1 Introduction
Traditionally, economists have focused on a rational decision maker – the “homo economicus” – to model human behavior. The observation of various deviations of behavior from the benchmark of optimizing rational decision making has motivated an entire field, behavioral economics. Research in this field has identified a plethora of different, partly distinct and partly interacting, behavioral biases, which are related to cognitive limitations, stress, limited memory, preference anomalies, and social interactions, among others. These biases are typically established by comparing actual behavior against a theoretical benchmark, often in simplistic, unrealistic, or abstract settings that are unfamiliar to the decision makers. Field evidence for behavioral biases among professionals is still scarce, mostly because of the difficulty to establish a rational benchmark in complex realworld settings. Consequently, most contributions focus on documenting a behavioral deviation in one particular dimension. This makes it often difficult to compare the behavioral biases documented in the literature. Moreover, deviations from rational behavior are usually seen as being related to suboptimal performance. However, this connotation often rests on a priori reasoning or value judgments because it is typically even harder or impossible to identify the consequences of deviations from the rational benchmark than the deviations themselves.
This paper breaks new ground in the documentation of behavioral deviations from the rational benchmark, as well as of their causes and consequences, by investigating the behavior of professional chess players. Conceptually, chess is ideally suited to address the research question about the typology and emergence of behavioral deviations from the rational benchmark. First, chess provides a clean and transparent decision environment that allows observing individual choices in a sequential game of perfect information. Second, chess players are typically seen as the prototypes of rational, forwardlooking strategic behavior. This is even more the case for professional chess players participating in tournaments with high stakes and incentives to win a game. Third, chess provides a unique source of information about behavior at extremely high resolution and accuracy. Fourth, the availability of chess engines makes it possible to construct a clean rational benchmark for behavior.
We develop a new methodology for identifying deviations from rational behavior as well as their implications for performance that makes use of artificial intelligence techniques embodied in modern chess engines. This methodology is based on the fact that chess engines provide a detailed quantitative and objective evaluation of a given configuration of pieces on the board in terms of the associated winning odds for each player, and of the complexity of a decision in terms of a precise measure of the difficulty of identifying the optimal move in a given configuration. Importantly, the logic of chess engines is based on the notion of mutually best responses, thus applying rational decision making. Using this, we compare the actual moves of human decision makers to the best conceivable move in the respective configuration. This best conceivable move is determined by a “super chess engine” whose performance exceeds that of the best humans by far. To construct a rational benchmark, we also replicate each configuration observed in a large data set of chess games and determine the decision of a “restricted chess engine” that still plays mutual best response, but that is comparable in terms of playing strength to human players and simulates play against another chess engine of similar strength. Like the human decisions, these replicated decisions of the restricted chess engine are then evaluated in comparison to the moves suggested by the super chess engine. This makes it possible to identify deviations of human behavior from the rational benchmark, as well as the consequences of these deviations for performance using withinperson variation at the level of individual moves.
The comparison of the relative performance of humans to that of a comparably strong chess engine identifies deviations from fully rational behavior by decision makers that are experts in the respective decision environment. This methodology allows us to investigate the exact circumstances and factors that lead individual players to deviate from rational behavior. In particular, the detailed information about the evaluation of a given configuration, the time left for decision making, complexity, and the time used for a given move provides a unique possibility to decompose different candidates for behavioral biases within the same person and game. Moreover, the difference between the relative performance of humans in comparison to the super chess engine and the relative performance of the restricted engine in comparison to the super chess engine shed light on the consequences of the behavioral deviations on performance. This allows us to investigate not only whether humans behave differently, but whether they perform better, compared to the rational benchmark, and under which circumstances.
The results document several systematic deviations of human behavior from the rational benchmark. These deviations can be related to different behavioral biases that have been discussed in the previous literature. In particular, we find systematic deviations of human behavior from the rational benchmark in relation to the current standing reflected in terms of an advantage or disadvantage. Being in a better position induces deviations from the rational benchmark that are associated with worse performance than stipulated by the rational benchmark, while being in a worse position is associated with more deviations that are associated with higher performance. A smaller remaining time budget in the game leads to more frequent deviations and worse performance, suggestive of the detrimental effects of time pressure. We also find evidence for the role of fatigue over the course of a game, which reduces the likelihood of deviations with better performance. Stress induced by cognitive limitations in the context of complex configurations leads to more frequent deviations from the rational benchmark but not to a systematic deterioration in performance.
When investigating the mechanisms, we find no systematic differences in the causes and consequences of behavioral deviations from the benchmark between weaker and stronger players. Strategic interactions or psychological factors, as reflected by the remaining time of the opponent, seem of limited importance. An analysis of decision times reveals that faster decisions are associated with more frequent deviations from the rational benchmark, but at the same time are associated with better performance. The evidence is suggestive of a superior assessment of humans, which is presumably related to intuition and experience.
Contribution to the Literature.
The results of this paper contribute to a substantial literature that has used chess as the prime example of how to think about and model rational behavior. Analyses of optimal strategic behavior in chess laid the grounds of game theory, with early proofs of the existence of winning strategies by
Zermelo (1913) and equilibrium by von Neumann (1928); see Schwalbe and Walker (2001) for an informative overview. Chess players have a long history as subjects of studies in psychology, starting with the work of de Groot (1946). Chase and Simon (1973) and Simon and Chase (1973) contain early discussions of theories of cognition derived from the study of chess players. Work in psychology on expert performance regularly uses chess players as subjects of study (Ericsson, 2006; Moxley et al., 2012). The view of professional chess players as the prototypes of rational decision makers led several empirical or experimental tests of rational behavior in economics to focus on chess players as subjects of interest. Examples include experiments with chess players to investigate the empirical relevance of subgame perfection and backward induction (PalaciosHuerta and Volij, 2009; Levitt et al., 2011), rational learning in repeated games (Gerdes and Gränsmark, 2010), or emotions and psychological factors (GonzálezDiaz and PalaciosHuerta, 2016). To our knowledge, this is the first study to analyze movebymove behavior of chess players relative to a rational benchmark provided by a chess engine of comparable chess strength to human players.Data from chess tournaments have also been used to analyze various other research questions. These include, in particular, gender differences in patience (Gerdes et al., 2011), gender effects in competitiveness (Backus et al., 2016), gender and attractiveness (Dreber et al., 2013), selfselection and productivity in tournaments (Bertoni et al., 2015; Linnemmer and Visser, 2016), consequences of political ideology (Frank and Krabel, 2013), collusion (Moul and Nye, 2009), cheating (Barnes and HernandezCastro, 2015; Haworth et al., 2015) and indoor air quality (Künn et al., 2019). Recent work used chess data to compare the relative performance and strength of chess players in different time periods (Guid and Bratko, 2011; Alliot, 2017). Anderson and Green (2018) use chess data at the game level to investigate the role of personal peak performance in the past in terms of ratings, as reference points for performance. Strittmatter et al. (2020)
use gamelevel data over the past 125 years to estimate the productivity potential over the life cycle and its dynamics over time and across cohorts. While the existing work in this literature typically analyzes human performance at the game level and sometimes uses a chess engine that vastly outperforms human chess players to benchmark behavior, the methodology developed here allows us to identify deviations from a rational benchmark of comparable strength on a movebymove basis, as well as the performance implications of these deviations.
The comparison of behavior to an objective benchmark in terms of the quality of a given move relative to the best possible move in a given configuration allows us to explore the empirical relevance of several behavioral biases identified in the literature within a single and comparable research design, as well as their implications for performance. Our results thereby complement findings of the detrimental effects of time pressure on the quality of decision making (Kocher and Sutter, 2006) and relate to findings of heterogeneous effects of time pressure in loss and gain domains (Kocher et al., 2013). Our findings also contribute to the literature that has emphasized the role of choking under pressure (Baumeister, 1985; CohenZada et al., 2017; Dohmen, 2008; Genakos et al., 2015) or limited attention (Föllmi et al., 2016) among professionals. The heterogeneity in the results for deviations from rational behavior depending on the current positional standing in the game in terms of advantage or disadvantage is also reminiscent of findings of reference dependence (Bartling et al., 2015) and observations from risk taking in tournaments (Cabral, 2003; Genakos and Pagliero, 2012). Likewise, the results add to the literature investigating the role of complexity and cognitive load for individual performance (Deck and Jahedi, 2015) and on the relationship between cognitive limitations and behavioral biases (Oechssler et al., 2009).
By identifying behavioral deviations from rationality that not necessarily imply worse performance but that can even lead to better performance than the benchmark, our evidence contributes to recent theoretical work on the behavioral foundations of deviations from a rational decision benchmark. For a long time, chess players have been thought to play according to intuition or heuristics rather than following rational optimizing strategies
(e.g. Simon and Chase, 1973), but to our knowledge there exists no clear evidence on the implications of these deviations for performance. Our results contribute evidence in line with predictions of recent theoretical work that has considered the optimal speed and accuracy of decisions in settings in which the relative evaluations of decision alternatives are unknown; the results of this work show that decision accuracy may actually decrease with longer decision time (Fudenberg et al., 2018). Likewise, the result that deviations from the rational benchmark can be associated with better performance is consistent with predictions of models of focusing and selective memory (Gennaioli and Shleifer, 2010; Bordalo et al., 2020) or casebased decision theory (Sahm and von Weizsäcker, 2016).2 Data and Measurement
2.1 Data from Professional Chess Players
In the terminology of game theory, chess is a twoperson, sequential, zerosum game with perfect information and alternating moves, for which the optimal strategy is strictly determined.^{1}^{1}1See Schwalbe and Walker (2001) for details and a discussion of the historical background. The data used in the empirical analysis have been collected from an internet platform that broadcasts all professional overtheboard chess tournaments (www.chess24.com) and contains detailed information for more than 100,000 moves from around 2,000 games that were played in 97 single roundrobin tournaments during the years 20142017. All games were played at regular time controls that allocate a time budget of a minimum of 2 hours thinking time to each player to conclude the game.^{2}^{2}2According to the regulations by the International Chess Federation FIDE, for a game to be rated each player must have a minimum of 120 minutes, assuming the game lasts 60 moves per player. The standard time control regime suggested by the International Chess Federation FIDE is 90 minutes per player per game plus 30 seconds added to each player’s time budget for each move played; additional 30 minutes are added to each player’s time budget after each player has played 40 moves (see https://handbook.fide.com, last accessed May 12, 2020). Tournaments that are not officially organized by FIDE use slight variations of the official FIDE time control regime. Appendix Table A1 provides an overview of the tournaments contained in the data set. The data contain detailed information about the players, including their performance statistics in terms of their ELO number.^{3}^{3}3The ELO number constitutes a method for calculating the relative playing strength of players (invented by the Hungarian mathematician Arpad Elo). The ELO number increases or decreases depending on the outcome of games between rated players. After every game, the winning player takes points from the losing player, while the total number of points remains fixed. According to international conventions, an ELO number of at least 2,500 is a requirement for being awarded the title of an international grandmaster (this requirement has to be fulfilled once during the career, but does not have to be maintained to keep the title, see https://handbook.fide.com/chapter/B01Regulations2017, last accessed April 20, 2020). We restrict our baseline analyses to games between professional players with an ELO number of at least 2,500 at the time of the game. Appendix Table A2 shows summary statistics on the game level.
In addition to the remaining time budget and time consumed for each move, the movebymove data comprises information about the exact configuration of pieces on the board. We use this information to compute an evaluation of this configuration in terms of the relative standing of each player, an evaluation of the complexity of the configuration, and an evaluation of move quality, as explained in more detail below. For the computation of performance, we exclude the first fifteen moves of each player in a game from our analysis. These are usually socalled “book moves”, which are studied intensively by players in the preparation of the game and are typically the result of routine openings.
2.2 Measuring Performance in Chess
To construct a benchmark for rational behavior, we make use of a chess engine, Stockfish 8
, which is an opensource program that computes the best possible move for a given configuration of pieces on the chessboard. This engine is considered to be one of the best available programs. The version we use has an estimated ELO rating of approximately 3150 points (in comparison, the incumbent World Champion Magnus Carlsen had an ELO rating of 2872 points in January 2020 according to the official rating list by the International Chess Federation FIDE).
^{4}^{4}4We limit Stockfish 8 to a search depth of 21 moves to economize on computing costs. The unconstrained version of Stockfish 8 has an ELO of approximately 3300 points (http://ccrl.chessdom.com). Based on an approximation by Ferreira (2013), the ELO strength of Stockfish 8 with search depth of 21 corresponds to approximately 3150 points. This engine can be restricted, such that the strength of play of the engine corresponds closer to the strength of play of human players. To find an objective benchmark for human players, we use engines with different strengths of play in the analysis as described in detail below.An engine behaves exactly like a fully rational agent in standard gametheoretic settings. This delivers a clean and transparent benchmark to evaluate human behavior. Figure 1 illustrates the decision algorithm solved by the engine. For each configuration, the engine creates a gametree for all possible moves by white and black for a prespecified length of moves ahead, the socalled search depth. Then, the configurations at the respective endnodes are evaluated in terms of pieces left on the board, safety of the king, mobility of pieces, pawnstructure and so on. Based on this evaluation, the engine then determines the best move using backward induction under the assumption of mutually best responses.^{5}^{5}5Modern chess engines are almost exclusively based on domainspecific algorithmic heuristics that were developed specifically to search the sequential gametree arising from a given configuration. Current chess engines use an enhanced version of the minmax algorithm that disregards branches of the search tree that have already been found to be dominated. This reduces the searchspace without impacting the final choice of the best move by the engine (https://www.chessprogramming.org/AlphaBeta
, last visited March 17, 2020). Only very recently more general machine learning techniques in the form of neural networks have been embodied in chess engines such as Google’s nonpublic AlphaZero
(Silver et al., 2018). Modern engines like Stockfish 8 calculate approximately 10100 million nodes per second on standard personal computing hardware.We use the chess engine to compute three measures that are central to the empirical analysis. First, the engine delivers a measure of the relative standing for a given configuration of pieces on the board, which reflects an evaluation of the current position of a player and represents a proxy of the winning odds. The evaluation of the current position is the result of the engine computing, for each configuration observed in the data set, the best continuation. The relative standing is measured in socalled pawn units, where one unit approximates the advantage of possessing one more pawn.^{6}^{6}6This measure is relative and indicates an advantage for the player with white pieces for positive numbers, and for the player with black pieces for negative numbers. For example, if the evaluation is 1.00 pawn units, black is better “as if one pawn up.” Second, we compute a measure of performance, in terms of the quality of play of a given player, by comparing the actual move made by the respective player to the best move suggested by the chess engine. This move is not necessarily the absolute best move that is possibly conceivable but on average the move suggested by the engine is better than conceivable by any human player. In the data, relative performance can be measured by a binary indicator of whether a player makes the optimal move (or one of the optimal moves in case of several moves with equal winning odds) as suggested by the chess engine in a given configuration. Alternatively, one can construct a measure of the quality of a move by computing the deviation of player ’s move (in terms of pawn units) from the best move identified by the chess engine.^{7}^{7}7Concretely, we configure the engine to computes the corresponding evaluations for the six moves that it evaluates as best in a given configuration. Further increasing the number of moves that are evaluated comes at a prohibitively large computational cost. If the actual move played is one of these six moves, the performance is calculated as the difference in evaluation between the best and the actual move played. If the move played is not among the six best moves, we compute performance as the difference in the evaluation right before and right after the respective move of the player. Third, we use the engine to compute, for each observed configuration, a measure of complexity of the configuration. The more complex the configuration, the longer the engine needs to search the gametree. The time consumed by the engine to compute the best strategy for the next moves ahead can therefore be used as a measure of the complexity of a given configuration.^{8}^{8}8As baseline measure of complexity, we use the computation time needed by the super chess engine to reach a searchdepth of 21 moves. Alternatively, we use the number of branches (nodes) of the gametree that the engine has to calculate to reach a searchdepth of 21 as a measure of the branching factor and thus complexity of a given configuration. The (unreported) results are qualitatively similar and available upon request. Figure 2 contains a concrete illustration of how these measures are computed.
Appendix Table A3
documents the descriptive statistics of the movebymove data used in the analysis.
3 Empirical Strategy
3.1 Conceptual Approach
We denote by the performance of a move by human player in a given configuration of pieces relative to the optimal benchmark suggested by the super chess engine. This benchmark is based on the backward induction algorithm described before and constitutes a first natural piece of information for isolating deviations from rational behavior. The relative performance measure is not sufficient for isolating the role of subjective factors that lead to behavioral deviations from the rational benchmark, however, because the measure does not account for the fact that the super chess engine is superior to human players in terms of strength of play. Thus, the objective human evaluation of a given configuration and the resulting optimal human move under rationality might differ systematically from the suggested optimal move of the super chess engine as a result of cognitive limitations, but not necessarily as a result of human psychological factors.
To address this issue, the empirical strategy applies a differenceindifferences logic that compares the performance of humans to the performance of an equally strong but fully rational benchmark where, in both cases, performance is measured relative to the best possible move based on the assessment of the super chess engine. To construct such a directly comparable benchmark of rational behavior with similar playing strength as humans, we replicate each decision problem faced by humans (for each configuration observed in our data set) using a restricted chess engine that is calibrated to have approximately the same strength of play as the humans observed in the data set. This implies that, for each observed configuration , we construct a benchmark measure that reflects performance under fully rational behavior for a strength of play comparable to that of the human players (with ELO numbers between 2500 and 2880) relative to the best possible move suggested by the super chess engine.^{9}^{9}9In particular, we restrict Stockfish 8 to a search depth of 12 moves, which corresponds to a play strength equivalent to an ELO of around 2700 when comparing performance differences between human players and the restricted engine (see Appendix Figure A1).
By construction, the restricted engine plays rational (best response) strategies such that the move played by the restricted chess engine only depends on objective, movespecific characteristics related to the configuration on the board, but not on subjective playerspecific or gamehistoryrelated factors. Deviations of performance of this restricted engine from the best possible move suggested by the super engine can thus be due only to differences in strength of play, but not due to deviations from rational behavior. The relative performance of the restricted chess engine delivers a valid performance benchmark under fully rational behavior against which the performance of humans can be compared. Notice that a plain comparison of moves between humans and the restricted chess engine would not be sufficient, because it would not be possible to evaluate the direction – and thus the performance consequences – of these differences. This is only achieved by the comparison to the best possible move suggested by the super chess engine.
3.2 Parameters of Interest
To illustrate the identification strategy, let the potential relative performance under no behavioral deviation in configuration be denoted by . This is a potential variable that is not observed; we only observe the realized relative performance in the data, which might differ from because of behavioral deviations. Performance differences due to behavioral deviations are defined by , where implies no behavioral deviation from the rational benchmark. Notice that also
is a potential variable that is unobserved. For ease of notation, define the dummy variable
, with being the indicator function, as an indicator of any deviation from the rational benchmark.The goal of the empirical analysis is to identify subjective (psychological) factors that can be associated with deviations from the rational performance benchmark. The conditional expectation of behavioral deviations from rationality is
(1) 
where
denotes the conditional probability of a deviation from the rational benchmark. The right hand side of equation (
1) makes use of the discrete law of iterated expectations and the fact that . The marginal effects of subjective factors on behavioral deviations can be decomposed into effects along the extensive and intensive margin conditional on deviation, since(2) 
Deviations from the rational benchmark in terms of performance differences as reflected by are sensitive to the respective metric in which they are measured (e.g., pawn units). The extensive margin effects have the advantage to not depend on the particular metric of . To explore the consequences of behavioral deviations at the extensive margin, we denote positive deviations, i.e., deviations from the rational benchmark that are associated with better performance than the rational benchmark, by . Likewise, negative deviations, i.e., behavioral deviations from the benchmark that are related to worse performance are denoted by . Furthermore, we denote the conditional probability of a behavioral deviation from the benchmark that implies better performance by and the conditional probability of a behavioral deviation that implies worse performance by , with . The partial effects along the extensive margin can then be decomposed into partial effects on the probabilities of behavioral deviations associated with positive and negative consequences for performance,
3.3 Identification
We now sketch an identification strategy that allows identifying the parameters of interest. Let denote the relative performance in configuration by the restricted chess engine (with strength of play similar to that of humans) in comparison to the performance under an optimal move suggested by the super engine. Furthermore, we denote the difference between the relative performance of humans and the restricted chess engine by .
As a first step in the identification of the effects of subjective factors , we focus on the effects along the extensive margin. For this purpose, we construct a binary measure of whether the relative performance of a human player differs from the restricted chess engine, . This binary measure represents the observable analogue to and the identification of the effects along the extensive margin relies on the assumption that
(3) 
This assumption is fundamentally not testable, because deviations from the rational performance benchmark, , are unobservable. The assumption implies that the conditional probability that human players deviate from the restricted engine is equal to the conditional probability that human players deviate from the rational performance benchmark, which is a natural assumption in our setting. In the data, 60% of all moves exhibit the same relative performances for humans and the restricted chess engine. Accordingly, there is a mass point in the distribution of , which is otherwise a continuously distributed variable. Notice that the exact calibration of the strength of the restricted engine might influence the results by influencing the empirical measure of . However, as discussed below, extensive robustness tests show that the results are insensitive to variations in the calibration of the restricted engine.
Under assumption (3), the marginal effect of a subjective (psychological) factor on the probability of observing a deviation from the rational benchmark is given by^{10}^{10}10This follows from
This effect corresponds to an effect along the extensive margin and contains no information about the performance implications of this deviation.
Next, consider the marginal effects on the probability of behavioral deviations that imply better or worse performance than the benchmark, respectively. For this purpose, define and . Under the assumption , the marginal effects of factors on correspond to marginal changes in the probability of behavioral deviations with better performance,
Similarly, under the assumption , the marginal effects on changes in the probability of behavioral deviations with worse performance are given by
Using the categorical measure , these insights can be combined to obtain the net effect on the probability of deviations with positive and negative performance consequences,
provided that the previous assumptions hold.^{11}^{11}11Note that the assumptions and together are somewhat stronger than assumption (3).
Finally, reconsider the total marginal effect of the subjective factors as described in equation (2). Using the performance measure , this effect can be identified under the assumption , such that
which is a combination of the effects along the extensive and intensive margin.^{12}^{12}12In particular,
The interpretation of the intensive margin effects conditional on deviation is problematic, however, because the subjective factors affect the performance consequences of deviations and the probability of observing a deviation at the same time, thus giving rise to a sample selection problem (see, e.g., Heckman, 1979).
3.4 Estimation
In practice, we use movebymove data with an observation for the positional configuration of pieces on the board faced by individual player in game . The estimation model is then given by
(4) 
with the error term . denotes the different performance measures (, , , , ) described above. All specifications include interacted playergame fixed effects (where
indicates the playergamelevel) to account for systematic variation in style of play, environmental factors related to the game, or strategic aspects related to particular pairings. Inference is based on gamelevel clustered standard errors to account for interdependencies in the performances of both players.
The parameter vector
represents the partial effects of different subjective (psychological) factors , . In view of earlier work, we primarily focus on four subjective (psychological) factors that might affect deviations from rational behavior: stress related to the current standing (being in a better or worse position), time pressure (remaining time budget), fatigue (number of moves played by each player before the current move), and complexity (related to cognitive limitations).4 Empirical Results
4.1 Main Results
Table 1 contains the results of multivariate regression analyses of the empirical model in equation (4) for different dependent variables. Column (1) shows coefficient estimates for regressions with the binary measure of any deviation from the benchmark, , as dependent variable. Compared to an approximately balanced positional standing, human players are more likely to deviate from the rational benchmark when being in a better position relative to their opponent. In contrast, they are not more likely to deviate in a worse position. The results for remaining time reveal a positive but only marginally significant effect on the probability to deviate from the rational benchmark. This suggests that players deviate more often from the rational benchmark if they have more time available, rather than under greater time pressure. Contradicting intuition regarding a potential influence of fatigue, the probability to deviate from the benchmark is smaller later on in the game. Greater complexity of the configuration is associated with a higher probability to deviate from the rational benchmark.
Dependent Variable:  
(binary)  (binary)  (binary)  (categ.)  
(1)  (2)  (3)  (4)  
Current Position  
Better position (>0.5 pawnunits)  
Worse position (<0.5 pawnunits)  
Time Pressure  
Remaining time (hours)  
Fatigue  
Num. previous moves  
Complexity  
Seconds to reach fixed depth  
PlayerGame Fixed Effects  
Move Observations  
PlayerGame Observations 
Note: OLS estimates. Evaluations of performance are based on the Stockfish 8 chess engine (super engine and restricted engine). The variable Num. previous moves is calculated as the number of previous moves per player. Standard errors are clustered on the game level. : , : , : .
Columns (2) and (3) present the results for the binary measures of deviations from the benchmark that also contain information about the direction in terms of the associated consequences for performance, and . Here, a somewhat richer picture emerges. Whereas being in a better position is not associated with human players deviating in a way that their performance is better than the benchmark (Column (2)), the effect on deviations that imply worse performance than the benchmark is positive and significant (Column (3)). A possible explanation for this finding is that human players might decide to play suboptimal moves that are associated with lower risk or complexity, but also worse performance, when in a better position. The opposite picture emerges when players are in a worse position. In this case, humans are more likely to make deviations that imply better performance than the benchmark (Column (2)), but are less likely to make deviations that imply worse performance than the benchmark (Column (3)). This finding is consistent with stronger incentives for higher performance, for instance due to loss aversion relative to a reference point of a balanced position. As a consequence, humans might become less focused or more adventurous when they are in a better position, but they excel when they are in a worse position.
The picture also becomes richer regarding the influence of time pressure. More remaining time increases the likelihood of deviations with better performance than the benchmark (Column (2)), whereas the likelihood of deviations with worse performance declines (Column (3)). This provides evidence that deviations with worse performance become more frequent with less remaining time, consistent with the hypothesis of choking under time pressure. These opposite effects for deviations with different consequences for performance also explain why remaining time only has a weakly positive effect on the probability of any deviation (Column (1)). Likewise, a clearer picture emerges regarding fatigue, proxied by the number of moves that have already been played during a game. In particular, later in the game, deviations from the benchmark that are associated with better performance become less frequent, whereas there is no significant effect on the likelihood of deviations that are associated with worse performance. Finally, the hypothesis that complexity affects deviations from the rational benchmark is supported by significant effects on deviations with both, higher and lower performance than the rational benchmark. This is consistent with the conjecture that it is harder for human players to determine the rational continuation in more complex settings. However, this does not necessarily imply strictly worse performance.
Column (4) of Table 1 presents results for the categorical measure as dependent variable. This measure allows making inference on the difference between the effects obtained for and
. In particular, the estimates confirm the findings that players in a better position are more likely to exhibit worse performance than the rational benchmark, whereas players that are in a worse position are more likely to deviate with better performance than the rational benchmark. Also the result for time pressure becomes more pronounced, indicating that less remaining time is associated with more frequent deviations and worse performance. Fatigue continues to imply more frequent deviations from the benchmark with performance deteriorating later in the game. Finally, the effect of complexity is significantly negative in the estimates for the categorical variable, but quantitatively small.
These results complement existing work by documenting the prevalence of various deviations from the rational benchmark that are related to reference points (see, e.g. Bartling et al., 2015), time pressure (see, e.g. Kocher and Sutter, 2006; Kocher et al., 2013), complexityrelated stress (see, e.g. Dohmen, 2008), fatigue, or cognitive load (see, e.g. Oechssler et al., 2009; Deck and Jahedi, 2015) within a single framework. In addition to identifying deviations, the results also point at the performance implications and, in particular, the possibility of enhanced performance as consequence of a deviation from the rational benchmark. This is consistent with the predictions of models of selective memory (Bordalo et al., 2020) or experiencebased intuition (Moxley et al., 2012; Sahm and von Weizsäcker, 2016).
4.2 Robustness
Alternative Model Specifications.
The results are broadly similar when considering specifications without playergame fixed effects (see Appendix Table A4). Moreover, controlling for the subjective factors in univariate specifications confirms the robustness of the main findings (Appendix Table A5 shows this exemplarily for the categorical variable ()). The pattern of the main results remain similar when we exclude moves in positions that the engine evaluates as exactly equal for both players, presumably because the optimal continuation results in a repetition of moves (see Appendix Table A6).
The estimation results obtained with a more flexible specification of the effect of relative positional standing, allowing for nonlinear effects, confirm the main findings and does not reveal evidence for pronounced nonlinearities in the effect (see Appendix Table A7). Figure 3 shows results for more flexible specifications of the subjective factors graphically (exemplarily for the dependent variable ). Figure 3(a) plots the estimates from a more flexible specification with respect to current relative standing. The results confirm the main findings of Table 1, which reports the results relative to balanced positional standings. Performance is worse for a positive evaluation of the current position compared to a balanced position, but relatively better for negative evaluations. As in the main analysis, the identification of these effects relative to the benchmark of the restricted engine rules out that this finding is driven by mechanical effects such as reversion to the mean. Figure 3(b) suggests laxer time budget lead to more frequent deviations with better performance, especially early in the game. Figure 3(c) and (d) confirm that fatigue and complexity lead to worse performance.
Alternative Measures for the Subjective Factors.
To investigate the robustness of the results, we also replicated the analysis with alternative proxy measures for the various dimensions of behavioral deviations. These include, in particular, relative standing measured in terms of a continuous measure (in pawn units), time pressure in terms of proximity to time control when additional time is added to players’ time budget, fatigue as proxied by elapsed time, and complexity in terms of the distance of the secondbest move to the firstbest move (in terms of pawn units). The results confirm the main results (see Appendix Table A8). In comparison to the baseline results, players in a worse position when using a continuous evaluation measure of relative standing are here even significantly less likely to deviate from the benchmark along the extensive margin (for ), but still exhibit better performance when using the categorical measure .
Calibration of Restricted Chess Engine.
Another potential concern regarding the empirical strategy is the calibration of the restricted chess engine. In particular, since identification relies on different assumptions that involve a comparison between human behavior and the rational benchmark of the restricted engine, the results might be sensitive to the particular calibration as it might induce measurement error in . The empirical specification already accounts for this by including interacted playergame fixed effects that capture potential measurement error that enters at the playergame level, e.g., because a particular player has a systematically higher or lower strength of play than the restricted chess engine. Moreover, the analysis is based on a fairly homogeneous sample of players with ELO ratings between 2,500 and 2,880 points. As discussed above, the results are robust even when the playergame level fixed effects are omitted (see Appendix Table A4
). Furthermore, measurement error in the response variable does not lead to bias in the coefficient estimates of
when it is statistically independent of the regressors, but might increase the variance
(see, e.g., in Wooldridge, 2010, the discussion about classical measurement error). Accordingly, statistically independent measurement error may lead to conservative inference.The most direct evidence for the insensitivity of the results with respect to the calibration of the restricted engine emerges from estimates conducted with subsamples for players with different strength of play. The results from the corresponding robustness checks document that the results are not sensitive to players with different strength of play or the exact specification of the chess engine. In particular, we find that the overall pattern of results is identical when including weaker players (with ELO ratings above 2000 instead of restricting to players with ELO ratings above 2,500), or when restricting to players with ratings between 2,400 and 2,600, or between 2,600 and 2,800 (Appendix Tables A9, A10 and A11).
Alternative Chess Engine.
To assess the robustness of the results with respect to the particular chess engine, we also replicated the analysis for an alternative engine to construct the rational benchmark. This engine (Komodo) is considered to have a different playing style than Stockfish 8.^{14}^{14}14We use Komodo 9, which is also considered to be among the world’s strongest chess engines. Komodo’s playing style is typically referred to as being more positional, focusing more on longterm strategic planning, than that of Stockfish. According to http://ccrl.chessdom.com (archived on September 10, 2019) it is estimated to have an ELO of 3235 in its unconstrained version. To replicate moves for the benchmark, we also restrict Komodo to a search depth of 12. To the extent that alternative engine exhibits a different playing style, it also potentially introduces different measurement error in the dependent variables than Stockfish 8, because it uses different computational heuristics. The results obtained with the alternative engine reveal similar patterns as the baseline results (see Appendix Table A12).
Total Effect/Intensive Margin.
The analysis so far focused on the extensive margin effects. To obtain estimates of the total effects and of the intensive margin effects conditional on deviation as in equation (2), we also estimate the model using a continuous performance measure as dependent variable. In particular, we consider deviations from the rational benchmark using the measure
in terms of pawn units. Since the distribution of pawn units is substantially skewed and since we consider a semicontinuous variable with a mass point at 0, we construct deviations from the rational benchmark in terms of logmodulus transformed performance,
.^{15}^{15}15In particular, we compute as , where is the difference in performance measured in pawn units. Recall that identification relies on the assumption that the conditional expectation of the relative performance of humans under rational behavior is equal to the conditional expectation of the relative performance of the restricted engine, . This implies a reliance on the particular metric used for measuring performance (here pawn units), in contrast to the identifying assumption for effects along the extensive margin stated in (3). The latter stipulates that the conditional probability of deviations of the relative performance of human players from the benchmark of the restricted chess engine is equivalent to the conditional probability of deviations from the rational performance benchmark, which does not rely on a particular metric. Moreover, the size of the estimated effects depends on the particular metric used, which effectively determines the scope of the intensive margin effect.Nevertheless, for completeness, we report the estimates of the total effect and the effect along the intensive margin conditional on deviation (see Appendix Table A13 Columns (1) and (2), respectively). In terms of interpretation, the total effect is an alternative measure for the overall performance consequences of behavioral deviations. Comparing the results accordingly to the baseline results for the dependent variable reveals mostly the same patterns for the total effect as for the categorical measure (see Appendix Table A13 Column (1)). The only exception in this pattern refers to the effect of being in a worse position, which exhibits a significantly negative total effect on performance. This effect is quantitatively smaller than the effect for being in a better position but of opposite sign compared to the extensive margin effect of being in a worse position. This suggests that being in a worse position increases the probability of deviations associated with better performance (in terms of ), but the negative performance effects along the intensive margin associated with worse performance dominate when using the logmodulus transformed performance measure. The other results are qualitatively comparable; complexity has no significant impact on the total effect. The results for the intensive margin effects conditional on deviation are also in line with the findings obtained of the categorical measure (see Appendix Table A13 Column (2)). The exception is again the effect of worse position, which is negative but quantitatively small and only marginally significant. The intensive margin effect for complexity is positive and significant, but also quantitatively small. These patterns are confirmed when using the alternative engine to construct the rational benchmark (see Columns (3) and (4) of Appendix Table A13).
In light of the more restrictive identification assumptions, the reliance on a particular performance metric, and the difficult interpretation because of sample selection (see, e.g., Heckman, 1979), we view these findings as reassuring for the overall pattern of results. We conclude that the main insights of the analysis are obtained from the qualitative results along the extensive margin, which have the advantage of a straightforward interpretation and of not relying on a particular performance metric. However, these findings also cast a note of caution regarding the interpretation of various and sometimes diverging findings in the literature, which might not be directly comparable as they result from different outcome measures and thus constitute estimates of effects that are not necessarily fully comparable.
4.3 Behavioral Heterogeneity
To shed light on the underlying behavioral mechanisms, we estimated various alternative specifications that allow for interactions between the factors that lead to behavioral deviations with time pressure, or for heterogeneity in the effects of the subjective factors. Time pressure in terms of less remaining time tends to amplify the probability of any deviations (in terms of ) associated with better or worse positions, but do not affect the deviations associated with fatigue or complexity. However, time pressure seems not to amplify the consequences of behavioral deviations on performance (in terms of ), except for complexity where less remaining time is associated with more frequent deviations and even worse performance (Appendix Table A14). These results complement earlier findings for asymmetric effects of time pressure (Kocher et al., 2013).
A conjecture that has been raised repeatedly in psychology is that stronger players benefit from better intuition (Simon and Chase, 1973; Moxley et al., 2012). To test this conjecture, we explore whether there is any heterogeneity in the effects of the subjective factors on deviations from the rational benchmark with respect to player strength, measured by ELO ratings. The results reveal no systematic patterns except that the deviations from rational behavior associated with time pressure are less pronounced for stronger players (see Appendix Table A15).
To study the potential role of reference dependence based on exante odds along the lines of earlier work (e.g., Bartling et al., 2015) or a potential role of emotional states as in work by (GonzálezDiaz and PalaciosHuerta, 2016), we also test for systematic heterogeneity in the performance of players playing with white or black pieces. Playing with white is typically associated with an inherent firstmover advantage at the outset of a game and therefore exhibits significantly higher exante winning odds. Alternatively, we test for heterogeneity across favorites and underdogs as defined by the relative rating of the two players prior to the game in terms of their ELO numbers. However, in our specification with playergame fixed effects, we find no evidence for significant differences in behavioral deviations along these dimensions (see Appendix Table A16).
To explore the role of strategic and psychological interactions, we also investigate the influence of the opponent’s remaining time or of the time spent on the previous move by the opponent, which reveals no statistically significant interactions between the opponents in terms of an impact on performance (see Appendix Table A17).
4.4 Decision Times
The results so far indicate that deviations from rational behavior do not necessarily imply worse performance. This suggests that human intuition and experience might be an important factor in determining a successful strategy. To explore this possibility, we investigate the role of another dimension of choice: the time players invest in making a decision about a move. If time allocation is determined by implicit costbenefit considerations, decision makers spend more time deliberating a move when the gap in the subjective evaluation between two options is relatively small (Chabris et al., 2009). Moreover, earlier studies found that additional time for deliberation improves performance (Moxley et al., 2012). Recent theoretical work has considered the optimal speed and accuracy of decisions in settings in which the relative evaluations of decision alternatives are unknown. This work has shown that faster decisions can also imply better performance when decision makers already have fairly precise information and the value of further information acquisition is low, or when decision makers face (subjectively) simple problems where information acquisition is fast (Fudenberg et al., 2018). Our setting allows us to provide new evidence for the relation between decision speed and performance. Under the premise that, for certain configurations, intuition and expert assessment based on experience lead to a fast and precise assessment of the best strategy with little gain from additional deliberation, this gives rise to the hypothesis that faster decisions are associated with more frequent deviations from the rational benchmark and with higher performance.
Figure 4 plots the frequency of deviations from the rational benchmark, in terms of the binary measure in and in terms of the categorical measure in relation to the time spent on the respective move. The pattern in Panel (a) of the figure reveals that longer deliberation times are associated with more frequent deviations from the rational benchmark. Panel (b) shows that these deviations are associated with worse performance on average. The figure also suggests that the relationship is not linear, consistent with an important role for intuition and experience in instantaneously grasping the best move in a particular setting.
To rule out that this pattern is driven by third factors such as playing style, competitor pairings or psychological factors such as position complexity or time constraints, and to investigate the relationship between deviations and performance consequences in more detail, we replicate the main analysis by including the time spent on making a move as an additional explanatory variable. Panel A of Table 2 presents the estimation results for the association of the time spent on a move with the four different measures of behavioral deviations, the binary measures , , , and the categorical measure , as dependent variables, in specifications with playergame fixed effects. The results support the hypothesis: spending more time on a move is associated with more frequent deviations from the rational benchmark (Column (1)). Moreover, as shown by the results in Columns (2) and (3), spending more time deliberating a move is associated with more frequent deviations with worse performance compared to deviations with better performance. This implies that, as illustrated by the findings for the categorical variable in Column (4), spending less time and thus deciding faster is associated with more frequent deviations from the rational benchmark that are associated with better performance.
Panel B of Table 2 reports results for an extended specification for the effects of subjective factors driving behavioral deviations that also accounts for the time spent on a move as an additional control variable. The results for deliberation times are qualitatively unchanged. Spending more time on a move is associated with more frequent deviations from the rational benchmark and worse performance. The coefficients for the subjective factors are qualitatively similar to the main results in Table 1 and seem to be unaffected by including the decision time spent on a move.
Panel A: Baseline Effect, No Subjective Factors  
Dependent Variable:  
(binary)  (binary)  (binary)  (categ.)  
(1)  (2)  (3)  (4)  
Time spent on move (min.)  
PlayerGame Fixed Effects  
Move Observations  
PlayerGame Observations  
Panel B: Full Specification  
Dependent Variable:  
(binary)  (binary)  (binary)  (categ.)  
(1)  (2)  (3)  (4)  
Time spent on move (min.)  
Current Position  
Better position (>0.5 pawnunits)  
Worse position (<0.5 pawnunits)  
Time Pressure  
Remaining time (hours)  
Fatigue  
Num. previous moves  
Complexity  
Seconds to reach fixed depth  
PlayerGame Fixed Effects  
Move Observations  
PlayerGame Observations 
Note: OLS estimates. Evaluations of performance are based on the Stockfish 8 chess engine. The variable Num. previous moves is calculated as the number of previous moves per player. Standard errors are clustered on the game level. : , : , : .
Additional results reveal that the time spent on a move also interacts with the psychological factors in determining deviations from the rational benchmark (see Appendix Table A18). In particular, more deliberation time in terms of time spent on a move tends to counteract the influence of being in a better or worse position, or of time pressure (in terms of less remaining time) on the likelihood of deviating from the rational benchmark. In terms of performance consequences, spending more time amplifies the positive performance consequences of being in a worse position and the negative performance consequences of time pressure. Moreover, longer deliberation time on a move tends to amplify the negative performance consequences associated with deviations from the rational benchmark due to fatigue by inducing more errors.
Additional results for decision time as dependent variable complement the previous results (see Appendix Table A19). The analysis of the determinants of decision time reveals that deliberation is most time consuming in configurations where the game is approximately balanced; players with clearly better or worse positional standings decide faster. This correlation is particularly strong when being in a worse position. A less constrained time budget in terms of more remaining time at the decision makers’ disposal is also associated with more time spent on a move. Later in the game decisions are made faster, which might indicate shorter deliberation as a consequence of fatigue. Finally, more complex situations induce slower decisions.
Together, these results indicate that faster decisions are associated with more deviations from rational behavior and, at the same time, better performance than stipulated by the rational benchmark. This suggests a superior positional assessment of humans, which is presumably related to intuition and experience, and which is particularly pronounced during critical phases of the game. These patterns are consistent with predictions of models of salience and selective memory (Gennaioli and Shleifer, 2010; Bordalo et al., 2020) under the assumption that chess experts have a very quick and intuitive perception of the best continuation and of critical positions that require longer deliberation times. This is also consistent with a twostep approach where decisions are taken either according to rational considerations or intuitively on a casebycase assessment (Sahm and von Weizsäcker, 2016).
5 Concluding Remarks
In this paper, we provide new evidence for the pervasiveness of deviations from rational behavior using data from professional chess players. In terms of methodology, our paper develops a new identification strategy for deviations from rational behavior by constructing a benchmark for rational behavior that utilizes the artificial intelligence embodied in chess engines and that can be used to analyze human behavior. This methodology might be useful for other applications in behavioral economics.
The empirical findings of this paper have farreaching implications. The results show that even professional chess players deviate from the benchmark of rationality. In particular, the results indicate that time pressure, fatigue, complexity, and pressure from being in a better or worse position induce deviations from rational behavior. However, the results also show that these deviations do not necessarily affect performance negatively, but often even entail superior performance. We also find that faster decisions are associated with more frequent deviations from the rational benchmark and, concurrently, better performance. In light of previous theoretical literature and additional empirical results, this suggests that the superior performance is presumably due to experience or intuition of experts that provides them with a fairly fast and precise assessment of the situation and the best continuation.
While this paper contributes a new methodology to identify deviations from a rational benchmark as well as its causes and consequences, the results are not conclusive about the underlying mechanisms. For instance, it is possible that deviations from the rational benchmark are entirely due to mechanisms related to cognitive processes that underly the decisions of an individual player. It is equally possible, however, that the deviations are part of a strategy that incorporates beliefs about likely deviations of the opponent from rational behavior, thus incorporating the notion that rational behavior might not be the optimal strategy, in analogy to the optimal strategy in a guessing game. Both possibilities are consistent with the empirical approach and results presented in this paper. A natural next step in the research agenda is to apply the methodology developed here to investigate the respective behavioral mechanisms in more detail.
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Appendix with Supplementary Material
For Online Publication
Additional Figures
Additional Tables
Games  Percent  
National Championships  
Russian Championship  165  8.3% 
U.S. Championship  114  5.8% 
Ukrainian Championship  78  3.9% 
French Championship  62  3.1% 
Armenian Championship  60  3.0% 
Other National Championships  115  5.8% 
Invited Tournaments 

Wijk aan Zee  450  22.7% 
Norway Chess  84  4.2% 
Poikovsky  81  4.1% 
Shamkir  76  3.8% 
Lake Sevan  69  3.5% 
Danzhou  42  2.1% 
Other Invited Tournaments  306  15.4% 
World Chess Federation Tournaments 

FIDE Grand Prix  280  14.1% 
Sum  1,982  100.0% 
Games  Mean  Std. Dev.  Min  Max  

Player strength  
Elo rating white player  1,982  2681.7  85.24  2500  2881 
Elo rating black player  1,982  2681.0  85.09  2500  2881 
Game result  
White player wins  1,982  0.279  0.448  0  1 
Draw  1,982  0.566  0.496  0  1 
Black player wins  1,982  0.156  0.363  0  1 
Duration  
Num. moves overall  1,982  41.05  14.55  15  98 
Duration game (hours) overall  1,982  3.332  1.443  0  8.113 
Note: The variable Num. moves overall is calculated as the number of moves per player in game.
Moves  Mean  Std. Dev.  Min  Max  

Game characteristics  
Elo rating player  106,391  2678.6  86.08  2500  2881 
Elo difference between players  106,391  0.103  71.07  284  284 
Performance measures  
(binary)  106,391  0.398  0.489  0  1 
(binary)  106,391  0.175  0.380  0  1 
(binary)  106,391  0.223  0.416  0  1 
(categ.)  106,391  0.0479  0.629  1  1 
(logmod)  106,391  0.0366  0.398  6.485  5.795 
Current position  
Evaluation current position (pawnunits)  106,391  0.146  24.30  327  327 
Better position (>0.5 pawnunits)  106,391  0.245  0.430  0  1 
Worse position (<0.5 pawnunits)  106,391  0.213  0.409  0  1 
Time pressure and time spent  
Remaining time (hours)  106,391  0.665  0.528  0  2.513 
Time spent on move (min.)  106,391  2.482  4.072  0  100.5 
Num. previous moves  106,391  31.86  13.68  15  98 
Duration game (hours)  106,391  2.763  1.409  0  8.113 
Remaining time (opp.)  104,409  0.654  0.525  0  2.513 
Time spent on move (opp.)  106,391  2.545  4.139  0  100.5 
Complexity position  
Seconds to reach fixed depth  106,391  30.33  24.14  0.00100  799.3 
Distance second best move  105,733  4.338  32.78  654  0 
Note: Descriptive statistics for the baseline sample. Evaluations of performance are based on the Stockfish 8 chess engine. The variable Distance second best move contains missing values for configurations where there is only one legal move available to the player. The variable Num. previous moves is calculated as the number of previous moves per player. The variable Remaining time (opp.) has missing values because the remaining time of the opponent is not recorded for the final move of a game.
Dependent Variable:  
(binary)  (binary)  (binary)  (categ.)  
(1)  (2)  (3)  (4)  
Game characteristics  
Elo player (divided by 100)  
Elo difference (divided by 100)  
Favorite Elo difference  
White player (dummy)  
Favorite (according to Elo)  
Current Position  
Better position (>0.5 pawnunits)  
Worse position (<0.5 pawnunits)  
Time Pressure  
Remaining time (hours)  
Fatigue  
Num. previous moves  
Complexity  
Seconds to reach fixed depth  
PlayerGame Fixed Effects  
Move Observations  
PlayerGame Observations 
Note: OLS estimates. Evaluations of performance are based on the Stockfish 8 chess engine. The variable Num. previous moves is calculated as the number of previous moves per player. Standard errors are clustered on the game level. : , : , : .
Dependent Variable:  
Performance: (categ.)  
(1)  (2)  (3)  (4)  (5)  
Current Position  
Better position (>0.5 pawnunits)  
Worse position (<0.5 pawnunits)  
Time Pressure  
Remaining time (hours)  
Fatigue  
Num. previous moves  
Complexity  
Seconds to reach fixed depth  
PlayerGame Fixed Effects  
Move Observations  
PlayerGame Observations 
Note: OLS estimates. Evaluations of performance are based on the Stockfish 8 chess engine. The variable Num. previous moves is calculated as the number of previous moves per player. Standard errors are clustered on the game level. : , : , : .
Dependent Variable:  
(binary)  (binary)  (binary)  (categ.)  
(1)  (2)  (3)  (4)  
Current Position  
Better position (>0.5 pawnunits)  
Worse position (<0.5 pawnunits)  
Time Pressure  
Remaining time (hours)  
Fatigue  
Num. previous moves  
Complexity  
Seconds to reach fixed depth  
PlayerGame Fixed Effects  
Move Observations  
PlayerGame Observations 
Note: In this table, configurations that are evaluated with 0.00 by the chess engine due to a mutally beneficial move repetition are excluded. OLS estimates. Evaluations of performance are based on the Stockfish 8 chess engine. The variable Num. previous moves is calculated as the number of previous moves per player. Standard errors are clustered on the game level. : , : , : .
Dependent Variable:  
(binary)  (binary)  (binary)  (categ.)  
(1)  (2)  (3)  (4)  
1 Slight advantage (+/=)  
2 Clear advantage (+/)  
3 Decisive advantage (+)  
4 Slight disadvantage (=/)  
5 Clear disadvantage (/+)  
6 Decisive disadvantage (+)  
Time Pressure  
Remaining time (hours)  
Fatigue  
Num. previous moves  
Complexity  
Seconds to reach fixed depth  
PlayerGame Fixed Effects  
Move Observations  
PlayerGame Observations 
Note: OLS estimates. Evaluations of performance are based on the Stockfish 8 chess engine. The variable Num. previous moves is calculated as the number of previous moves per player. Advantages and disadvantages are calculated based on the usual chess conventions: A configuration that is evaluated by a chess engine as less than 0.3 pawn units better for one side is considered equal (). A configuration that is evaluated as between 0.3 and 0.7 pawn units better for one side is considered as a slight advantage () or slight disadvantage (), respectively. A configuration that is evaluated as between 0.7 and 1.6 pawn units better for one side is considered as a clear advantage () or clear disadvantage (), respectively. Positions that are evaluated as 1.6 better for one side are considered as a decisive advantage () or decisive disadvantage (), respectively. Standard errors are clustered on the game level. : , : , : .
Dependent Variable:  
(binary)  (binary)  (binary)  (categ.)  
(1)  (2)  (3)  (4)  
Better Position (pawn units, continous)  
Worse Position (pawn units, continous)  
Time Pressure  
Less than 10 moves before first time control  
Fatigue  
Duration game (hours)  
Complexity  
Distance second best move  
PlayerGame Fixed Effects  
Move Observations  
PlayerGame Observations 
Note: OLS estimates. Evaluations of performance are based on the Stockfish 8 chess engine. Better/worse position are measured in absolute pawn units. Less than 10 moves before first time control is a dummy indicating that a player has less than 10 moves to play before reaching move 40 when additional time is added to each players’ time budget. In contrast to the variable remaining time in the main analysis, less than 10 moves before first time control implies more time pressure. Accordingly, the sign of the coefficients for time pressure variable is expected to be opposite of that in the baseline specification. Duration game is the overall time that both players have already spent thinking about their moves. Distance second best move measures how far in terms of pawn units the chess engine evaluates the current configuration to be worse in case the second best move is played compared to the best move. 658 observations are dropped compared to the baseline specification because there is no second legal move available to the player. Standard errors are clustered on the game level. : , : , : .
Dependent Variable:  
(binary)  (binary)  (binary)  (categ.)  
(1)  (2)  (3)  (4)  
Current Position  
Better position (>0.5 pawnunits)  
Worse position (<0.5 pawnunits)  
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