Editor's Note: This article was adapted from Mind Matters, www.ScientificAmerican.com/MindMatters, a column edited by Gareth Cook, a Pulitzer Prize–winning journalist at the Boston Globe, and Jonah Lehrer, the science writer behind the blog The Frontal Cortex, http://scienceblogs.com/cortex
The setting: a nursery. A baby speaks directly to the camera: “Look at this. I’m a free man. I go anywhere I want now.” He describes his stock-buying activities, but then his phone interrupts. “Relentless! Hang on a second.” He answers his phone. “Hey, girl, can I hit you back?”
This E*Trade commercial is only the latest proof of what comedians and movie directors have known for years: few things are as funny as a baby who talks like an adult because, as everyone knows, babies can’t do that. This comedic law obscures an important question: Why don’t young children express themselves articulately?
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By attempting to answer that question, researchers are uncovering clues about brain development and the mysterious process of learning a language. Recent work supports the seemingly counterintuitive idea that the way children learn to talk—in baby steps—remains the same no matter what age they are when they start to learn a language. In other words, a baby’s degree of mental development has very little to do with the fact that he or she does not speak in complete sentences.
Many people assume children learn to talk by copying what they hear. In other words, babies listen to the words adults use, and the situations in which they use them, and imitate accordingly. Behaviorism, the scientific approach that dominated the American study of cognition for the first half of the 20th century, made exactly this argument.
This “copycat” theory cannot explain why toddlers are not as fluent as adults, however. After all, when was the last time you heard literate adults express themselves in one-word sentences (“bottle,” “doggie”) or in short phrases such as “Mommy open box”? Of course, it is easy to show that a copycat theory of language acquisition cannot explain these strange patterns in child speech. Actually explaining one-word sentences is much harder. Over the past half a century scientists have settled on two reasonable possibilities.
First, the “mental development hypothesis” states that one-year-olds speak in baby talk because their immature brains cannot handle adult speech. Children do not learn to walk until their body is ready; likewise, they do not speak multiword sentences or use word endings and function words (“Mommy opened the boxes”) before their brain is capable of doing so.
Beyond the Copycat Stage
The second theory, the “stages of language hypothesis,” states that the incremental-step progress in child speech is a necessary process in language development. A basketball player cannot perfect his or her jump shot before learning to both jump and shoot, and children, similarly, learn to add and then to multiply—never in the reverse order.
In language learning there is also evidence of such necessary movements toward fluency. For instance, in a 1997 review article cognitive scientists Elizabeth Bates of the University of California, San Diego, and Judith C. Goodman of the University of Missouri–Columbia found that studies of young children consistently show that kids do not usually begin speaking in two-word sentences until after they have learned a certain number of words. Until they have crossed that linguistic threshold, the word-combination process does not kick in.
The difference between these theories boils down to this: under the mental development hypothesis, patterns in language learning should depend on a child’s level of cognitive development when he or she starts learning a language. Under the stages of language hypothesis, however, learning patterns should not depend on mental development. This prediction is difficult to test experimentally because most children learn language at around the same age—and thus at roughly similar stages of cognitive development.
In 2007 researchers at Harvard University found an ingenious way around this problem. More than 20,000 internationally adopted children enter the U.S. every year. Many of them are no longer exposed to their birth language after arrival, and they must learn English more or less in the same way infants do—that is, by listening and by trial and error. International adoptees do not take classes or use a dictionary when they are learning their new homeland’s tongue, and most of them do not have a well-developed first language. All these factors make them an ideal population in which to test these competing hypotheses about how language is learned.
The Adoption Effect
Harvard neuroscientists Jesse Snedeker, Joy Geren and Carissa L. 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 U.S. natives and therefore had more mature brains to bring to bear on the task. Even so, just as with American-born infants, their first English sentences consisted of single words and were largely bereft of function words, word endings and verbs. The youngsters then went through the same hallmark language stages as typical American-born children, albeit at a faster clip. The adoptees and native children started combining words in sentences when their vocabulary reached the same size, 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 a more mature brain did not help the adoptees avoid the toddler-talk stage—suggests that babies speak in baby talk not because they have a baby brain but because they only just got started learning and need time to accrue sufficient 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 that language as does the average child raised as a native speaker. Researchers have long suspected there is a “critical period” for language development after which it is unlikely to proceed with full success to fluency, yet we are still far from understanding this critical period. Nobody knows exactly when in a child’s life it occurs or why it ends—and some experts question its existence entirely.
Paradoxically, although Snedeker, Geren and Shafto may have explained why there are no talking babies—a prospect so absurd it makes us laugh if we see it in commercials or movies—we still need to explain how babies become eloquent adults.
