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 have known for years: few things are as funny as a baby who talks like an adult. This comedic law obscures an important question: Why don’t young children express themselves articulately? And why is the idea of toddler speaking in perfect sentences so hilarious?

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 American cognitive science for the first half of the 20th century, made exactly this argument.

This “copycat” theory can’t explain why toddlers aren’t as loquacious 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, showing that a copycat theory of language acquisition can’t explain these strange patterns in child speech is easy. Actually explaining one-word sentences is much harder. Over the past half-century, scientists settled on two reasonable possibilities.

Beyond the Copycat Stage

First, the “mental developmental hypothesis” states that one-year-olds speak in baby talk because their immature brains can’t handle adult speech. Children don’t learn to walk until their bodies are ready. Likewise, they don’t speak multi-word sentences or use word endings and function words (“Mommy opened the boxes”) before their brains are ready.

The second, the “stages-of-language hypothesis,” states that the stages of progress in child speech are necessary stages in language development. A basketball player can’t perfect his or her jump shot before learning to (1) jump and (2) shoot, and children, similarly, learn to add and then to multiply, not in the reverse order. There’s evidence, for instance, that children don’t usually begin speaking in two-word sentences until after they’ve learned a certain number of words. Until they’ve crossed that linguistic threshold, the word-combination process doesn’t kick in.

The difference between these theories boils down to this: under the mental development hypothesis, these patterns in language learning should depend on the child’s level of mental development when he or she starts learning a language. Under the stages-of-language hypothesis, however, they shouldn’t depend on such patterns. This prediction is difficult to test experimentally, because most children all learn language at around the same age, and thus at roughly similar stages of mental development.

The Adoption Effect

In 2007 researchers at Harvard University found an ingenious way around this problem. More than 20,000 internationally adopted children enter the U.S. each year. Many of them are no longer exposed to their birth language after arrival, and they must learn English more or less 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 are learning their new homeland’s tongue and most of them don’t have a well-developed first language. All of these factors make them an ideal population in which to test these competing hypotheses about language is learned.

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 U.S. natives and 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 adoptees 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 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 baby talk not because they have baby brains, 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 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 are still far from understanding this critical period or why it ends.

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.

Are you a scientist? Have you recently read a peer-reviewed paper that you want to write about? Then contact Mind Matters editor Jonah Lehrer, the science writer behind the blog The Frontal Cortex and the book Proust Was a Neuroscientist. His next book, How We Decide, will be available in February 2009.