Detecting the “We-Mode” in Observed Interactions: Infants’ Increasing Selectivity to Cues of “Truly” Joint Attention From 9 to 18 Months

Human social cognition is grounded in “we-mode” interactions, providing a psychological foundation for coordinating attention and knowledge, thereby facilitating human-specific cultural learning. While infants’ sensitivity to interpersonal sharedness is well-documented in participation-based “first-party” interactions, its ontogenetic origins in observed “third-party” settings remain poorly understood. Focusing on joint attention as a foundational mechanism of shared cognition, this study investigated when infants begin to appreciate the “we-dimension” within observed triadic interactions, addressing theoretical debates between leaner and richer accounts of joint attention. In a screen-based object-encoding task, 9-month-old infants and 18-month-old toddlers saw videos of two adults looking at an object. We systematically manipulated (a) partner-directed gaze (looking at partner vs. away) and (b) its temporal alignment (synchronously vs. asynchronously). This allowed us to contrast “truly joint attention” (coordinated mutual awareness, “we-mode”) with “parallel attention” (mere co-orientation) and a novel “social gaze” condition featuring uncoordinated partner-looks without sharedness. Results from a looking-time-based novelty-preference test revealed that 18-month-olds selectively encoded objects only when observing eye contact in the “synchronous–looking at partner” condition, indicative of “truly joint attention”. Conversely, 9-month-olds exhibited a broader sensitivity, encoding objects in all conditions featuring partner-directed gaze, including “social gaze” scenarios lacking markers of sharedness. These findings demonstrate that infants’ early sensitivity to third-party joint attention refines into a more selective appreciation of the observed “we-dimension” during the second year. This developmental progression marks a critical milestone in recognizing others’ collaborative engagement, providing the foundation for the specialized observational learning that underpins human-unique cultural transmission.