The possibility that early bilingualism affects children's language and cognitive development has long been a concern for parents and educators. In the first half of the 20th century, the prevailing view was that bilingualism and second-language acquisition early in life made children confused and interfered with their ability to develop normal cognitive functions and succeed in educational environments. These ideas were dramatically reversed in a landmark study by Peal and Lambert that showed a general superiority of bilinguals over monolinguals in a wide range of intelligence tests and aspects of school achievement. Recent research has been more balanced, identifying areas in which bilingual children excel and others in which bilingualism has no effect on their development.
Bialystok, E. (2008). Second-Language Acquisition and Bilingualism at an Early Age and the Impact on Early Cognitive Development. Encyclopedia on Early Childhood Development.