Changes in the Ability to Detect Ordinal Numerical Relationships Between 9 and 11 Months of Age

When are the precursors of ordinal numerical knowledge first evident in infancy? Brannon (2002) argued that by 11 months of age, infants possess the ability to appreciate the greater than and less than relations between numerical values but that this ability experiences a sudden onset between 9 and 11 months of age. Here we present 5 experiments that explore the changes that take place between 9 and 11 months of age in infants' ability to detect reversals in the ordinal direction of a sequence of arrays. In Experiment 1, we replicate the finding that 11- but not 9-month-old infants detect a numerical ordinal reversal. In Experiment 2 we rule out an alternative hypothesis that 11-month-old infants attended to changes in the absolute numerosity of the first stimulus in the sequence rather than a reversal in ordinal direction. In Experiment 3, we demonstrate that 9-month-old infants are not aided by additional exposure to each numerosity stimulus in a sequence. In Experiment 4 we find that 11-month-old but not 9-month-old infants succeed at detecting the reversal in a nonnumerical size or area-based rule, casting doubt on Brannon's prior claim that what develops between 9 and 11 months of age is a specifically numerical ability. In Experiment 5 we demonstrate that 9-month-old infants are capable of detecting a reversal in ordinal direction but only when there are multiple converging cues to ordinality. Collectively these data indicate that at 11 months of age infants can represent ordinal relations that are based on number, size, or cumulative area, whereas at 9 months of age infants are unable to use any of these dimensions in isolation but instead require a confluence of cues.