Reduced Affiliative Behavior in Autism Reflects Greater Dependence on Perceived Similarity

Social difficulties in autism are often framed as reduced motivation, yet this account does not explain when and why autistic individuals affiliate. We show that autism selectively alters the architecture—not the presence—of similarity-based social behavior. Across two independent samples (online, n = 714; in-person, n = 225), autistic and neurotypical adults exhibited comparable context-dependent (selective) preference for relatively similar others. In contrast, autistic individuals showed a markedly stronger global coupling between perceived similarity and affiliative behavior, such that low perceived similarity was associated with sharply reduced affiliation. This effect was strongest among those with lower trait empathy. Structural MRI revealed dissociable contributions of hippocampal and posterior cingulate cortex volumes to this coupling across groups. These findings demonstrate that autism preserves similarity attraction while amplifying its role as a stable heuristic for social engagement, supporting a model in which social motivation is restructured toward similarity-dependent engagement rather than diminished.