Most people today, I would think, would agree that being bilingual is a desirable skill, and that bilingualism grants a person a clear advantage in life.
Surprisingly, this was not always the prevalent opinion. In studies conducted in the early 20th century, there appeared to be a clear consensus that bilingual children had distinct disadvantages when it came to their cognitive development. In these studies bilinguals performed poorly when compared to monolinguals on intelligence tests.
Then, later in the century, the results suddenly changes and bilinguals began to outperform monolingual children, and the common wisdom became what it is today: Bilinguals have advantages – not all the advantages, as we’ll see, but many advantages.
These sorts of studies are notoriously difficult. Intelligence tests, whether verbal or non-verbal, must take into account a host of factors, ranging from the economic background of the children to cultural biases to flaws in the formulation of questions. IQ tests in general are not universally accepted as useful measurements of intelligence because of these problems. I think it is very likely that the earlier studies were badly flawed and possibly even biased against bilinguals.
Advantages and Disadvantages
Today it’s pretty clear that bilinguals excel at both linguistic reasoning (that is, analysing languages and being able to separate meaning from structures) and in other problem-solving tests. While this is only one aspect of overall intelligence and capability, it is a clear advantage.
However, monolinguals do test better by a fair margin in one aspect of language skills: Vocabulary recognition. In tests where children are asked to pair the correct word with a picture or drawing, monolinguals show a clear advantage. This makes sense, as bilinguals must sort through two vocabularies when seeking the right word. This may not seem like much of an advantage, but anyone who has seen a bilingual hesitate while they seek the correct word for a thought knows that it can in fact have real-world consequences.
Of course, language skills in general are not really much of a measurement of overall intelligence. One can speak many languages very easily and still not be particularly bright, after all. Language is often acquired on an almost primal level, especially in young children. And, of course, there is a lot of daylight between being able to get by in a language and real fluency. One should never make the error of assuming someone is smart just because they speak an extra language!