Infants Discriminate the Affective Expressions of their Peers: The Roles of Age and Familiarization Time.

Research examining infants' discrimination of affect often uses unfamiliar faces and voices of adults. Recently, research has examined infant discrimination of affect in familiar faces and voices. In much of this research, infants were habituated to the affective expressions using a "standard" 50% habituation criterion. We extend this line of research by examining infants' discrimination of unfamiliar peers', that is, 4-month-olds, dynamic, facial, and vocal affective expressions and assessing how discrimination is affected by changing the habituation criterion. In two experiments, using an infant-controlled habituation design, we explored 3- and 5-month-olds' discrimination of their peers' dynamic audiovisual displays of positive and negative expressions of affect. Results of Experiment 1, using a 50% habituation criterion, revealed that 5-month-olds, but not 3-month-olds discriminated the affective expressions of their peers. In Experiment 2, we examined whether 3-month-olds' lack of discrimination in Experiment 1 was a result of insufficient habituation (i.e., familiarization). Specifically, 3-month-olds were habituated using a 70% habituation criterion, providing them with longer familiarization time. Results revealed that using the more stringent habituation criterion, 3-month-olds showed longer habituation times, that is increased familiarization, and discriminated their peers' affective expressions. Results are discussed in terms of infants' discrimination of affect, the role of familiarization time, and limitations of the 50% habituation criterion.