One misleading belief that resulted from a century's worth of writing on bilingualism is that children's intelligence will suffer if they are bilingual. Some of the earliest research to bilingualism looked at whether bilingual children were behind or ahead of monolingual children in IQ tests. From the 1920s to the 1960s, the tendency was to believe that monolingual children were ahead of bilingual children in IQ tests. The conclusion was that bilinguals were mentally confused. It was said that having two languages in one brain impaired efficient thought processes. It was argued that having one more developed language was better than having two half developed languages.
Early research was misconceived. First, such research often gave bilinguals an IQ test in their weaker language - usually English. Had bilinguals been tested in Welsh or Spanish or Hebrew, a different result may have been found. The testing of bilinguals was thus unfair. Second, like was not compared with like. Bilinguals tended to come from, for example, impoverished New York or rural Welsh backgrounds. The monolinguals tended to come from more middle class, urban families. Working class bilinguals were often compared with middle class monolinguals. So the results were more likely to be due to social class differences than language differences. The comparison of monolinguals and bilinguals was unfair.
The most recent research suggests that bilinguals are, at the least, equal to monolinguals on IQ tests. When bilinguals have two well developed languages (in the research literature called balanced), bilinguals tend to show a slight superiority in IQ scores compared with monolinguals. This is the received psychological wisdom of the moment and is the good news for raising bilingual children. Far from making people mentally confused, bilingualism is now associated with a mild degree of intellectual superiority.
The current state of psychological wisdom about bilingual children is that, where two languages are relatively well developed, bilinguals have thinking advantages over monolinguals.
Research across different continents of the world shows that bilinguals tend to be more fluent, flexible, original and elaborate in their answers to this type of open-ended question. The person who can think of a few answers tends to be termed a convergent thinker. They converge onto a few acceptable conventional answers. People who think of lots of different uses for unusual objects (e.g. a brick, tin can, cardboard box) are called divergers. Divergers like a variety of answers to a question and are imaginative and fluent in their thinking.
While many monolinguals are divergers, there is a tendency for bilinguals to be ahead of monolinguals on such tests of creativity and divergent thinking. Having two or more words for each object and idea may mean there is more elasticity in thinking. A child may have different associations for the word ‘brick' in each language. For example, a Welsh/English bilingual has the word ‘school' and its Welsh equivalent ‘ysgol'. ‘Ysgol' also means ladder. The idea of school is thus extended to an image of schooling being a ladder. There is a sequential climb through school learning with the aim of getting to the top rung.
There are other dimensions in thinking where bilinguals with two well developed languages may have temporary and occasionally permanent advantages over monolinguals: increased sensitivity to communication, a slightly speedier movement through the stages of cognitive development, and being less fixed on the sound of words and more centred on the meaning of words. For example, imagine young children are asked: what is more like the word ‘cap', ‘cat' or ‘hat'? There is a tendency for bilinguals to centre more on similarity of meaning (i.e. the word ‘hat') than similarity of sound (i.e. the word ‘cat'). Such ability to move away from the sound of words and to fix on the meaning of words tends to be a (temporary) advantage for bilinguals around the ages four to six. This advantage may mean an initial head start in learning to read and learning to think about language.