Chimpanzee choice rates in competitive games match equilibrium game theory predictions

Abstract

The capacity for strategic thinking about the payoff-relevant actions of conspecifics is not well understood across species. We use game theory to make predictions about choices and temporal dynamics in three abstract competitive situations with chimpanzee participants. Frequencies of chimpanzee choices are extremely close to equilibrium (accurate-guessing) predictions, and shift as payoffs change, just as equilibrium theory predicts. The chimpanzee choices are also closer to the equilibrium prediction, and more responsive to past history and payoff changes, than two samples of human choices from experiments in which humans were also initially uninformed about opponent payoffs and could not communicate verbally. The results are consistent with a tentative interpretation of game theory as explaining evolved behavior, with the additional hypothesis that chimpanzees may retain or practice a specialized capacity to adjust strategy choice during competition to perform at least as well as, or better than, humans have.

Figure 1. The trial progression, touch-panel setup, and game payoffs.

(A) Two players interacting through touch-panel screens are shown a self-start key (circle) at the beginning of each trial. After both players press the start key, two action choices are displayed, represented by squares on the left and right sides of the screens. After both players make a choice, payoffs are dispensed to the winner and both players get feedback about their opponent's choice. (B) Payoff matrices for the 3 games in this study. (C) Subjects sit perpendicular to each other facing touch-panel screens that are embedded in the walls of the experimental booth (photo credit: Chris Martin).

Figure 2. Frequencies of R choices for all pairs in both roles show that chimpanzee behavior is close to game theoretic (NE) predictions.

(A, B) Chimpanzees in the symmetric and asymmetric MP games. (C) Chimpanzees and two human groups in the Inspection game. Deviations from Nash equilibrium among chimpanzees average .02 (individual std error .025); deviations among humans average .13 (individual std error .059). Two-sample t-test for the difference in absolute deviation is t(23) = 6.38 (p < .001). (D) Average behavior over all chimpanzees compared to NE for all three games.

Figure 3. Chimpanzees respond to history and payoff structure more than humans do.

(A) History-responsiveness of choices based on learning rate (y-axis) and overall learning model fit which includes Matcher-Mismatcher response differences to payoff structure (x-axis). Chimpanzees have a higher learning rate and a better model fit. (B) Cumulative distribution functions of the likelihood-ratio (LR) statistic (x-axis) showing the improvement in fit of a model with learning compared to a no-learning benchmark. Higher LR numbers indicate more learning; one human outlier removed, see Supplementary Information). The chimpanzee distribution is shifted to the right (i.e., it stochastically dominates the human distribution). This shift means that for every level of detectable learning, more chimpanzees are likely to show that level of learning or greater, compared to humans (p = .040 by an ELR bootstrap test; see Supplementary Information Sect. IV).