Posterior parietal cortex mediates rarity-induced decision bias and learning under uncertainty

Optimal decision-making under uncertainty is critical for survival, yet real-world decisions often deviate from optimality. Here, we report a rarity-induced bias in humans and mice, where rare events exert a stronger and more persistent impact on future decisions than common events. Optogenetic manipulations demonstrate two opposing roles of the posterior parietal cortex (PPC): generating rarity-induced bias throughout learning, and driving optimal choices in the expert stage. In vivo recordings support both roles: preceding rare rewards enhance the activity of PPC neurons encoding the biased choice while suppressing neurons encoding the opposite choice, causing bias; learning enhances the stimulus-encoding capacity of PPC neurons, promoting optimal choices. A data-driven metacognitive model recapitulated the bias and learning process, and predicted PPC’s causal role in learning, as validated by optogenetics. Our study uncovers an evolutionarily conserved rarity impact in decision-making, and elucidates essential roles of PPC in mediating rarity-induced bias and learning under uncertainty.