During the past twenty years research has shown, with some degree of consistency, that learning a second language in childhood, either by simultaneous acquisition or in the context of bilingual education, is associated with positive cognitive gains. In both bilingual—monolingual comparisons and in studies using “within-bilingual” designs, children's bilingualism is positively related to concept formation, classification, creativity, analogical reasoning, and visual-spatial skills, to name a few (Diaz, 1983; Hakuta, Ferdman, & Diaz, 1987). In addition, as is evident in several chapters of the present volume, bilingual children have demonstrated a particularly refined awareness and control of the objective properties of language, commonly referred to as metalinguistic skills. Ben-Zeev (1977), for example, found that bilingual children approached linguistic tasks with a special sensitivity to language structure and detail. More recently, Bialystok (1986) has shown that children's bilingualism positively affects their increasing ability to solve problems involving high levels of control of linguistic processing.

Even though we have substantial documentation of the cognitive and metalinguistic advantages of childhood bilingualism, an important issue remains unresolved; namely, researchers have not yet developed and tested the validity of an explanatory model of how or why bilingualism has such positive effects. To date, it is not clear, for example, how bilinguals' metalinguistic skills are related to advantages in cognitive abilities not directly related to language, such as classification or visual spatial skills.