Two studies of 5-month-old infants explored whether a phenomenon reported by K. Wynn (1992) reflects a familiarity preference instead of a mathematical competence. Experiment 1 was a conceptual replication of Wynn's study. When data were analyzed with the relatively liberal statistical approach used by Wynn, the original phenomenon was replicated. However, an analysis of variance revealed that girls and boys behaved in different ways, and that boys did not behave as Wynn would have predicted. Experiment 2 was identical to Experiment 1, with one exception that should not have influenced computation: Infants in this study were completely familiarized with the test displays before testing. Again, the data revealed an interaction involving sex: Boys tended to be influenced by their familiarity with the test displays, whereas girls tended to behave as Wynn would have predicted. These findings are discussed with reference to the literature on sex differences--specifically the finding that male infants are typically immature relative to their female peers--as well as to articles that have been critical or supportive of Wynn's conclusions.