Factors in the Child´s Growing Environment Influencing Early Vocabulary Development

Studies of early language development show a wide variability between children, especially in vocabulary size. Despite several earlier studies, there are still many open questions about which factors influence the variation in early vocabulary. Trajectories of language development are already established during the first few years, which is why this age is optimal to study.
The main aim of the thesis was to study environmental and biological factors influencing vocabulary development between 13 and 24 months of age. We also investigated whether these factors relate differently to vocabulary development in boys and girls.
All four studies in the thesis are sub-studies to Steps to the Healthy Development and Well-being of Children (the STEPS Study), a longitudinal prospective birth cohort study. The participants were recruited from an eligible cohort of 9,811 families, of which 1,797 with 1,805 children chose to participate in the STEPS Study. Number of participating children in Study I was 646, Study II 420, Study III 685, and Study IV 719 children.
Studies I and II, analysed the significance of recurrent respiratory tract infections (RTIs) on vocabulary size at 13 and 24 months of age and on vocabulary growth between 13 and 24 months of age in boys versus girls. Study III focused on the influence of paternal factors on vocabulary growth and Study IV analysed vocabulary growth in lexical categories in relation to child and family factors. The results of the studies show that recurrent RTIs are not a risk factor for vocabulary growth. The study results also show a significant difference between vocabulary growth in boys and girls. A difference between vocabulary growth in boys and girls was also observed in relation to risk and background factors. Vocabulary size at 13 months of age predicted vocabulary growth between 13 and 24 months of age in boys, but not in girls. Boys with fathers working less than full time had a larger vocabulary growth compared to boys with fathers fully employed. A maternal high level of occupation predicted larger vocabulary growth in boys compared to boys with mothers of lower occupational status. Girls with fathers having a high level of occupation had again, a larger vocabulary growth compared to girls with fathers of lower occupational status. The differences, between boys and girls, in how vocabulary growth is related to environmental and biological factors was also observed in
the growth of lexical word classes. In boys, a family burden of late onset of speech related negatively to vocabulary growth in all lexical categories except for sound effects. Boys in day care at 24 months of age had a larger vocabulary growth in the lexical category of sound effects. Girls attending day care at 24 months of age were predictive of larger growth in the lexical categories sound effects, nouns, people words, and games and routine words. Firstborn girls had a larger growth in the lexical categories of descriptive and functional words.
The present thesis shows that variation between boys and girls can be found not only in vocabulary size but also in vocabulary growth already at 13–24 months of age. It emphasizes environmental and biological factors related to early vocabulary growth and demonstrates that early vocabulary growth in boys and girls relates differently to these factors. The thesis highlights the need to analyse factors influencing language development separately in boys and girls. Doing so will give us a more comprehensive picture of differences between vocabulary growth in boys and girls and help ensure the best possible trajectory in language development.