When I was preparing my first book on bilingualism some thirty years ago, I was confronted with opposing views on the effect of bilingualism on children. Studies in the first half of the last century appeared to show that bilingual children had lower IQs and that they were outperformed by monolingual children in both verbal and nonverbal intelligence tests. Most of those studies concluded that bilingualism had a negative effect on the child's linguistic, cognitive and educational development.
Midway through the last century, the opinions changed rather suddenly and researchers found that bilingualism was, after all, a real asset for the child. Many studies came to the conclusion that bilinguals are more sensitive to semantic relations between words, are better able to treat sentence structure analytically, are better at rule-discovery tasks, have greater social sensitivity, and so on.
Why was there such a discrepancy between the studies of the first and the second half of the century? We now know that one of the main problems lay in making sure that the monolingual and bilingual groups used in the studies were truly comparable in every aspect, apart from their linguistic skills. Even though studies in the latter part of the century controlled for many factors, a slight bias may have favored bilingual children at that time.
I returned to the question three years ago when I was preparing my recent book on bilingualism. I contacted developmental psycholinguist Ellen Bialystok, the best-known authority in the field, and she kindly sent me papers to read and brought me up to date. What emerges from recent research is that the differences between bilinguals and monolinguals, when any are found, are specific to a particular task and can be quite subtle.
It is now clear that bilingualism enhances problem solving where the solutions depend on selective attention or inhibitory control (abilities of the executive control system, according to Bialystok). This advantage seems to continue throughout the bilingual's lifespan and is even present in elderly bilinguals. (I will write a post on this later).
The advantage shown by bilinguals - as discussed by Ellen Bialystok in a recent interview - is found also in certain metalinguistic abilities, that is, our capacity to analyze different aspects of language (sounds, words, syntax and so on) and, if needed, to talk about these properties. But the advantage is present only when selective attention or inhibitory control are needed to do the task. This is the case when a problem contains a conflict or an ambiguity such as counting words in a correct sentence, using a new (or made-up) name for an object in a sentence, judging that a sentence such as "apples grow on noses" is syntactically grammatical even though it contains a semantic anomaly, and so on.
When the metalinguistic task requires the analysis of representational structures, then monolinguals and bilinguals obtain similar results. This occurs when the task is to explain grammatical errors in a sentence, substitute one sound for another, interchange sounds, etc.
One domain where it would appear that bilinguals do less well than monolinguals is in vocabulary tests such as choosing a picture that illustrates the word spoken by the experimenter. This is not surprising, however, as bilingual children start being affected by the complementarity principle (see here) which states that bilinguals usually acquire and use their languages for different purposes, in different domains of life, and with different people. When bilingual children are evaluated in terms of both their languages, then the results improve greatly.
So where do we stand today on the effects of bilingualism? Ellen Bialystok and Xiaojia Feng give a reply: "The picture emerging from these studies is a complex portrait of interactions between bilingualism and skill acquisition in which there are sometimes benefits for bilingual children, sometimes deficits, and sometimes no consequence at all." (p. 121).
In sum, we now have a fuller and more complex picture of what the differences are between monolinguals and bilinguals - when differences exist!
Bialystok, E. & Feng, X. (2010). Language proficiency and its implications for monolingual and bilingual children. In A. Durgunoglu & C. Goldenberg (Eds.). Dual language learners: The development and assessment of oral and written language. (pp. 121-138). New York: Guilford Press.
Grosjean, F. Effects of bilingualism on children. Chapter 18 of Grosjean, F. (2010). Bilingual: Life and Reality. Cambridge, Mass: Harvard University Press.
"Life as a bilingual" posts by content area: http://www.francoisgrosjean.ch/blog_en.html
François Grosjean's website: www.francoisgrosjean.ch