How The Brain Learns A Second Language : Short Wave Becoming fluent in a second language is difficult. But for adults, is it impossible? Short Wave hosts Maddie Sofia and Emily Kwong dissect the "critical period hypothesis," a theory which linguists have been debating for decades — with the help of Sarah Frances Phillips, a Ph.D. student in the linguistics department at New York University.

You can watch a related video about Emily learning Mandarin here. It's part of the Where We Come From series.

'I'm Willing To Fight For It': Learning A Second Language As An Adult

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So, Maddie, you and I have known each other for a while now, and I think we're ready to take it to the next level.

SOFIA: Oh, my God. Are we going whitewater rafting?


SOFIA: Are we doing it?

EMILY KWONG: (Laughter) No, not today. But I have brought you something just as invigorating and just as vulnerable - a Kwong family home movie.


EMILY KWONG: (Unintelligible).

UNIDENTIFIED PERSON #1: Yes, I think there's more eggs.

SOFIA: Oh, my.


UNIDENTIFIED PERSON #1: Do you see more eggs?



SOFIA: Baby Kwong.

EMILY KWONG: So I'm 2 years old, and we're on an Easter egg hunt. I got my floral Easter dress. I got my grandparents, Hui and Edgar Kwong, and they are all about this right now.




EDGAR KWONG: Here's chocolate for you.




SOFIA: Honestly, you still react that way to chocolate. Let's be real.

EMILY KWONG: It's true.


TIMOTHY KWONG: Emily, you want an egg?

EMILY KWONG: That's my uncle, Timothy Kwong.


UNIDENTIFIED PERSON #2: (Speaking Mandarin).

EMILY KWONG: And you'll notice, Maddie, throughout these home movies - and I brought a few today - that there are two languages being spoken by our family, right? There's English, but there's also Mandarin Chinese.


EDGAR KWONG: (Speaking Mandarin).

EMILY KWONG: Those are my grandparents during Christmas. But for years, all I could say in Mandarin was hello, thank you and goodbye. English was the only language I knew...


EMILY KWONG: (Speaking Mandarin).


EMILY KWONG: (Speaking Mandarin).

...Until now.

(Speaking Mandarin).

UNIDENTIFIED PERSON #3: (Speaking Mandarin).

EMILY KWONG: (Speaking Mandarin).


EMILY KWONG: All year, I've been taking Mandarin classes virtually...

Oh, no, I got this (speaking Mandarin).

...Trying to learn this language. And in the back of my brain, I'm wondering, of course, you know, am I too old to try?

Please hold. (Speaking Mandarin).

Can you really learn another language as an adult?

I have to say restaurant first, don't I, right?

UNIDENTIFIED PERSON #3: I don't know. What do you think?

SOFIA: So today on the show, we ask some big questions about second-language acquisition and get answers from neurolinguist Sarah Phillips.

EMILY KWONG: This is SHORT WAVE, the daily science podcast from NPR.


SOFIA: All right, Emily Kwong, today we are talking about the science of learning a second language because you are learning Mandarin Chinese, which, like, as far as a pandemic hobby goes, more power to you, Emily. More power to you.

EMILY KWONG: (Laughter) Right? For real, though, it is a hard language to learn. Language itself, actually, is an incredible ability, if you think about it, that we humans have. It involves many parts of the brain, and the study of language spans across many different disciplines.

SARAH PHILLIPS: So bilingualism gets studied in at least three different fields - linguistics, psychology and cognitive neuroscience.

EMILY KWONG: Sarah Phillips is a Ph.D. student in the linguistics department at New York University and exactly the person I wanted to call up to talk about language learning.

SOFIA: Oh, yeah. I remember Sarah from our episode on P600, like how the brain responds to sentences with confusing grammar or syntax.

EMILY KWONG: Yeah. Brains and language are her jam. So her parents met in Korea while her father was serving in the Marine Corps, and they raised her bilingual here in the U.S.

PHILLIPS: Learning Korean was very important to be able to communicate with my mom's side of the family in the same way that growing up speaking African American English was very important in being able to communicate and be a part of my dad's side of the family.

EMILY KWONG: She's got a really interesting backstory. And I told her about my project, about taking Mandarin class for two hours every Monday, flashcards on the other nights, watching movies I can't understand. And listen to this.

PHILLIPS: Someone who is engaging in learning a second language, thereby uses another language on a pretty regular basis, that means you're a developing bilingual. So in essence, you are bilingual.


PHILLIPS: But, you know, we would probably qualify that.

EMILY KWONG: I'm a baby bilingual.

PHILLIPS: Exactly, a baby bilingual. Maybe as an alternative to baby bilingual, maybe we should think of this as a developing bilingual.

SOFIA: Oh, that's pretty cool, though. You're a developing baby bilingual.

EMILY KWONG: And Sarah says, more specifically, 'cause she's a scientist, that I am a developing sequential bilingual, meaning I'm learning a second language after acquiring a first language. But that's really different from a simultaneous bilingual like Sarah, who developed...

SOFIA: Right.

EMILY KWONG: ...The ability to speak two or more languages in the earliest years of life. And one of the reasons I never tried to learn my heritage language, honestly, is because of something called the critical period hypothesis. Have you ever heard of this?

SOFIA: I think so. Is that the idea that you can only become fluent in a language when you're young, like there's this critical window for language learning?

EMILY KWONG: Yeah, it's a theory that dates back to the 1950s...


EMILY KWONG: ...And basically argues there's a magic window for a person to learn a first language, somewhere between age 2 and puberty. Scientists debate the cutoff age. But the key idea is there's a biological window where language learning is the most automatic.

PHILLIPS: Where this comes from actually starts really early on with work done with zebra finches and how zebra finches - and maybe even other types of birds, but the literature that I'm familiar with points to zebra finches, where early on in their development, they have to learn certain songs or calls that are particular to their kind. And these calls are important for things like mating and, you know, detecting trouble. In essence, they're important for communicating certain things that are important for their communities.


EMILY KWONG: And, Maddie, researchers found that if baby zebra finches were separated from adults for long enough, they couldn't produce the same calls as their parents, which isn't good - right? - when you think about...

SOFIA: Right.

EMILY KWONG: ...How important these calls are for mating and socialization in zebra finch communities.

SOFIA: Dang. OK, so does the same thing happen with humans? Like, I don't know that you could ethically study that, but I'm curious.

EMILY KWONG: Well, there have been cases where children were denied language before puberty because of abusive parents or extreme social isolation. And when many of those children tried to learn their first language past puberty, they couldn't pick up the grammar.

SOFIA: Got you. Got you. OK, but how does this apply to second-language acquisition and your earlier question? Like, how late is too late to learn another language?

EMILY KWONG: Yeah, this has been the big question because the critical period hypothesis has totally entered our popular consciousness as kind of this rule of second-language learning, too, that you can't really learn a language fluently when you're older.

SOFIA: Right.

EMILY KWONG: And scientists kind of disagree with this. Let's unpack why by looking at the developing baby brain.

SOFIA: Oh, neuroscience. We love it.

EMILY KWONG: So little humans experience an explosive amount of language learning in the first few years of life. Our brain cells change over time, and that change is most rapid when we're little...

SOFIA: Right.

EMILY KWONG: ...As our bodies produce neurological structures and connections we'll use throughout our lifetime. Researchers at the Center for the Developing Child at Harvard University estimate that in the first three years of life, your brain was developing 1 million new neural connections per second.

SOFIA: Every second?


SOFIA: That's feels like - that's too many. Take it easy, brain. You know what I mean? That's a lot.

EMILY KWONG: Baby brains got to grow. But here's the thing. Your brain doesn't stop building neural connections after you're pubescent, right?

SOFIA: Right, right, right.

EMILY KWONG: And in the '90s and the early 2000s, researchers took note of that. They began to argue that second-language learning is not bound to a biological period. How could it be? And this idea emerged that the critical window should actually be called a sensitive window, when you're most susceptible to picking up a new language. Sarah agrees with that.

PHILLIPS: When we think about the critical period, we really want to think about this period of time where our brains are going through an explosive amount of growth and change. And so it's easier and even optimal to then want to learn as many things, including languages, during that time period because our brains are so quick and easy to soak up information.

EMILY KWONG: And once you're past that window, like me, you can still become fluent in another language. It will just take way more conscious effort. That's the distinction.

SOFIA: Got you. Like, you're essentially rewiring your brain a little bit.

EMILY KWONG: Yeah, or a lot. And most scientists agree that this process becomes more difficult with age because your body, including your brain, has already developed certain habits, and habits are hard to break.

PHILLIPS: If we think about language, like, it's not just our brains involved, right? We also have to use our eyes to perceive what we see, and we use our mouths if we're oral producers of language. You know, we have to finagle our mouths to do the right things, right? And these are all habits that we've developed during our early childhood years. So once you become an adult, now you have to learn how to break those habits to adopt a new way of speaking and doing. And so it's a little harder, but it's not impossible.

EMILY KWONG: That's very comforting. I hear you that I'm going to have to fight for it.

SOFIA: Emily Kwong, you're always fighting for stuff. You're always fighting for stuff. You're a fighter. You got this.

EMILY KWONG: And I'm willing to fight for this one, you know?

SOFIA: Yeah. Yeah.

EMILY KWONG: Like, contemporary research shows there are a lot of factors that influence language learning beyond your age. There's education and exposure and the chance to practice in your community. And I'm not going for total fluency here. I just want to know enough for my relatives to tell me how bad I am and to be able to say, (speaking Mandarin), which means, have you eaten?

SOFIA: Nice.

EMILY KWONG: Thank you. And have you eaten is kind of a common refrain in a lot of Asian languages. It's kind of a way of saying I love you.

SOFIA: Oh, I really love that. That's nice.

EMILY KWONG: So, Maddie, in response, if you've eaten, you would say (speaking Mandarin). Now, one area I'm kind of self-conscious about is pronunciation. So if you're listening, do not come for my tone. I already know. I already know. Mandarin is a tonal language, and some of these tones my mouth has never made before.

SOFIA: Right, right.

EMILY KWONG: And Sarah said that's an area where childhood speakers have a clear, unmistakable advantage.

PHILLIPS: The sound system is really the first things we learn about our languages, right? So the rise and fall and intonation and pitch and those kinds of things, as well as the actual speech sounds of our language - those are literally some of the first things that we learn in our infancy.

EMILY KWONG: Which is why adults struggle to produce the speech sounds of another language. But when it comes to pronunciation and accent, Sarah kind of pushed back on my questions, asking me, who do you imagine as a perfectly native speaker anyway? Is it fair to compare yourself to that person?

PHILLIPS: I'm willing to bet that your lived experiences are going to be dynamically different from the person who you envision as your native speaker. And so you might not ever actually become native, like, in your pronunciation. But I don't think that that should be something that people stress over. And the reason being is that the way that we use language fits our identity.

EMILY KWONG: So I can let go of the idea of sounding just like my grandparents, who grew up in Beijing...

SOFIA: Right.

EMILY KWONG: ...Because it's here in the U.S., among my extended family and other Chinese Americans, that I long to be understood.

PHILLIPS: Are you saying it well enough to be understood? That should be, really, the threshold upon which you want to cross.

SOFIA: Oh, my gosh. I love that. That's so comforting. Like, this is like language therapy right now - like learning a new language therapy - because everybody worries about that, pronunciation, when they're trying to speak in a different language, right?

EMILY KWONG: Yeah, it took the pressure off enormously.

SOFIA: Right.

EMILY KWONG: And I should share with you, my grandma was trying to teach me Mandarin in the years before she and my grandfather died.


HUI KWONG: (Speaking Mandarin).

EMILY KWONG: So I feel like I kind of owe it to them to try.

(Speaking Mandarin).


SOFIA: Emily, thank you so much for bringing us a story that's as personal as it gets - your heritage, your family, your brain chemistry. Thank you.

EMILY KWONG: Thank you, Maddie. And I have something special to share that I have been working on all year. So on Sunday in the SHORT WAVE feed, we're dropping a brand-new episode that's all about the social side of linguistics, my journey to learn my heritage language and my relationship to my dad. It's a bit more intimate than what we typically do on SHORT WAVE, and I hope you'll like it.

SOFIA: Yes. We love you, Christopher Kwong. Getting the C (ph). Getting the C (ph). There's a video version of this wonderful story out right now on We'll put the link in the feed description.

EMILY KWONG: This work is part of a new series at NPR called "Where We Come From," featuring stories from immigrant families of color, like mine. We'll share the link to the full series in the feed, too.


SOFIA: Today's episode was produced by Thomas Lu, edited by Viet Le and fact-checked by Indi Khera. The audio engineer for this episode was Alex Drewenskus.

EMILY KWONG: Special thanks to sociolinguist Amelia Tseng, Fluent City language school, Dennis Yueh Yeh Li (ph), Megan Arias (ph) and my family, especially Christopher Kwong, Timothy Kwong, Linda Kwong and Amanda Kwong. The team at "Where We Come From" is Anjuli Sastry, Michael Zamora, Julia Furlan, Diba Mohtasham, Nicole Werbeck and Yolanda Sangweni.

SOFIA: This is SHORT WAVE, the daily science podcast from NPR.


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