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To determine whether infants can abstract invariant face expressions across different persons (i.e., can form face expression categories), groups of 18-, 24-, and 30-week-old infants (18 boys and 18 girls per group) were habituated by the infant control procedure to photographs of 4 different female faces all wearing an identical expression (happy or surprise). In an immediately following test phase, categorization was inferred from greater generalization of habituation (less recovery of fixation) to 2 new female faces in the familiarized expression than to the same new faces in the altered (novel) expression. To rule out the possibility that generalization at test might be due to failure to discriminate the new persons, control groups of 18 boys and 18 girls at each age saw the same test faces following repeated presentations of only 1 of the 4 habituation faces. The results indicated that not until 30 weeks could infants differentiate happy and surprise expressions on a categorical basis. At 24 weeks they could distinguish a surprise expression following habituation to happy faces, but could not do the reverse. At 18 weeks they could do neither. Overall, the performance of girls was superior to that of boys. The findings are consistent with recent evidence suggesting that the ability to extract invariant configural information relative to the human face does not emerge until about 7 months of age.
Caron et al. (Sun,) studied this question.