A home-based progressive-ratio task for assessing food reinforcement in infancy: Developmental sensitivity at 9 and 12 months.

The relative reinforcing value of food (RRVFood) is theorized to emerge early and contribute to obesity risk. We examined whether a matched set of solid foods elicited differentiation between food and nonfood reinforcement at 9 (n = 110) and 12 (n = 133) months of age. We hypothesized that infants would respond for both reinforcers at both age points with enough variability for individual-differences analyses, and that RRVFood would relate to food consumption, appetitive traits, and weight-for-length z-score (WLZ) more strongly at 12 months. In a longitudinal subsample (n = 70), we further hypothesized stability in responding across age points and explored how responding to each reinforcer changed over time. Infants completed a progressive-ratio lever-press task at 9 and 12 months to earn food (cereal) and nonfood (block) reinforcers. Absolute reinforcing value (ARV) was operationalized as lever presses for each reinforcer; RRVFood was operationalized as ARVFood/ARVNonfood ratio. Mothers completed the Children's Eating Behavior Questionnaire-Toddler (CEBQ-T) to index food enjoyment and food responsiveness. Food reinforcer consumption and WLZ were also assessed. At 9 months, ARVFood and ARVNonfood positively correlated, but RRVFood was not associated with food reinforcer consumption, appetitive traits, or WLZ. By 12 months, RRVFood was positively associated with enjoyment of food, and ARVFood was positively associated with WLZ. Findings suggest that food reinforcement is more meaningfully differentiated from nonfood reinforcement at 12 months than at 9 months. Whether this reflects genuine developmental consolidation of food motivation or measurement limitations inherent to testing younger infants remains to be determined.