Evidence for a magnitude effect in probability discounting with pigeons

Area of Science:

• Behavioral Economics
• Animal Cognition
• Decision Making

Background:

• The magnitude effect in probability discounting, where larger rewards are devalued more by uncertainty than smaller rewards, is established in humans.
• Previous research on probability discounting in non-human animals has yielded mixed results regarding the magnitude effect.
• Understanding how reward magnitude and probability influence choice is crucial for behavioral economics and animal cognition.

Purpose of the Study:

• To investigate whether pigeons demonstrate a magnitude effect in probability discounting analogous to humans.
• To examine the influence of reward magnitude and probability on pigeons' choices between probabilistic food rewards.
• To test the predictive power of an extended cumulative decision model for these effects.

Main Methods:

• Two experiments using a 2-component concurrent-chains procedure with pigeons choosing between probabilistic food rewards.
• Varying reward magnitudes (4-s vs. 2-s access to food) and probabilities (100% vs. 50%) across terminal links.
• Analyzing preference for larger vs. smaller rewards under different probability conditions.

Main Results:

• Pigeons showed a greater preference for the larger reward in the 100% probability component, indicating a magnitude effect.
• The value of the larger reward decreased more when its probability was 50% compared to the smaller reward.
• Results were consistent regardless of whether probabilistic outcomes were signaled, and were predicted by the cumulative decision model.

Conclusions:

• Pigeons exhibit a magnitude effect in probability discounting, mirroring human behavior.
• The findings support a unified model of decision-making that accounts for both delay and probability discounting in non-human animals.
• The cumulative decision model successfully predicts the influence of reward magnitude and probability on choice, resolving prior discrepancies.