Decision making and risk attitude of the common marmoset in a gambling task.

To analyze decision making under uncertainty of monkeys, common marmosets were trained to choose and remove one of two colored caps on wells arranged side by side. Each well contained constant reward (3 grains of puffed rice) or risky reward (0 or 6 grains; probability, 50%:50%). For each marmoset, white or black color was assigned randomly as a symbol of non-risky or risky choice. Arrangement of white and black caps was determined randomly in each trial. After 200 trials (5 trials per day), the marmosets were classified according to the pattern of their choice. Eight of 18 marmosets (44.4%) were risk-aversive, whereas 5 marmosets (27.8%) were risk-prone. The remaining 5 marmosets (27.8%) preferred to choose one side (left n=4, right n=1). These results showed individual differences in decision making of marmosets. An additional task with reduction in the expected value of the preferred choice revealed that risk-aversive marmosets were slower to adjust their choices to such reductions than risk-prone animals.