What was the serious flaw in early studies claiming that bilingualism stunts cognitive development?

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Early studies claiming that bilingualism stunts cognitive development had a serious flaw, in that the bilinguals were often immigrants whose poor command of English gave them a disadvantage during the tests. This was not a fair comparison between monolingual and bilingual children, as the monolingual children were prepared in advance for the tests, whereas the bilingual children were not given this opportunity. Later studies have found that bilingualism does not stunt cognitive development and can actually have cognitive benefits.

What Was the Serious Flaw in Early Studies Claiming That