Bilingual children are no more prone to language problems, learning delays, or diagnosed language disorders than monolingual children (see Paradis, Genesee, & Crago, 2010; Petitto & Holowka, 2002). Many parents have the opposite impression – they feel that their child has a delay due to their bilingualism. However, scientific findings indicate otherwise: although bilingual children usually know fewer words in each of their languages compared to monolingual children, this difference disappears when considering the "conceptual vocabulary" of both languages (Marchman et al., 2010). This means that if you add up the words children know in both languages and at the same time exclude synonyms in both languages (e.g., "dog" and "perro"), bilingual children know roughly the same number of words as monolingual children (Pearson, Fernández, & Oller, 1993; Pearson & Fernández, 1994).
For instance, if a Slovak-English bilingual child knows 50 Slovak words and 50 English words, it likely won't communicate as effectively as its monolingual peer who knows 90 English words. However, if we assume that 10 of these Slovak words are also known in English, then this child has a conceptual vocabulary of 90 words, which is the same as its peer's. Moreover, research shows that 14-month-old bilingual children are equally capable of learning word-object associations as monolingual children (Byers-Heinlein, Fennell, & Werker, 2013).
Just as some monolingual children may have a language delay or disorder, a similar percentage of bilingual children will have a language delay or disorder. If one bilingual child has a language difficulty, it does not mean that bilingualism in general leads to language difficulties. The challenge for pediatricians and speech therapists is to determine whether a bilingual child truly has a problem, or if their mistakes are part of normal development. Assessing bilingual children based on monolingual criteria can lead to misdiagnosing delays.
