Facing variation in early vocabularies: How group classification of autism and individual looking behaviours to faces influence vocabulary outcomes in the first years of life

Language forms the foundation of our thoughts and enables us to communicate and connect with others. Therefore, learning to comprehend and use language is important for building social connections and integration. Infants start to understand words at around six months, and this is followed by speaking at around one year of age. There is significant variability in the pace at which children learn words, and this can influence their future academic and employment opportunities. Early language difficulties may result in long-term challenges with communication. This PhD thesis has two main objectives: to examine how infants at higher likelihood of or later diagnosis of autism differ in vocabulary outcomes from typical likelihood or non-diagnosed infants, and to investigate how individual differences in infants’ gaze behavior towards faces relate to their vocabulary outcomes. The thesis draws on data from two cohorts: Eurosibs, focusing on children more likely to be diagnosed with autism, and the Youth cohort, tracking children's development from birth until adolescence. In Chapter 2, this thesis reviews existing literature to understand how both the expressive and receptive vocabularies of children with an elevated likelihood or later diagnosis of autism compare to typical likelihood or non-diagnosed children. The findings indicate that elevated likelihood/diagnosed infants have smaller vocabularies than typical likelihood/non-diagnosed peers. These differences become more pronounced with age, especially between diagnosed and non-diagnosed children. Chapter 3 then investigates the agreement between two methods of assessing language skills in 14-month-old infants: parent questionnaires (CDI) and behavioral assessments (MSEL). A moderate level of agreement is found between the two methods, suggesting that they are related, but may assess different aspects of language skills. The thesis then explores how infants’ preference to look at faces develops in the first year and how this preference might relate to later vocabulary outcomes. Chapter 4 examines how looking behaviours to faces might predict vocabulary outcomes, with studies showing that infants who pay more attention to faces tend to have larger vocabularies later on. Chapter 5 then investigates how looking behaviours to faces develops between the first 4 and 7 months after birth, finding that face preference and duration of looking to faces increases with age. Then, Chapter 6 investigates how looking behaviours to faces and object exploration at 4-7 months relate to vocabulary at 2.3-4 years, finding that quicker gazes to faces and more exploration of objects are associated with larger vocabularies. The significance of these relationships varies with age. This PhD thesis explores the variability in vocabulary outcomes during early childhood. It examines differences between autistic and non-autistic children and investigates how individual differences in looking behaviour to faces and object exploration relate to later vocabulary outcomes. The findings indicate that the timing of assessments is crucial for detecting group differences and individual differences in vocabulary outcomes. Future research should consider how the impact of factors varies with the child's age at measurement. This insight is also important when designing effective interventions for vocabulary-related disorders, designing approaches that are adapted over the course of development.