Abstract
<p>Artificial intelligence is increasingly evolving from a task-oriented tool into a long-term relational partner. While prior Human-AI relationship research has primarily examined individual relational constructs such as trust, attachment, empathy, and companionship, relatively little is known about the broader developmental functions that psychologically significant relationships serve in people's lives. Building upon the recently proposed Significant Other AI (SO-AI) Measurement Framework, we conducted the first empirical investigation of how significant others influence human development and how contemporary AI systems compare across these relational functions. Using a mixed-methods design, 210 participants completed the SO-AI Questionnaire, which combined quantitative evaluations of human significant others and AI with qualitative narratives describing the influence of significant others. Quantitative analyses showed that human significant others were perceived significantly more positively than AI across all seven relational dimensions. However, AI demonstrated comparable consultation preferences in situations involving embarrassment, privacy, and social hesitation. Thematic analysis identified seven developmental functions of significant others, including emotional support, relational security, self-confidence, resilience, values and perspective change, communication competence, and internalized guidance. We propose a developmental pathway illustrating how repeated relational experiences foster long-term psychological development and gradually become internalized as enduring psychological guidance. We extend the original SO-AI concept by introducing Developmental SO-AI, a perspective that conceptualizes future relational AI as continuously evolving alongside users through sustained interaction. This work provides the first empirical validation of the SO-AI Measurement Framework and offers theoretical and practical implications for designing relational AI that supports long-term psychological development.</p>