DIAZ, RAFAEL M. Bilingual Cognitive Development: Addressing Three Gaps in Current Research. CHILD DEVELOPMENT, 1985, 56, 1376-1388. 100 Spanish(L1)-English(L2) bilingual children attending kindergarten and first grade bilingual education programs were tested at 2 points in time, 6 months apart. Children in the sample (ages 5-7 years) represented extreme groups of high and low secondlanguage proficiency. At each time of testing, children's proficiency in Spanish and English was assessed and a battery of cognitive tasks measuring visual and verbal abilities was administered. The findings indicate that observed cognitive differences between second-language-proficiency groups can be attributed to group differences in socioeconomic variables. The relationship between degree of bilingualism and cognitive abilities was positive and rather strong for children of low secondlanguage proficiency. This pattern of results questioned the validity of Cummins's threshold hypothesis and suggested a new, alternative hypothesis-namely, that degree of bilingualism is related to variability in cognitive measures only before a certain threshold of proficiency in the second language is attained. In addition, 2 sets of statistical analyses on the longitudinal data gave support to a cause-effect model in which degree of bilingualism appears as the causal factor affecting children's cognitive abilities.