We experimentally show that current models of reciprocity are incomplete in a systematic way using a new variant of the ultimatum game that provides second-movers with a marginal-cost-free punishment option. For a substantial proportion of the population, the degree of rst-mover unkindness determines the severity of punishment actions even when marginal costs are absent. The proportion of these subjects strongly depends on a treatment variation: higher xed costs of punishment lead to harsher

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... ponses. The fractions of purely selfish and inequity-averse participants are small and stable. Among the variety of reciprocity models, only one accommodates (rather than predicts) parts of our ndings. The treatment eect is unaccounted for. We discuss ways of incorporating our ndings into the existing models.