Cognitive Scientists Define
Critical
Period for Learning Language
Study shows children remain adept learners until the age of 17 or 18.
By Anne Trafton | MIT News Office
Period for Learning Language
Study shows children remain adept learners until the age of 17 or 18.
By Anne Trafton | MIT News Office
May 1, 2018 -- A great deal of evidence suggests that it is more
difficult to learn a new language as an adult than as a child, which has led
scientists to propose that there is a “critical period” for language learning.
However, the length of this period and its underlying causes remain unknown.
A new study performed at MIT
suggests that children remain very skilled at learning the grammar of a new
language much longer than expected — up to the age of 17 or 18. However, the
study also found that it is nearly impossible for people to achieve proficiency
similar to that of a native speaker unless they start learning a language by
the age of 10.
“If you want to have native-like
knowledge of English grammar you should start by about 10 years old. We don’t
see very much difference between people who start at birth and people who start
at 10, but we start seeing a decline after that,” says Joshua Hartshorne, an
assistant professor of psychology at Boston
College
People who start learning a
language between 10 and 18 will still learn quickly, but since they have a
shorter window before their learning ability declines, they do not achieve the
proficiency of native speakers, the researchers found. The findings are based
on an analysis of a grammar quiz taken by nearly 670,000 people, which is by
far the largest dataset that anyone has assembled for a study of
language-learning ability.
“It’s been very difficult until now
to get all the data you would need to answer this question of how long the
critical period lasts,” says Josh Tenenbaum, an MIT professor of brain and
cognitive sciences and an author of the paper. “This is one of those rare
opportunities in science where we could work on a question that is very old,
that many smart people have thought about and written about, and take a new
perspective and see something that maybe other people haven’t.”
Steven Pinker, a professor of
psychology at Harvard
University
Quick learners
While it’s typical for children to
pick up languages more easily than adults — a phenomenon often seen in families
that immigrate to a new country — this trend has been difficult to study in a
laboratory setting. Researchers who brought adults and children into a lab,
taught them some new elements of language, and then tested them, found that
adults were actually better at learning under those conditions. Such studies
likely do not accurately replicate the process of long-term learning,
Hartshorne says.
“Whatever it is that results in
what we see in day-to-day life with adults having difficulty in fully acquiring
the language, it happens over a really long timescale,” he says.
Following people as they learn a
language over many years is difficult and time-consuming, so the researchers
came up with a different approach. They decided to take snapshots of hundreds
of thousands of people who were in different stages of learning English. By
measuring the grammatical ability of many people of different ages, who started
learning English at different points in their life, they could get enough data
to come to some meaningful conclusions.
Hartshorne’s original estimate was
that they needed at least half a million participants — unprecedented for this
type of study. Faced with the challenge of attracting so many test subjects, he
set out to create a grammar quiz that would be entertaining enough to go viral.
With the help of some MIT
undergraduates, Hartshorne scoured scientific papers on language learning to
discover the grammatical rules most likely to trip up a non-native speaker. He
wrote questions that would reveal these errors, such as determining whether a
sentence such as “Yesterday John wanted to won the race” is grammatically
correct.
To entice more people to take the
test, he also included questions that were not necessary for measuring language
learning, but were designed to reveal which dialect of English the test-taker
speaks. For example, an English speaker from Canada
Within hours after being posted on
Facebook, the 10-minute quiz “Which English?” had
gone viral.
“The next few weeks were spent
keeping the website running, because the amount of traffic we were getting was
just overwhelming,” Hartshorne says. “That’s how I knew the experiment was
sufficiently fun.”
A long critical period
After taking the quiz, users were
asked to reveal their current age and the age at which they began learning
English, as well as other information about their language background. The
researchers ended up with complete data for 669,498 people, and once they had
this huge amount of data, they had to figure out how to analyze it.
“We had to tease apart how many
years has someone been studying this language, when they started speaking it,
and what kind of exposure have they been getting: Were they learning in a class
or were they immigrants to an English-speaking country?” Hartshorne says.
The researchers developed and
tested a variety of computational models to see which was most consistent with
their results, and found that the best explanation for their data is that
grammar-learning ability remains strong until age 17 or 18, at which point it
drops. The findings suggest that the critical period for learning language is
much longer than cognitive scientists had previously thought.
“It was surprising to us,”
Hartshorne says. “The debate had been over whether it declines from birth,
starts declining at 5 years old, or starts declining starting at puberty.”
The authors note that adults are
still good at learning foreign languages, but they will not be able to reach
the level of a native speaker if they begin learning as a teenager or as an
adult.
"Although it has long been
observed that learning a second language is easier early in life, this study
provides the most compelling evidence to date that there is a specific time in
life after which the ability to learn the grammar of a new language
declines," says Mahesh Srinivasan, an assistant professor of psychology at
the University of California at Berkeley, who was not involved in the study.
“This is a major step forward for the field. The study also opens surprising,
new questions, because it suggests that the critical period closes much later
than previously thought."
Still unknown is what causes the
critical period to end around age 18. The researchers suggest that cultural
factors may play a role, but there may also be changes in brain plasticity that
occur around that age.
“It’s possible that there’s a
biological change. It’s also possible that it’s something social or cultural,”
Tenenbaum says. “There’s roughly a period of being a minor that goes up to
about age 17 or 18 in many societies. After that, you leave your home, maybe
you work full time, or you become a specialized university student. All of
those might impact your learning rate for any language.”
Hartshorne now plans to run some
related studies in his lab at Boston
College
The researchers also hope that
other scientists will make use of their data, which they have posted online, for additional studies.
“There are lots of other things
going on in this data that somebody could analyze,” Hartshorne says. “We do
want to draw other scientists’ attention to the fact that the data is out there
and they can use it.”
The research was funded by the
National Institutes of Health and MIT’s Center for Minds, Brains, and Machines.
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