Standard economic models assume people exclusively pursue material self‐interests in social interactions. However, people exhibit social preferences; that is, they base their choices partly on the outcomes others obtained in a social interaction. People care about fairness, and reciprocity affects behavior. This study examines the differences in negative reciprocity (costly punishment for unfair divisions) as a function of age. Sixty‐one kindergarteners (5‐year‐olds), 53 second graders (8‐year‐olds), and 57 sixth graders (12‐year‐olds) played a dictator game or a mini–ultimatum game either with a human proposer or with a random machine that determined the division between the two players. By keeping the divisions between the players constant and varying the source of the unfair proposal, we were able to differentiate between reciprocity‐based and inequality‐aversion preferences. We found that kindergarteners proposed and accepted unfair divisions regardless of the source of the offer, behaving according to the standard economic model. Children in the sixth grade tended to reject unfair offers from a human proposer but accept unfair divisions from a random device, indicating the emergence of negative reciprocity preferences by age eight (and contrary to inequality aversion). Children at this age also tended to give more fair offers in the ultimatum game than in the dictator game, indicating the emergence of strategic thinking. Copyright © 2012 John Wiley & Sons, Ltd.