n ) P (n ), then h i (n ). I4 No imaginary actions: If n h i (n), then A i n A i n. I5 Distinct action names in disjoint information sets: For a subtree, if there a decision nodes n, n D with A i n = A i n, then h i (n ) = h i (n). I6 Perfect recall: Suppose that player i is active in two distinct nodes n and n k, and there is a path n, n,..., n k such that at n player i takes the action a i. If n h i (n k ), n n k, then there exists a node n n and a path n, n,..., n l = n such that h i (n ) = h i (n ) and at n player i takes the action a i. I7 Information sets consistent with own payoff information: For any i I, if h i (z) then h i (z) Z. Moreover, if z h i (z) then u i (z ) = u i (z). 5 he numbering is consistent with Heifetz, Meier, and Schipper (03).

13 Properties (I), (I4), and (I5) are standard for extensive-form games, and properties (U0), (U), and (I6) generalize standard properties of extensive-form games to our generalized setting. At each information set of a player, property (I3) confines the player s anticipation of her future view of the game to the view she currently holds (even if, as a matter of fact, this view is about to be shattered as the game evolves). (I7) is new. It makes information sets of terminal nodes akin to feedback functions in the literature on self-confirming equilibrium. At any terminal node, a player considers only terminal nodes. hat is, she knows that the game ended. Moreover, any two terminal nodes that a player cannot distinguish must yield her the same payoff because otherwise she could use her payoffs to distinguish among these terminal nodes. his implies that at the end of the game each player knows her own payoff. Note that this assumption does not rule out imperfect observability of opponents payoffs. It also does not rule out that the player may not perfectly observe the terminal node. Heifetz, Meier, and Schipper (03) already illustrated properties I to I6 with graphic examples. In Figure 7 we illustrate properties U0-U. hese properties have already been discussed in Heifetz, Meier, and Schipper (03). For the illustration of I7 in Figure 7 assume that the player moving at the node that is immediately preceding the terminal nodes is the player whose payoffs are indicated by the first component of the payoff vectors that are attached to the terminal nodes. Figure 7: Properties U0, U, and I7 U0 n n n n U n n n n I7,,,,, 3 h (z),, h (z) We denote by H i the set of i s information sets in all trees. For an information set h i H i, we denote by hi the tree containing h i. For two information sets h i, h i in a given tree, we say that h i precedes h i (or that h i succeeds h i) if for every n h i there is a path n,..., n in 3

14 such that n h i. We denote it by h i h i. he following property is implied by I and I4 (see Heifetz, Meier, and Schipper, 03, Remark ): If n, n h i where h i = h i (n) is an information set, then A i n = A i n. If n h i we write also A hi for A i n. Properties U0, U, I, and I6 imply no absent-mindedness (see Heifetz, Meier, and Schipper, 03, Remark ): No information set h i contains two distinct nodes n, n on some path in some tree. he perfect recall property I6 and no absent-mindedness guarantee that with the precedence relation player i s information sets H i form an arborescence: For every information set h i H i, the information sets preceding it {h i H i : h i h i } are totally ordered by. Confined awareness (U0) and Perfect recall (I6) imply that a player cannot become unaware during the play (see Heifetz, Meier, and Schipper, 03, Remark 6). Awareness may only increase along a path. Formally, if there is a path n,..., n in some subtree such that player i is active in n and n, and h i (n) while h i (n ), then. o model unawareness proper, we impose as in Heifetz, Meier, and Schipper (03) additional properties. Different from that earlier paper, these properties are now also applied to information sets at terminal nodes. hey parallel properties of static unawareness structures in Heifetz, Meier, and Schipper (006): U4 Subtrees preserve ignorance: If, n, h i (n) and contains the copy n of n, then h i (n ) = h i (n). U5 Subtrees preserve knowledge: If, n, h i (n) and contains the copy n of n, then h i (n ) consists of the copies that exist in of the nodes of h i (n). It is known that U5 implies U3, see Heifetz, Meier, and Schipper (03, Remark 3): U3 Subtrees preserve awareness: If n, n h i (n),, and contains a copy n of n, then n h i (n ). Properties U3 to U5 are illustrated in Figure 8 with an example and counterexample each. For trees, we denote whenever for some node n and some player i P (n) it is the case that h i (n). Denote by the transitive closure of. hat is, if and only if there is a sequence of trees,,..., satisfying. For instance, in Figure 3 we have and. Clearly,. An extensive-form game with unawareness Γ consists of a join-semilattice of subtrees of a tree satisfying properties 3 above, along with information sets h i (n) for every n with and i P (n), and payoffs satisfying properties U0, U, U4, U5, and I-I7 above. 4

15 Figure 8: Properties U3 to U5 U3 U4 U5 For every tree, the -partial game is the join-semisublattice of trees including and all trees in Γ satisfying, with information sets as defined in Γ. A -partial game is a extensive-form game with unawareness, i.e., it satisfies all properties 3, U0, U, U4, U5, and I-I7 above. For instance, in Figure 3 the sublattice {, } together with all information sets in those trees forms the -partial game. In fact, it is the game with unawareness of Figure. We denote by Hi the set of i s information sets in the -partial game,. his set contains not only i s information sets in the tree but also in all trees with. Further, we denote by Hi D (H,D i, resp.) the set of i s information sets of decision nodes (in the -partial game, resp.) and by Hi Z (H,Z i, resp.) the set of i s information sets of terminal nodes (in the -partial game, resp.). 3. Strategies For any collection of sets (X i ) i I 0 we denote by X = X i, X i = i I 0 X j, X i0 = j I 0 \{i} j I\{i} X j 5

16 with typical elements x, x i, and x i0, respectively. For any collection of sets (X i ) i I 0 and any tree, we denote by Xi the set of objects in X i restricted to the tree and analogously for X, X i, and X i0, where restricted to the tree will become clear from the definitions below. A pure strategy for player i s i S i := h i H D i A hi specifies an action of player i at each of her information sets h i Hi D s 0 S 0 := n D 0 A 0 n of decision nodes. We let denote the strategy of nature, with D 0 denoting the decision nodes of nature. With the strategy s i, at node n D n i define player i s action at n to be s i (h i (n)), for i I. hus, by U and I4 the strategy s i specifies what player i I does at each of her active nodes n D n i, both in the case that n h i (n) and in the case that h i (n) is a subset of nodes of a tree which is distinct from the tree n to which n belongs. In the first case, when n h i (n), we can interpret s i (h i (n)) as the action chosen by player i in node i. In the second case, when n / h i (n), s i (h i (n)) cannot be interpreted as the action chosen consciously by player i in n since he is not even aware of n. Instead, his state of mind at n is given by his information set h i (n) in a tree lower than n (denoted by hi ). hus, s i (h i (n)) is the physical move of player i in n in tree n induced by his consciously chosen action at his information set h i (n) in tree hi (n) (with n hi (n)). As an example, consider player in the game of Figure. At his first decision node in the upper tree, the root of the tree, player s information set consists of the corresponding node in the lower tree. the strategy of player may assign r to his information set in the lower tree. But it also induces action r at the root of the upper tree. In an extensive-form game with unawareness Γ the tree represents the physical paths in the game; every tree in that contains an information set represents the subjective view of the feasible paths in the mind of a player, or the view of the feasible paths that a player believes that another player may have in mind, etc. Moreover, as the actual play in unfolds, a player may become aware of paths of which she was unaware earlier, and the way she views the game may alter as well. hus, in an extensive-form game with unawareness, a strategy cannot be conceived as an ex ante plan of action. Formally, a strategy of player i is a list of answers to the questions what would player i I do if h i were the set of nodes she considered as possible?, for h i H i (and analogous for nature). A strategy of a player becomes meaningful as an object of beliefs of other players. How much of a player s strategy other players can conceive off depend on their awareness given by the tree in which their information set is located. his leads to the notion of -partial strategy. For a strategy s i S i and a tree, we denote by 6

17 s i the strategy in the -partial game induced by s i (i.e., s i (h i) = s i (h i ) for every information set h i H i of player i in the -partial game). A mixed strategy of player i, σ i (S i ), specifies a probability distribution over player i s set of pure strategies. With this notation, we let σ 0 the probability distribution over strategies of nature. We don t consider mixed strategies as an object of choice of players; this notion will just be used in proofs in a technical way. A behavioral strategy for player i I, π i Π i = h i H D i (A hi ) is a collection of independent probability distributions, one for each of player i s information sets h i Hi D of decision nodes, where π i (h i ) specifies a mixed action in (A hi ). With the behavioral strategy π i, at node n D n i define player i s mixed action at n to be π i (h i (n)). hus, the behavioral strategy π i specifies the mixed action of player i I at each of her active decision nodes n D n i, both in the case that n h i (n) and in the case that h i (n) is a subset of nodes of a tree which is distinct from the tree n to which n belongs. In this latter case, we have automatically that π i does not assign probabilities to actions in A n \ A hi (n). (I.e., at the decision node n of the richer tree n player i may have more actions than she is aware of at h i (n). In such a case, she is unable to use actions that she is unaware of.) In extensive-form games with unawareness there are two distinct notions of a strategy profile being consistent with a node that we call reaching a node and visiting a node, respectively. he difference between these two notions is relevant when we consider information sets that players believe are consistent with a strategy and information sets that are actually consistent with a strategy. Former is relevant for extensive-form rationalizability while latter is relevant for self-confirming equilibrium. We say that a strategy profile s = (s j ) j I S reaches a node n if the players actions ( ) and nature s moves s j (h j (n )) in nodes j P (n ) n lead to n. Notice that by property (I4) ( no imaginary actions ), s j (h j (n )) j I is indeed well defined: even if h j (n ) / for ( ) some n, s j (h j (n )) is a profile of actions which is actually available in to the j P (n ) active players j P (n ) and possibly nature at n. We say that a strategy profile s S reaches the information set h i H i if s reaches some node n h i. We say that the strategy s i S i reaches the information set h i if there is a strategy profile s i S i of the other players (and possibly nature) such that the strategy profile (s i, s i ) reaches h i. Otherwise, we say that the information set h i is excluded by strategy s i. Analogously, we say that the strategy profile s i S i reaches the information set h i if there exists a strategy s i S i such that the strategy profile (s i, s i ) reaches h i. For each player i I, denote by H i (s) the set of information sets of i that are reached by the strategy profile s. his set typically contains information sets in more than one tree. 7

18 For the second notion of a strategy profile being consistent with a node, recall that a strategy s i specifies not only what player i I does at nodes n h i (n) but also in node n where n might be located in a tree more expressive than hi (n). We say that a strategy profile s = (s j ) j I S visits a node n in the upmost tree if the players actions and nature s moves (s j (h j (n ))) j P (n ) in nodes n lead to n. We extend the notion of a strategy profile visiting a node to any node in any tree by saying that a strategy profile s = (s j ) j I S visits a node n if s visits n with with n = n. We say that a strategy profile s S visits the information set h i H i if s visits some node n h i. We say that the strategy s i S i visits the information set h i if there is a strategy profile s i S i of the other players (and possibly nature) such that the strategy profile (s i, s i ) visits h i. Analogously, we say that the strategy profile s i S i visits the information set h i if there exists a strategy s i S i such that the strategy profile (s i, s i ) visits h i. Define the path p(s, ) induced by strategy profile s in tree by the sequence of nodes in visited by s. Further, for any strategy profile s and tree, define H i (p(s, )) := {h i H i : h i = h i (n) for n on the path p(s, ) with i P (n)}. Note that information sets in H i (p(s, )) may lie in different trees weakly less expressive than. Figure 9: Information Sets Visited versus Reached o clarify the subtle but important difference between the notions of visit and reach as well as the definitions of Hi (p(s, )) and H i (s) consider the example in Figure 9. 6 here are two trees,. here are two players, and. Player moves first. As long as he moves right, player moves second and the game ends. Otherwise the game ends. When player moves left, player remains unaware of her action middle. his is shown in Figure 9 by the blue arrows and ovals (i.e., information set h in ) upon player moving left. Otherwise, if player moves right, player becomes aware of middle (information set h ). (Player s initial information sets are indicated by green intermitted ovals.) Consider the strategy of player indicated by 6 For implicitly, we neglect payoffs and information sets of player at terminal nodes in this example. 8

19 the red intermitted edges. his strategy reaches h but visits h in. Information set h in tree is neither reached nor visited by the strategy. Let s denote any strategy profile in which player follows the strategy indicated by the red intermitted edges. We have h H (p(s, )), h H (p(s, )), h, h / H (s), h H (s), h, h / H (p(s, )), h, h / H (p(s, )). It should be clear that in a standard extensive-form game (without unawareness), a strategy profile reaches a node n if and only if it visits n. We extend the definitions of information set reached and visited to behavioral strategies in the obvious way by considering nodes/information sets reached/visited with strict positive probability. Similarly, we let p(π, ) denote now the set of paths that have strict positive probability under the behavioral strategy profile π in. Hi (p(π, )) is now the set of information sets along paths in p(π, ). In standard games without unawareness but with perfect recall, Kuhn s theorem asserts that for every mixed strategy profile there is an equivalent behavioral strategy profile. Kuhn s theorem can be extended to extensive-form games with unawareness using a notion of equivalence based on the notion of reaching nodes. For any node n, any player i I, and any opponents profile of strategies s i (including nature if any), let ρ(n π i, s i ) and ρ(n σ i, s i ) denote the probability that (π i, s i ) and (σ i, s i ) reach node n, respectively. For any player i I 0, a mixed strategy σ i and a behavioral strategy π i are equivalent if for every profile of opponents strategies s i S i and every node n N of the extensive-form game with unawareness ρ(n σ i, s i ) = ρ(n π i, s i ). Schipper (07) proves an extension of Kuhn s heorem according to which in every extensive-form game with unawareness (with perfect recall), for every player and for every mixed strategy of that player there exists an equivalent behavioral strategy. 3. Belief Systems A belief system of player i, β i = (β i (h i )) hi H i ( is a profile of beliefs a belief β i (h i ) S h i i h i H i ) ( S h i i ) about the other players strategies (and possibly nature) in the hi -partial game, for each information set h i H i, with the following properties: β i (h i ) reaches h i, i.e., β i (h i ) assigns probability to the set of strategy profiles of the other players (including possibly nature) that reach h i. If h i precedes h i (i.e., h i h i ) then β i (h i ) is derived from β i (h i ) by Bayes rule whenever possible. 9

20 Note that different from Heifetz, Meier, and Schipper (03) a belief system specifies also beliefs about strategies of opponents and nature at information sets of terminal nodes. his is an essentially feature that we will require for defining self-confirming equilibrium. Denote by B i the set of player i s belief systems. 7 For a belief system β i B i, a strategy s i S i and an information set h i H i, define player i s expected payoff at h i to be the expected payoff for player i in hi given β i (h i ), the actions prescribed by s i at h i and its successors, assuming that h i has been reached. We say that with the belief system β i and the strategy s i player i is rational at the information set h i Hi D if either s i does not reach h i or there exists no strategy s i which is distinct from s i only at h i and/or at some of h i s successors in hi and yields player i a higher expected payoff in the hi -partial game given the belief β i (h i ) on the other players strategies S h i i. his extends the definition of rationality in Pearce (984) or Battigalli (997) to games with unawareness. Player i s belief system on behavioral strategies of opponents µ i = (µ i (h i )) hi H i (Π hi i ) h i H i is a profile of beliefs a belief µ i (h i ) (Π h i i ) about the behavioral strategies of other players (incl. possibly nature) in the hi -partial game, for each information set h i H i, with the following properties µ i (h i ) reaches h i, i.e., µ i (h i ) assigns probability to the set of behavioral strategy profiles of the other players (incl. possibly nature) that reach h i. If h i precedes h i (i.e., h i h i ) then µ i (h i ) is derived from µ i (h i ) by Bayes rule whenever possible. We denote by M i the set of player i s belief systems over behavioral strategies of opponents. For a belief system µ i M i, a behavioral strategy π i Π i and an information set h i H i, define player i s expected payoff at h i to be the expected payoff for player i in hi given µ i (h i ), the mixed actions prescribed by π i at h i and its successors, assuming that h i has been reached. We say that with the belief system µ i and the behavioral strategy π i player i is rational at the information set h i Hi D if either π i does not reach h i or there exists no behavioral strategy π i which is distinct from π i only at h i and/or at some of h i s successors in hi and yields player i a higher expected payoff in the hi -partial game given the belief µ i (h i ) on the other players behavioral strategies Π h i i. 7 In some applications, we may want to fix prior beliefs about moves of nature. In such a case, we would consider B i to consist only of belief systems in which for every belief the marginal on nature is consistent to the fixed prior belief about moves of nature. 0

21 4 Self-Confirming Equilibrium he discussion of the example in Section in the introduction made clear that the challenge for a notion of equilibrium is to deal with changes of awareness along the equilibrium paths. In a steady state of conceptions, awareness should not change. We incorporate this requirements into our definition of self-confirming equilibrium. For simplicity, we first consider a notion of self-confirming equilibrium in pure strategies. Definition (Self-confirming equilibrium in pure strategies) A strategy profile s S is a self-confirming equilibrium if for every player i I: (0) Awareness is self-confirming along the path: here is a tree such that for all of player i s visited information sets h i H i (p(s, )) we have h i. here exists a belief system β i B i such that (i) Players are rational along the path: With belief system β i, strategy s i is rational at all visited information sets in H i (p(s, )). (ii) Beliefs are self-confirming along the path: For the information set of terminal nodes h i Hi Z H i (p(s, )) visited by the strategy profile s, the belief system β i is such that β i (h i ) assigns probability to the subset of profiles of opponents and nature s strategies of S h i i that reach h i. Moreover, for any preceding (hence non-terminal) information set h i h i, β i (h i ) = β i(h i ). Condition (0) requires that awareness is constant along the equilibrium path. Players do not discover anything novel in equilibrium play. his is justified by the idea of equilibrium as a stationary rest-point or stable convention of play. Implicitly, it is assumed that discoveries if any are made before equilibrium is reached. Condition (i) is a basic rationality requirement of equilibrium. Note that rationality is required only along information sets that occur along the path of play induced by the equilibrium strategy profile. he equilibrium notion is silent on off-equilibrium information sets (in particular on information sets that could be visited with s i but are not visited with s i ). Condition (i) does also not require that players believe others are rational along the path, believe that others believe that etc. It is just a minimal rationality requirement in an extensive-form game. Condition (ii) consists of two properties. First, at the end of the game the player is certain of strategies of opponents and nature that allow her to reach the particular end of the game. hat is, terminal beliefs are consistent with what has been observed during play (and hence at the end of the play). Second, beliefs do not change during the play. hat is, beliefs at any information set reached during the play are consistent with what is observed at any point

22 during the play and in particular with what is observed at the end of the game. Again, the idea is that everything that could have been learned on this path has been learned already in the past. his is justified by the idea of equilibrium as a stationary rest-point or stable convention of play as a result of prior learning. Note that this notion of equilibrium is silent on beliefs off equilibrium path. 8 It should be obvious that pure self-confirming equilibria may not exist even in standard games (i.e., consider as a simple counterexample the matching pennies game which fits our framework as we allow for simultaneous moves). hus, we consider also an analogous notion of self-confirming equilibrium in behavioral strategies. Definition (Self-confirming equilibrium in behavioral strategies) A behavioral strategy profile π Π is a self-confirming equilibrium if for every player i I: (0) Awareness is self-confirming along the path: here is a tree such that for all of player i s visited information sets h i H i (p(π, )) we have h i. here exists a belief system 9 µ i M i such that (i) Players are rational along the path: With belief system µ i, behavioral strategy π i is rational at all visited information sets in H i (p(π, )). (ii) Beliefs are self-confirming along the path: For the information set of terminal nodes h i Hi Z H i (p(π, )) visited by the behavioral strategy profile π, the belief system µ i is such that µ i (h i ) assigns probability to {π i Π i : π j (h j) = π j (h j ) for j I 0 \ {i} and h j H j (p(π, hi ))}. Moreover, for any preceding (hence non-terminal) information set h i h i, µ i (h i ) = µ i(h i ). he interpretation of properties (0) to (ii) is analogous to previous Definition. For property (ii), note that {π i Π i : π j (h j) = π j (h j ) for j I 0 \ {i} and h j H j (p(π, hi ))} is the set of behavioral strategy profiles of opponents of player i and nature that are behaviorally indistinguishable from π at all information sets conceived by i and found relevant by i along actual paths of play induced by π. 8 Note that in self-confirming equilibrium profile s it is not the case that if s visits information h i then s reaches it. For instance, in the game of Figure 9 player s strategy indicated by the red intermitted edges may be his equilibrium strategy. Although h is visited with this strategy, it is not reached with this strategy. By Definition (ii) player may believe at h that player uses a strategy that assigns left to his information set in. But this belief is never falsified by information received in equilibrium. 9 Note that we do not require that player i believes that opponents mix independently. his is because we don t believe this assumption is easy to motivate. he literature on self-confirming equilibrium knows both assumptions. For instance, independence is assumed in Fudenberg and Levine (993) but not in Rubinstein and Wolinsky (994).