Even Simultaneous Bilinguals Do Not Reach Monolingual Levels of Proficiency in Syntax

While there is no doubt that children raised bilingual can become extremely proficient in both languages, theorists are divided on whether bilingualism effectively monolingualism twice (the “Two Monolinguals One Brain” hypothesis) or differs some fundamental way from monolingualism. A strong version of the Monolinguals” hypothesis predicts bilinguals achieve monolingual-level proficiency either (or both) their languages. Recently, Bylund and Abrahamsson argued evidence lower syntactic simultaneous was due to confounds language dominance; when tested primary language, any difference disappears. We find for this hypothesis. Meta-analysis Monte Carlo simulation show variation published results fully consistent with sampling error, method mattered. Meta-analytic estimates strongly indicate performance by relative monolinguals. Re-analysis a large dataset (N = 115,020) confirms finding, even controlling dominance. Interestingly, effect relatively small, challenging current theories.