Here's an article "Looking for Baby Sitters: Foreign Language a Must" by Jenny Anderson in the NYT that starts off as the usual Bogus Trend story and goes off in an interesting direction:

Parents cite different reasons for hiring baby sitters and nannies to speak a second language with their children. Some struggled to pick up foreign languages and want to make life easier for their children. Some believe it makes them smarter. And naturally, this being the melting pot that is New York, many parents have a connection to another language and want to reinforce it...

One other reason is to discriminate against African-Americans when advertising for a nanny. Putting in a foreign language requirement is a legal way to state No African-Americans Need Apply.

Indeed, not long ago, many parents insisted that their foreign-language-speaking nannies refrain from using their native tongue and speak only English with their children, for fear that another language might muddle their English-language development....

In fact, research shows that learning a second language makes it easier to learn additional languages.

In recent years, a number of neuroscientists and psychologists have tried to untangle the impact of bilingualism on brain development. ”It doesn’t make kids smarter,” said Ellen Bialystok, a professor of psychology at York University in Toronto and the author of ”Bilingualism in Development: Language, Literacy and Cognition.”

”There are documented cognitive developments,” she said, ”but whatever smarter means, it isn’t true.”

Ms. Bialystok’s research shows that bilingual children tend to have smaller vocabularies in English than their monolingual counterparts, and that the limited vocabulary tends to be words used at home (spatula and squash) rather than words used at school (astronaut, rectangle). The measurement of vocabulary is always in one language: a bilingual child’s collective vocabulary from both languages will probably be larger.

”Bilingualism carries a cost, and the cost is rapid access to words,” Ms. Bialystok said. In other words, children have to work harder to access the right word in the right language, which can slow them down – by milliseconds, but slower nonetheless.

At the same time, bilingual children do better at complex tasks like isolating information presented in confusing ways. In one test researchers frequently use, words like ”red” and ”green” flash across a screen, but the words actually appear in purple and yellow.

Bilingual children are faster at identifying what color the word is written in, a fact researchers attribute to a more developed prefrontal cortex (the part of the brain responsible for executive decision-making, like which language to use with certain people)....

One arena in which being bilingual does not seem to help is the highly competitive kindergarten admission process.

”It doesn’t give you a leg up on the admissions process,” said Victoria Goldman, author of the sixth edition of ”The Manhattan Family Guide to Private Schools.” It is one piece of the bigger puzzle, which includes tests scores, interviews and the ability of a child to follow directions. ”Speaking another language is indicative that you are verbal, but you have to be behaved.”

George P. Davison, head of school at Grace Church School, a competitive downtown school, said that bilingualism tended to suppress verbal and reading comprehension test scores by 20 to 30 percent for children younger than 12. ”If anything, it can have a negative effect on admissions,” he said.

I love how politically incorrect the NYT is when it comes to providing Reader Service about the single most burning issue for NYT subscribers: getting your kid into an exclusive private kindergarten.

I researched this topic a decade ago for an article on Canada's experience with bilingualism. Being Anglo-French bilingual gives you huge advantages in getting to the top of the Canadian civil service pyramid (and Canadians love civil service jobs). Public schools that conduct half their classes in French and half their classes in English were very fashionable. One problem, though, was that boys often struggled, and wound up dropping out. But upper middle class girls tended to thrive in a dual immersion system.

And the extra cognitive demands of Anglo-French bilingualism tended to keep the working class kids out of these public schools, so that was all to the good from a Stuff White Canadian People Like perspective.

So, if you have a bright, highly chatty little girl, slowing her talking down a few milliseconds by having her learn a foreign language probably is a tradeoff worth taking. On the other hand, if you have a little boy who can't talk like Robert Downey Jr., well, it's something to watch out for.

In general, however, this whole discussion, which I remember having in 1972 with my parents when we had to choose a foreign language for me to study, seems to be getting less and less important for Americans. Americans speak English and English is ever more the world-dominant language. I don't know if that's such a great thing for the world, but it does make life more convenient for native English-speakers.