This weekend Johnson enjoyed an American holiday in Berlin: the children's Halloween party held by neighbours, a half-German, half-American couple. Besides mermaid tails, ladybug antennas or monster horns, nearly every one of the nippers at the party had another accessory: a second language.
Johnson’s own nipper is still pre-verbal at nearly 18 months, meaning that every request not immediately understood and satisfied may quickly turn into a piercing shriek. But we take comfort that Johnson, junior, is cognitively just fine. If his language comes a little late, that is probably because, for one thing, he is male, and for another, he is surrounded every day by three languages: English and Danish at home, and German all day at nursery. More confusingly still, the three languages are closely related: is it bread, Brot or brød? Apple, Apfel or æble? House, Haus or hus? The earthy words in English are mostly Germanic, meaning these triplets are coming up in his world all the time.

Children raised bilingual or multilingual show similar results. In early days they will mix languages. They make errors by using the syntax of one language and the words of another. (“Touch the guitar”, my old Spanish teacher’s daughter would say, instead of “Play the guitar”.) But these problems disappear quickly. By three or four, children reliably separate the languages, knowing which can be spoken with whom. Their fluency in each would be the envy of any adult language-learner.
Many parents once believed that a second language was a bad idea, as it would interfere with developing the first and more important one. But such beliefs are woefully out of date today. Some studies (such as this one) seem to show that bilinguals have smaller vocabularies in each language (at early stages) than monolinguals do. But other studies (such as this one) find no vocabulary shortfall in either language. In any case, the influence of mono- or bilingualism on vocabulary size is later overtaken by the importance of education, socio-economic status, reading and writing habits. In short, there is little evidence that raising a child bilingual will hurt their primary language.

The benefits, by contrast, are both strong and long-lasting. Bilingual children as young as seven months outperform monolinguals at tasks requiring “executive function”: prioritising and planning complex tasks and switching mental gears. This is probably because monitoring the use of two languages is itself an exercise in executive function. Such studies control for socio-economic status, and in fact the same beneficial effects have been shown in bilingual children of poor families. Finally, the effects appear to be lifelong: bilinguals have later onset of Alzheimer’s disease, on average, than do monolinguals.

All this is hot evidence for a mental exercise that could give children a lifelong advantage. Should you then run out and sign your child up for whatever language you can find? Alas, no. As the saying goes, “for whosoever hath, to him shall be given.” Multiple languages are best for you when you’ve had them from birth. The dramatic studies here work with “crib” bilinguals, children raised with both languages spoken by natives in their homes.

And even having a native speaker among the parents at home is not by itself enough. If a child is raised by one monoglot Anglophone and one bilingual in an English-speaking country, the child's second language may atrophy if the bilingual parent isn’t strict about conducting all exchanges with the child in this language. This is the root of the “one parent, one language” theory that many bilingual families swear by. By this theory, consistency is important for the learning brain.
But one researcher on the topic, François Grosjean (who blogs here), disagrees that one-parent, one-language is a must. Instead, he says, “the need factor is crucial”—that is, the child must experience regular monolingual situations in each language. If there are no domains (school, travel, grandparents) where only one of the two languages will do, “children are very good at judging whether it is worth maintaining a language or letting it wither away.” One option he recommends is to speak only one language at home and the other outside the home. (This requires both parents to be fairly fluent in both languages, though.)

For parents who cannot make their children “crib bilinguals”, there are, of course, still many reasons to teach children foreign languages, and many benefits. Here, still, time is not on parents’ and teachers’ sides. The earlier children begin the second language, the better they will learn it.

Norway has already introduced English in the first year of school, and Denmark is soon to do so. These countries, unlike France, Germany or Spain, have very small languages of their own, so they know language ability is crucial to their future competitiveness. Talk about the “need factor”.