I want to be careful about these sort of research. Too many times I've seen these kind of "research" is often funded by commercial entities that teach foreign languages. It's like what PG had described in his "Submarine" essay.
There is a big industry to teach foreign languages to toddlers and even new born. There is quite a bit of reputable research to back up positive effects however no one seems to be doing research on if there are any negative effects. This is like every research starts with a hypothesis like there are either positive effects or nothing . But on the other hand there are proven facts that toddlers/babies learn only certain number of words in a given period. This implies kids at that age have limited capacity to memorize and recall things. So wouldn't that imply that bilingual training at that age would halve that capacity? Could bilingual babies have less volculbary in each language and late start in figuring out deeper nitty gritty of grammar? Is it possible that bilingual training at early age exchanges depth for a breadth?
What you say about research being partially corrupted by agendas is generally true and applies to pretty much every area of science to varying degrees.
The claim that second language acquisition is defined solely by this is, however completely false. It is a large and well-researched academic discipline and there exists plenty of research regarding negative effects. Here is a pretty decent summary, with a very compact abstract:
Bilingualism: The good, the bad, and the indifferent
> The present paper summarizes research showing that bilingualism affects linguistic and cognitive performance across the
lifespan. The effect on linguistic performance is generally seen as a deficit in which bilingual children control a smaller
vocabulary than their monolingual peers and bilingual adults perform more poorly on rapid lexical retrieval tasks. The effect
on cognitive performance is to enhance executive functioning and to protect against the decline of executive control in aging.
These effects interact to produce a complex pattern regarding the effect of bilingualism on memory performance. Memory
tasks based primarily on verbal recall are performed more poorly by bilinguals but memory tasks based primarily on
executive control are performed better by bilinguals. Speculations regarding the mechanism responsible for these effects are described.
There is a big industry to teach foreign languages to toddlers and even new born. There is quite a bit of reputable research to back up positive effects however no one seems to be doing research on if there are any negative effects. This is like every research starts with a hypothesis like there are either positive effects or nothing . But on the other hand there are proven facts that toddlers/babies learn only certain number of words in a given period. This implies kids at that age have limited capacity to memorize and recall things. So wouldn't that imply that bilingual training at that age would halve that capacity? Could bilingual babies have less volculbary in each language and late start in figuring out deeper nitty gritty of grammar? Is it possible that bilingual training at early age exchanges depth for a breadth?