后来，语言学家David Birdsong和Jan Vanhove又做了一次实验，这一次，他们得到了与Johnson实验完全不同的结论！
附原文：Can You Learn a Second Language After Childhood?
Rethinking the critical period hypothesis
Judy was eight years old when she arrived in the United States from China. (“Judy” was the “American” name her mother had chosen for her.) Her father was a graduate student, and he’d arrived a couple of years before. She and her mother then joined him once their finances were secure.
At first, Judy spoke no English, and she was deeply impressed by how well her father used the language. Even her mother seemed to know enough English to get by in the supermarket and to make small talk with the neighbors. Although her parents arranged play dates with children of other Chinese families, Judy found it hard at first to interact with the kids in her neighborhood and at school.
Within a few months, though, Judy had made friends. She still spoke Chinese with her parents at home, but she could now express herself in English, and her fluency was improving rapidly. Sometimes kids would tease her for the funny way she talked, but her friends said it was cute.
A year after her arrival, Judy no longer admired her father’s English ability. He had a heavy accent and said things in a weird way. He didn’t sound at all like an American, and she noticed that people sometimes had difficulty understanding him.
And as for her mother—well, that was just embarrassing. No matter how many times Judy corrected her, she kept making the same silly mistakes over and over again. More often now, her mother relied on Judy to translate for her.
Judy’s father graduated and got a job in Silicon Valley. They moved to a house in the suburbs, but there were still plenty of other Chinese families nearby. Her grandparents came over from China and moved in with them. They spoke very little English, although they did take English language classes at the Chinese church on Saturdays. Mostly though, they socialized with other retired Chinese immigrants in the community.
By the time Judy graduated from high school, most people assumed she’d been born in this country. If you knew her history, you could still detect a few subtle oddities of pronunciation and grammar. Otherwise, you’d likely dismiss them as personal quirks—that’s just the way Judy talks.
Judy’s situation is typical of the immigrant experience in this country and elsewhere around the world. Those in the younger generation (like Judy) pick up the new language quickly, and it eventually becomes their dominant language. Those in the middle generation (like Judy’s parents) learn the new language to the extent that they need it in their careers and daily lives. And those in the older generation (like Judy’s grandparents) pick up little more than a few words and expressions, preferring instead to limit their social interactions to family and fellow immigrants.
The fact that it gets harder to learn a second language the older you are is well documented. However, the reasons are less understood, although there’s plenty of speculation. The most common explanation you’ll hear from scholars of language is that there’s a “critical period” for language learning. In other words, there’s a window of opportunity in which picking up a new language is easy, but once that window shuts, language learning becomes much more difficult, if not impossible.
The classic study demonstrating a critical period for language learning was conducted by Johnson and Newport (1989). The researchers used a grammaticality judgment task to assess English proficiency in a group of Korean immigrants. That is, the participants listened to sentences and indicated whether each was grammatically correct or not. Those who’d arrived in the U.S. before puberty got virtually all of the questions right, just like native speakers. But there was a steep decrease in performance after puberty.
However, as linguists David Birdsong and Jan Vanhove point out in the new book Bilingualism Across the Lifespan, subsequent attempts to replicate this study with different immigrant groups have yielded inconsistent results. Sometimes the “inflection point”—the sudden drop in performance—comes earlier in childhood, and other times the data show no clear inflection point at all. When the data from all of the studies are combined, Birdsong and Vanhove maintain, what you get is a downward-sloping line. In other words, the older you are, the less likely you are to master a second language. But we already knew that.
No doubt maturational factors play a role. Many things get harder to learn as we get older, like playing a sport or a musical instrument. This doesn’t mean you can’t learn tennis or the piano as an adult. You’re never going to play like a pro. But then again, nearly all the kids taking tennis or piano lessons will never play like a pro, either.
Birdsong and Vanhove maintain that situational and motivational factors are just as important in determining ultimate attainment in a second language. Consider the typical situation in which children and adults learn a new language. Kids learn it on the street and in the school, interacting with peers and elders. Grownups enroll in a language class. In other words, youngsters learn language in a natural environment, while their elders attempt to do so under artificial circumstances.
Motivation also plays an important role. Children need to master the language of the society they live in. They have a strong motivation to fit in. Younger adults, though, typically have one foot in the old culture and another in the new. They learn enough of the new language to get by. Furthermore, older adults, especially after retirement, tend to restrict their social circles to family and a few close friends, even when they’re living in their own culture. Older immigrants have little motivation to learn the new language, since they’ll have little interaction with members of the new society.
If you believe there’s no way you could ever learn a second language, consider the following thought experiment. Imagine that you’re the sole survivor of a shipwreck, and you’ve washed ashore on an uncharted island. None of the inhabitants speak English. How long will it take you to learn their language? You’ll never speak like one of them, but within weeks you’ll be finding ways to express yourself, and within months you’ll be having conversations in your new language.
In the end, language learning is a normal part of the human experience. The vast majority of Americans have little motivation to learn another language, because they don't have to. But when the situation demands it, people pick up as much of a new language as they need to get by, whether that be speaking like a native, fluent but with a heavy accent, or just enough to do the shopping.