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Because multilingual environments are the norm rather than the excep tion, infants must have the capacity very early in life to distinguish one language from another. Without such a capacity, infants might acquire linguistic systems that amalgamate properties of different languages. The ensuing confusion would be overpowering. Fortunately, this never arises, despite the intuitive fears of monolingual parents. Infants raised in multilingual societies do not become dysphasic. Nor do children who learn more than one language at once pay a high price in terms of time or effort.
Mehler et al. (Tue,) studied this question.