Rationality and cognitive bias in captive gorillas' and orang-utans' economic decision-making

Abstract

Human economic decision-making sometimes appears to be irrational. Partly, this is due to cognitive biases that can lead to suboptimal economic choices and context-dependent risk-preferences. A pertinent question is whether such biases are part of our evolutionary heritage or whether they are culturally acquired. To address this, we tested gorillas (Gorilla gorilla gorilla) and orang-utans (Pongo abelii) with two risk-assessment experiments that differed in how risk was presented. For both experiments, we found that subjects increased their preferences for the risky options as their expected gains increased, showing basic understanding of reward contingencies and rational decision-making. However, we also found consistent differences in risk proneness between the two experiments, as subjects were risk-neutral in one experiment and risk-prone in the other. We concluded that gorillas and orang-utans are economically rational but that their decisions can interact with pre-existing cognitive biases which modulates their risk-preference in context-dependent ways, explaining the variability of their risk-preference in previous literature.

Fig 1. ’Single cup’ design and ’multiple cups’ design.

Apparatus for the ’single cup’ design (A) as used in [24], leading to risk-aversion or weak risk-proneness. Subjects have to choose between one safe cup and one risky cup yielding an unknown amount of reward, with an unknown probability of yield. Apparatus for the ’multiple cups’ design (B) as used in [40], leading to high risk-proneness. Subjects have to choose between one safe cup and one to four risky cups, where only one risky cup contains the reward.

Fig 2. Results of Experiment 1.

Mean percentage of trials where subjects selected the risky option according to the value of the risky option (A) the probability to win (B) and the expected value of the risky option (C) for gorillas (black) and orang-utans (grey). Error bars indicate 95% confidence intervals.

Fig 3. Results of Experiment 2.

Mean percentage of trials where subjects selected the risky option according to the value of the risky option (A) the probability to win (B) and the expected value of the risky option (C) for the high-valued reward (grapes, in thick line) and low-valued reward (vegetables, in narrow line). Error bars indicate 95% confidence intervals. Only trials with P<1 were considered for (C).

Fig 4. Performance comparison between experiments.

Mean percentage of trials where subjects selected the risky option for Experiment 1 (solid line) and Experiment 2 (dotted line, low-valued food type): according to the value of the risky option (A) and the probability to win (B). Error bars indicate 95% confidence intervals.