Getting the proper grip: A longitudinal study of how infants learn to adapt action Plans.

Across ages and cultures, planning actions adaptively during tool use is a hallmark of human intelligence and a critical factor in human survival and proper function. Previous cross-sectional studies showed that adaptive planning begins in infancy and improves with age and experience. However, little is known about how developmental improvements in adaptive planning occur. Do infants gradually adapt their action planning with one object and then generalize this skill to other objects, or does learning remain tool-specific? Here, we longitudinally tested nine infants in weekly sessions across the age range when tool use is rapidly developing. Infants were presented with a familiar tool (spoon) and three unfamiliar tools (brush, hammer, magnet) with handles pointing to the right or left. For each trial, we scored whether infants used an adaptive radial grip (evidence of action planning) or an inefficient ulnar grip (no evidence of planning). Across several months of testing, every infant gradually learned to use an adaptive radial grip for the spoon, but none showed improvement for the unfamiliar tools. Adaptive planning with the spoon was further limited to self-directed actions (bringing food to their own mouth) rather than other-directed actions (feeding a puppet). Learning was characterized by high variability before stable achievement of an efficient grip. Across all tools, right-pointing handles elicited more radial grips than left-pointing handles. Our findings replicate previous cross-sectional research and provide new insights into the longitudinal progression of adaptive planning during tool use in infancy. Specifically, the development was gradual rather than abrupt, and learning remained highly tool-specific without generalization, emphasizing the critical role of specific and extensive experience with particular tool-action combinations.