Word frequency as a cue for identifying function words in infancy.

While content words (e.g., 'dog') tend to carry meaning, function words (e.g., 'the') mainly serve syntactic purposes. Here, we ask whether 17-month old infants can use one language-universal cue to identify function word candidates: their high frequency of occurrence. In Experiment 1, infants listened to a series of short, naturally recorded sentences in a foreign language (i.e., in French). In these sentences, two determiners appeared much more frequently than any content word. Following this, infants were presented with a visual object, and simultaneously with a word pair composed of a determiner and a noun. Results showed that infants associated the object more strongly with the infrequent noun than with the frequent determiner. That is, when presented with both the old object and a novel object, infants were more likely to orient towards the old object when hearing a label with a new determiner and the old noun compared to a label with a new noun and the old determiner. In Experiment 2, infants were tested using the same procedure as in Experiment 1, but without the initial exposure to French sentences. Under these conditions, infants did not preferentially associate the object with nouns, suggesting that the preferential association between nouns and objects does not result from specific acoustic or phonological properties. In line with various biases and heuristics involved in acquiring content words, we provide the first direct evidence that infants can use distributional cues, especially the high frequency of occurrence, to identify potential function words.