Since the mid-20th century, there have been dramatic changes in our conception of how bilingualism affects children’s cognitive development, moving from one of certain negativity, to unlimited advantage, and finally resting in a current state defined largely by confusion because of the complexity in how bilingualism is defined. However, the question has great consequences, so it is important to evaluate the evidence to understand the impact. Such information determines how families make decisions about their home language, particularly regarding the maintenance of heritage languages; how schools offer programs based on alternative languages; how clinicians assess children for learning or other special needs; and how communities offer services to diverse members. By defining the concepts more precisely than has typically been the case, the complexity of the relation between bilingualism and cognition becomes clear. The evidence shows that bilingualism impacts cognitive level and brain function across the lifespan, but the nature and extent of those effects are modified by the type and degree of bilingualism and the nature of the task. Understanding the conditions under which various effects emerge is essential for interpreting the effects of bilingualism on children’s cognitive development and their potential role in education.
