Vocal communication in the first 18 months of life.

Fifty-one normally developing infants aged birth to 18 months, 10 or 11 in each of five age groups, were videorecorded in their homes before and after an expected change in the form of their vocalizations and under a set of conditions that reflected common daily occurrences. The vocalizations produced were coded according to their communicative contexts, defined in nonvocal behavioral terms. Communicative codes were assigned to seven major categories. The distribution of codes across categories was found to be different for different age groups. It varied between the first and second observations; however, the pattern of change differed across age groups. Data from individuals were transformed to proportions, to control for individual differences in productivity. They were then found to reflect differences in level of development of vocal communication. It was concluded that vocal communication follows an orderly developmental sequence in normally developing infants in the first 18 months of life.