Cognitive influences on risk-seeking by rhesus macaques

Abstract

Humans and other animals are idiosyncratically sensitive to risk, either preferring or avoiding options having the same value but differing in uncertainty. Many explanations for risk sensitivity rely on the non-linear shape of a hypothesized utility curve. Because such models do not place any importance on uncertainty per se, utility curve-based accounts predict indifference between risky and riskless options that offer the same distribution of rewards. Here we show that monkeys strongly prefer uncertain gambles to alternating rewards with the same payoffs, demonstrating that uncertainty itself contributes to the appeal of risky options. Based on prior observations, we hypothesized that the appeal of the risky option is enhanced by the salience of the potential jackpot. To test this, we subtly manipulated payoffs in a second gambling task. We found that monkeys are more sensitive to small changes in the size of the large reward than to equivalent changes in the size of the small reward, indicating that they attend preferentially to the jackpots. Together, these results challenge utility curve-based accounts of risk sensitivity, and suggest that psychological factors, such as outcome salience and uncertainty itself, contribute to risky decision-making.

Figure 1

Gambling task: After monkey fixated on a small central square, two eccentric response targets appeared. Following a 1 second delay, the central cue was extinguished, indicating that gaze must be shifted to either of the two targets. Following choice, all stimuli were extinguished and reward was delivered. In the Alternation Task, choices were either safe, risky, or alternating. In the Variance Task, one side offered a certain juice volume while the other side offered a risky volume.

Figure 2

Monkeys preferred risky to alternating options with the same value. When monkeys chose between risky and alternating options with the same average expected value, they strongly preferred the risky option. The unpredictability of the risky option thus contributes to its appeal. Error bars indicate one standard error.

Figure 3

Monkeys attended to changes in the large reward in a gamble. Average likelihood of choosing the risky option increased when the value of the large reward increased and decreased when the value of the large reward decreased (solid line). Average likelihood of choosing the risky option was uncorrelated with the value of the small reward (dashed line). Note that vertical scale is magnified to emphasize the effect. Error bars indicate one standard error above and below the mean.