In fact, studies carried out in the U.S., Ireland, and Wales until the 1960s did report that bilingual children were significantly disadvantaged linguistically compared to monolingual children, learned command of language more slowly, and ended up with smaller vocabularies in each language. But it was eventually realized that that interpretation was confounded by other variables correlated with bilingualism in those studies. In the U.S. more than in other countries, bilingualism is associated with poverty. When American bilingual children were compared with American monolingual English-speaking children, the latter tended to be from more affluent communities, to be attending better schools, and to have more educated and wealthy parents working at higher occupational levels and with larger vocabularies. Those correlates of bilingualism alone might have accounted for the lower language skills of the bilingual children.
More recent studies in the U.S., Canada, and Europe control for those other variables, by comparing bilingual and monolingual children attending the same school and matched for parental socio-economic status. It turns out that bilingual children and monolingual children matched in other respects pass milestones of language acquisition (e.g., age to say first word, first sentence, or to acquire a 50-word vocabulary) at the same age. Depending on the study, either bilingual and monolingual children end up as adults with essentially the same vocabulary size and word-retrieval rate, or else the monolingual children end up with a slight advantage (vocabulary up to 10% larger in their sole language). However, it would be misleading to summarize this result by saying, “Monolingual children end up with a slightly larger vocabulary: 3,300 versus only 3,000 words.” Instead, the result is, “Bilingual children end up with a much larger vocabulary: a total of 6,000 words, consisting of 3,000 English words plus 3,000 Chinese words, instead of 3,300 English words and no Chinese words.”
Studies to date have not demonstrated generalized cognitive differences between bilingual and monolingual people. It is not the case that one group is on the average smarter or thinks more quickly than the other group. Instead, there appear to be specific differences, such as (perhaps) slightly faster word retrieval and ability to name objects on the part of monolinguals (because they don’t have the problem of selecting among different names, all correct but in different languages familiar to them). Among these specific differences, the most consistently established to date involves what cognitive scientists term “executive function,” and that difference is in favor of bilinguals.
To understand the meaning of executive function, picture a person doing anything at all, e.g., crossing a street. Reflect that we are constantly bombarded by sensory information in many modalities, including sight, sound, smell, touch, and taste, plus our own thoughts. Into the pedestrian’s senses flood the sight of billboards and of clouds overhead, the sounds of people talking and birds singing, the smells of the city, the touch sensation of the pedestrian’s feet on the pavement and of his arms swinging at his sides, and thoughts of what his wife said to him at breakfast that morning. If he were not crossing a street, the pedestrian would concentrate on the words of people or on the sight of billboards or on his wife’s most recent words. When crossing a street, though, his survival requires that he concentrate on the sights and sounds of cars approaching at different speeds from both directions, and on the feeling of his feet stepping off the curb. That is, to do anything at all in life requires inhibiting 99% of one’s sensory input and thoughts at any moment, and paying attention to the 1% of input relevant to the task currently at hand. That brain process of executive function, also known as cognitive control, is believed to reside in the brain area known as the pre-frontal cortex. It’s what permits you to pay selective attention, to avoid being distracted, to concentrate on solving a problem, to shift between tasks, and to call up and use the word or bit of information needed at the moment out of your huge stockpile of words and information. That is, executive control is a big deaclass="underline" it’s crucial to our functioning competently. In children, executive control develops especially over the course of the first five or so years of life.
Bilingual people have a special issue of executive control. Monolingual people hearing a word compare it with their single stock of words, and when uttering a word they draw it from their single stock. But bilingual people must and do keep their languages separate. Every time they hear a word pronounced, they must instantly know according to which set of arbitrary rules to interpret the meaning of those sounds: for instance, a Spanish/Italian bilingual has learned that the sounds b-u-rr-o mean “donkey” in Spanish but “butter” in Italian. Every time bilinguals wish to say something, they must call up the words from the language being used in the current conversation, and not from their other language. Multilingual people participating in a group bilingual conversation, or Scandinavian shop assistants, must switch those arbitrary rules every few minutes or even more often.
The importance of executive control for multilingual people was brought home to me by a disconcerting failure of it on my part. When I went to work in Indonesia in 1979 and began learning the Indonesian language, I had already lived for extended periods in Germany, Peru, and Papua New Guinea, and I had become comfortable at speaking German, Spanish, or Tok Pisin without confusing those languages with each other or with English. I had also learned some other languages (especially Russian) but had never lived in their countries long enough to gain experience of speaking them continually. When I was initially talking with Indonesian friends, I was astonished to discover that my intention of pronouncing an Indonesian word often resulted in my uttering the Russian word with the same meaning, despite the Indonesian and Russian languages being completely unrelated! I had evidently learned to separate English, German, Spanish, and Tok Pisin into four well-controlled pigeonholes, but I was still left with an undifferentiated fifth pigeonhole equivalent to “language other than English, Spanish, German, and Tok Pisin.” Only after more time in Indonesia was I able to inhibit the stock of Russian words lurking out of control in my mind and ready to creep into my Indonesian conversations.
In short, bilingual or multilingual people have constant unconscious practice in using executive control. They are forced to practise it whenever they speak, think, or listen to other people talking—i.e., constantly throughout their waking hours. In sports, art performance, and other arenas of life, we know that skills improve with practice. But: which are the skills that practice of bilingualism improves? Does bilingualism merely develop bilinguals’ specific skill at switching between languages, or is bilingualism more generally useful to them?
Recent studies have devised tests to explore this question by comparing problem-solving by bilingual and monolingual people ranging from 3-year-old children to 80-year-old adults. The overall conclusion is that bilinguals of all those ages have an advantage at solving only a specific type of problem. But it is a broad specific problem: solving tasks that are confusing because the rules of the task change unpredictably, or because there are misleading and irrelevant but glaringly obvious cues that must be ignored. For instance, children are shown a series of cards depicting either a rabbit or a boat that is either red or blue, and that does or doesn’t have a gold star. If a gold star is present, children must remember to sort the cards by color; if a gold star is absent, they must remember to sort the cards by the object depicted. Monolingual and bilingual subjects are equally successful at such games as long as the rule remains the same from trial to trial (e.g., “sort by color”), but monolinguals have much more difficulty than bilinguals at accommodating to a switch in rules.