Every human being learns at least one native language as an ordinary part of growing up. Exceptions are rare – severe abuse, for example, or physical or mental disorders that interfere with ordinary first-language acquisition. Not only that, but the process of learning a first language is remarkably uniform across individuals and communities: children typically go through similar stages in a similar order, although the exact timing of those stages is different for every child. And the outcome is always the same: every adult is a fluent, competent speaker of his or her native language.

But learning a second language is a very different thing. As discussed in Chapter 7, people vary widely in whether they speak multiple languages; and if so, how those languages are learned, and why. In some communities, it's ordinary and expected that people will continue to learn new languages into adulthood; in others, learning a second language is a rare accomplishment. And the outcome is anything but uniform: some people eventually learn a second language so well that they're almost indistinguishable from native speakers, while others have a strong accent even after decades of experience.

Second-language learning, then, leaves us with two mysteries: Why isn't it as successful as learning a first language? And why is it so much easier for some people than for others? In this chapter, we will explore these two related questions; our case study will focus on whether there is a ‘critical period’ for second-language learning – a restricted age range outside of which it's simply impossible to learn a new language as well as a native speaker. To put it differently: if you start learning a new language at 40, are you doomed to failure? Or, if you don't succeed, are you just not trying hard enough?

Age and second-language acquisition

The critical period hypothesis

The most obvious difference between first- and second-language acquisition is age. Your first language, of course, is the one (or perhaps more than one) that you learn as an infant and very young child. By definition, though, a second language is one that you learn after you already have a first language; we can't meaningfully talk about second-language acquisition until a person is old enough to already have a first language.