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3 experiments examined 7-month-old infants' ability to discriminate the facial expressions of happy vs. fear. Experiment 1 revealed that infants demonstrated discrimination of happy vs. fear expressions when posed by a single model but that this discrimination was affected by the order of stimulus presentation. In experiment 2 infants were shown 2 models posing the happing and fearful expressions and did not demonstrate generalization of the discrimination of happy versus fear across these models. The third and main experiment varied the test procedure. Here, infants demonstrated reliable generalized discrimination of the 2 expressions across different models. As in experiment 1, however, these results were constrained by the order in which the stimuli were presented. These consistent order effects were not due to the initial salience of the 2 expressions but, instead, appeared to reflect differential rates of habituation to happy vs. fear expressions.
Nelson et al. (Sat,) studied this question.