After years of research suggesting that the “critical period” to learn ends before the age of 10, an enormous new study of well over half a million learners 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 very difficult 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, who conducted this study as a postdoc at MIT.
The study, performed at MIT by researchers from three Boston universities including renowned psychologist/linguist Steven Pinker, found that 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 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.
Data collected included a person’s current age, language proficiency, and time studying English. The investigators calculated they needed more than half a million people to make a fair estimate of when the “critical period” for achieving the highest levels of grammatical fluency ends. So they turned to the world’s greatest experimental subject pool: the internet.
“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.”
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 five 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.
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.”
Steven Pinker, a professor of psychology at Harvard University, is also an author of the paper, which appeared in the journal Cognition on May 1.