Rational expectations and kinematic information in coordination games

Successful coordination often requires integrating strategic reasoning with real-time observations of others' actions, yet how humans resolve conflicts between these information sources remains unclear. This study aimed to fill this gap by examining how people coordinate in a strategic game when observing partial kinematic information from their partner's actions. Participants played a HI-LO game with a virtual partner, coordinating their payoff choices based on the initial portion (10–40% of movement length) of their partner's grasping movements toward an invisible large or small target. Hand movements were presented as schematic animations, with partners grasping targets linked to higher or lower payoffs across two configurations. Participants relied exclusively on kinematic cues from hand shape changes in maximum grip aperture to infer their partner's choices. We found that although participants typically favored the higher payoff in line with rational game-theoretic expectations, they revised these expectations whenever the partial kinematic cues suggested otherwise. When early grip aperture changes indicated the partner was reaching for a large target associated with a lower payoff, participants revised their preference for higher payoffs, achieving high coordination success. These findings show that people prioritize kinematic evidence about others' actions over abstract assumptions about rational payoff maximization. Even very early movement cues can shift beliefs about what a rational agent is likely to choose, highlighting the central role of action perception in strategic coordination.