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Ostracism, or social exclusion, is widespread and associated with a range of detrimental psychological and social outcomes. Ostracism is typically explained as instrumental punishment of free-riders or deviants. However, this instrumental account fails to explain many of the features of real-world ostracism, including its prevalence. Here we hypothesized that ostracism can emerge incidentally (non-instrumentally) when people choose partners in social interactions, and that this process is driven by simple learning mechanisms. We tested this hypothesis in four experiments (n = 456) with economic games in dynamic social networks. Contrary to the instrumental account of ostracism, we find that the targets of ostracism are not primarily free-riders. Instead, incidental initial variability in choosing partners for social interactions predicts later ostracism better than the instrumental account. Using computational modelling …