[thedavinci2000]

Sounds like something a monolingual would say/snark

Anecdotes are just that. I know plenty of people that can dominate two languages fluently.

Recently read a study where a stroke patient can recover faster if multilingual or that dementia can be kept at bay longer.

[cryptotype]

Regarding fluency, bilinguals and monolinguals often have often have similar total vocabulary sizes, but bilinguals tend to have a significantly smaller vocabulary in each language.[1, 2] In other words, vocabulary size(L1, monolingual) ~= size(L1 + L2, bilingual), and size(L1, monolingual) < size(L1, bilingual).

1. Portocarrero et al. (2007). http://www.sciencedirect.com/science/article/pii/S0887617707...

2. Gatt et al. (2015). http://ijb.sagepub.com/content/early/2015/03/13/136700691557...

[beachstartup]

> Sounds like something a monolingual would say/snark

how do you think i knew they had poor grammar in both languages?

most people can't even speak or write a single language well, of course a huge number of people of average intelligence/aptitude who are forced to speak two languages are going to be terrible at both.

how many people can 'dominate' their native tongue? not many.