Why Don'；t Babies Talk Like Adults？
Over the past half-century，scientists have settled on two reasonable theories related to babytalk.Onestates that a young child'；s brain needs time to master language，in the sameway that it does to master other abilities such as physical movement.Thesecond theory states that a child'；s vocabulary level is the key factor.According to this theory，some key steps have to occur in a logical sequencebe-fore sentence formation occurs.Children'；s mathematical knowledge developsin the same way.
In 2007，researchers at Harvard University，who were studying the twotheories，found a clever way to test them.More than 20，000internationallyadopted children enter theU.S.each year.Many of them no longer heartheir birth language after they arrive，and they must learn'；English more orless the same way infants do-that is，by listening and by trial and error.International adoptees don'；t take classes or use a dictionary when they arelearning their new tongue and most of them don'；t have a well-developed firstlanguage.All of these factors make them an ideal population in which to testthese com-peting hypotheses about how language is learned.
Neuroscientists JesseSnedeker，Joy Geren and Carissa Shafto studied the language development of 27children adopted fromChinabetween the ages of two and five years.These children began learn-ing Englishat an older age than US natives and had more mature brains with which to tacklethe task.Even so，just as with American-born infants，their first Englishsentences consisted of single words
and were largely bereft（缺乏的）of functionwords，word endings and verbs.Theadoptees then went through the same stages as typical American-born children，though at a faster clip.The adoptees and native children started combing wordsin sentences when their vocabulary reached the same sizes，fur-ther suggestingthat what matter is not how old you are or how mature your brain is，but thenumber of words you know.
This finding-that havingmore mature brains did not help the adoptees avoid the toddler-talkstage-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 enoughvocabulary to be able to expand their conversa-tions.Before long，the one-wordstage will give way to the two-word stage and so on.Learning how to chat likean adult is a gradual process.
But this potential answeralso raises an even older and more difficult question Adult immigrants wholearn a second language rarely achieve the same proficiency in a foreignlanguage as the average child raised as a native speaker.Researchers have longsuspected there is a "；critical period"；for language developm'；ent，after which it cannot proceed with full success to fluency.Yet we still do notunderstand this critical period or know why it ends.
What is the writer'；s main purpose in Paragraph 2？（）