Observed reaching speed signals stimulus value and informs foraging

Optimal foraging requires agents to strike a balance between potential costs and rewards of interacting with stimuli in the environment. Research on human and animal foraging shows that the value an agent assigns to a stimulus is correlated with the speed of their reaching movement towards that stimulus (Shadmehr et al., 2019). Humans and other animals learn about the value of stimuli in their environment by observing others acting (Pyke, 1984; Boyd, Richerson & Henrich, 2011). Considering that humans are able to derive specific mental states such as intentions, emotions or confidence from specific movement parameters (Becchio et al., 2012), we aimed to investigate whether observers can use an actor's movement speed to: 1) infer the value of a foraging stimulus; and 2) use such cues to inform their own foraging behavior. The current study first replicated the effect of stimulus value on reaching movements in a novel foraging task (Exp. 1, N = 34). In three further experiments, we demonstrate that, depending on the speed by which an actor reaches for stimuli, observers infer the value of these stimuli (Exp. 2, N = 54), express foraging preferences (Exp. 3, N = 54), and invest time and effort to forage (Exp. 4, N = 105). This demonstrates that observers optimize their own explore-exploit decisions by inferring the value of a stimulus from the manner by which an actor approaches it, highlighting the fundamental role that action understanding plays in successful foraging.