This thesis focuses on contingency in parent-child interaction, investigating it in the light of the linguistic capacity of the child and the status of the caregiver. Further, the thesis covers the development of two tools to assess the developmental maturity level of expressive phonology. A functional emergentist perspective on language acquisition is taken, which includes a phonetic perspective on phonological development. Both infant language development and factors that influence parent responsiveness are explored. 

The thesis contains four studies. In the first study, durations of parents’ utterances and pauses in interaction with their 18-month-old infants were related to the infant’s vocabulary size. Recordings of interactions of fifteen children and their parents were made at home in daily life situations. The children were divided into three groups according to their vocabulary size: large, typical or small. The main finding is that parents in the large vocabulary size group responded faster to their children compared to the parents in the typical size vocabulary group, who in turn responded faster than the parents in the small vocabulary size group. 

In study two, duration in vocal turn-taking between 6-month old infants and their caregivers was investigated, in terms of the status of the caregiver and the sex of the infant. Caregivers’ pauses were measured in 10-minute caregiver-infant interactions recorded at home. It was found that primary caregivers responded faster to their infants compared to secondary caregivers, and that in turn, infants responded faster to the primary caregiver than to the secondary caregiver. 

Study three introduces the Word Complexity Measure for Swedish (WCM-SE), a tool for calculating phonological complexity in words or utterances. Calculations are based on ten parameters describing speech structures that are considered phonetically complex to produce. In the development of  the WCM-SE, both language-specific and language-general descriptions of speech development were considered, as well as universal acoustic and aerodynamic principles. 

Study four documents the selection of Swedish words for the word lists in the test Profiles of Early Expressive Phonological Skills for Swedish (PEEPS-SE). The selection was based on criteria of age of acquisition and word complexity, as measured by the WCM-SE. 

The findings presented in this thesis contribute to our knowledge of early interaction and parents’ potential impact on the child’s early language and communication development. Further, the tools developed for the assessment of Swedish are valuable contributions both to the research field of early phonology and to clinical work in Sweden.