Chimpanzee decisions in the prosocial choice paradigm depending on the position of tokens in the feeder.

The prosocial choice paradigm is a widely used method in studies of prosocial behavior in chimpanzees, but findings produced using this approach have been inconsistent. One potential source of these discrepancies is the lack of control for motor lateralization. The present study examined the influence of motor lateralization on decision-making in four chimpanzees (Pan troglodytes) housed at the Warsaw Zoo. Subjects chose between prosocial (1/1) and selfish (1/0) tokens under three experimental conditions: (a) tokens randomly mixed within the feeder, (b) tokens arranged in single-color stacks on opposite sides, and (c) mixed tokens without a partner (ghost condition). Each individual made 200 decisions per condition over a three-month period. Results indicated that chimpanzees were significantly more likely to choose the prosocial option when token placement minimized lateralization effects through random mixing. In contrast, when tokens were presented in fixed locations, choices were systematically influenced by the physical convenience of reaching, reflecting individual motor preferences. These findings highlight the importance of controlling motor lateralization in studies using the prosocial choice paradigm. Even with motor lateralization controlled, the presence of a partner proved essential for demonstrating prosociality. Prosociality was not detected in the so-called ghost condition, where no partner is present. Future research should increase the number of trials, distribute sessions over time, randomize token placement, and examine the relationship between task-based decisions and broader patterns of social interaction.