Development is a complex process, involving interactions between various domains across multiple levels of description. In order to understand both typical and atypical development, a number of large consortia have been established. The London Down Syndrome (LonDownS) Consortium is a multidisciplinary team of human geneticists, cellular biologists, psychiatrists, psychologists, neuroscientists, and mouse geneticists, whose aim is to understand the link between Down syndrome and Alzheimer’s disease, and to identify protective and risk factors that could inform interventions. I will present data from this large-scale study of Down syndrome with an emphasis on individual differences in infants and toddlers. I will then discuss whether running a large battery of tasks necessarily means embracing complexity. Many of our traditional developmental paradigms aim to isolate specific domains. Yet, everyday experiences emerge from the complex interactions of various domains – such as motor ability, attention allocation, and the actions of other social agents. I will introduce some of my plans to put these interactions at the very core of my research and explain why I believe it has the potential to reconceptualise our understanding of neurodevelopmental disorders.