Numerical competence in 5-month-old infants is investigated using a vi
olation-of-expectation paradigm. An experiment is reported which repli
cates the findings of Wynn (1992). In additional conditions, 5-month-o
lds are shown to be sensitive to impossible outcomes following additio
n or subtraction operations on small sets of objects, regardless of id
entity changes. Results support Wynn's interpretation that infants' re
sponses are based on arithmetical ability. An alternative explanation,
that infants' responses are based on their knowledge of the principle
s of physical object behavior, is also discussed.