Why do so many children who hear two languages speak just a single language?

Twenty years ago De Houwer (1999) asked why young children reared with two languages speak just a single language. At the time, there was little research that could address the question. This contribution reviews research from the last two decades that either directly or indirectly addresses the problem of single language use by bilingually raised children. Amongst others, it focuses on the role of parental input patterns, the quantity and quality of language input, parental discourse strategies, the role of institutions such as day care centers and preschools, and child agency