Bilingual young people are able to process verbal information better than monolingual children.
The hypothesis is that mutual interference between the bilingual child’s 2 languages forces the child to develop particular coping strategies which in some ways accelerate cognitive development. The sample consisted of groups of Hebrew-English bilingual children, 1 group tested in the United States and the other group tested in Israel, and 2 groups of monolingual Jewish children, with those tested the United States speaking only English and those tested in Israel speaking only Hebrew. In groups parent occupation and education level were similarly high. WISC IQ was estimate from 4 subtests-Similarities, Digit Span, Picture Completion, and Picture Arrangement-a statistically controlled. In spite of lower vocabulary level, bilinguals showed more advanced processing of verbal material, more discriminating perceptual distinctions, more propensity search for structure in perceptual situations, and more capacity to reorganize their perception in response to feedback
Ben-Zeev, S., 1977. The influence of Bilingualism on Cognitive Strategy and Cognitive Development. Child Development, 48(3), pp. 1009-1018.