Neuroscientists Jesse Snedeker, Joy Geren and Carissa Shafto studied the language development of 27 children adopted from China between the ages of two and five years. These children began learning English at an older age than US natives and had more mature brains with which to tackle the task. Even so, just as with American-born infants, their first English sentences consisted of single words and were largely bereft (缺乏的) of function words, word endings and verbs. The adoptees then went through the same stages as typical American-born children, though at a faster clip. The adoptees and native children started combining words in sentences when their vocabulary reached the same sizes, further suggesting that what matters is not how old you are or how mature your brain is, but the number of words you know.
This finding -- that having more mature brains did not help the adoptees avoid the toddler-talk stage -- suggests that babies speak in babytalk not because they have baby brains, but because they have only just started learning and need time to gain enough vocabulary to be able to expand their conversations. Before long, the one-word stage will give way to the two-word stage and so on. Learning how to chat like an adult is a gradual process.
But this potential answer also raises an even older and more difficult question. Adult immigrants, who learn a second language rarely, achieve the same proficiency in a foreign language as the average child raised as a native speaker. Researchers have long suspected there is a critical period for language development,after which it cannot proceed with full success to fluency.Yet we still do not understand this critical period or know why it ends.
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