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.
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
http://www.yumingschool.org/wp-content/uploads/2011/07/Bilin...
> 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.