It is not unusual for an adult to learn a second language with nearly perfect mastery of grammar and vocabulary. But once past adolescence, most people never manage to imitate the new language's pronunciation perfectly, no matter how long and hard they work at it. In other words, they speak with foreign accents. Even when another speaker tries to help with Say it like this, not that, the learner is unable to learn what changes to make. Most people who change countries in their later teens never learn to speak the way the natives do.
Why do adults find it so hard to learn how to pronounce new word sounds? Some like to say that this reflects a general decline in the learning capacities of older people, but that appears to be a myth. Instead, I suspect this particular disability is caused, more or less directly, by a genetically programmed mechanism that disables our ability to learn to make new connections in or between the agents we use to represent speech sounds. There is evidence that our brains use different machinery for recognizing language sounds than for recognizing other sorts of sounds, particularly for the little speech- sound units that language scientists call phonemes. Most human languages use less than a hundred phonemes.
Why should we be able to learn many different speech sounds before the age of puberty but find it so much harder to learn new ones afterward? I suspect that this link to puberty is no coincidence. Instead, one or more of the genetically controlled mechanisms that brings on sexual maturity also acts to reduce the capacities of these particular agencies to learn to recognize and make new sounds! But why did this peculiar disability evolve? What evolutionary survival advantage would favor individuals whose genes reduce, after that age, this particular ability to learn? Consider this hypothesis:
The onset of the childbearing age is the biological moment when a person's social role changes from learner to teacher. The evolutionary purpose of suppressing speech-sound learning may simply serve to prevent the parent from learning
the child's speech — thus making the child learn the adult's speech instead!
Wouldn't parents want to teach the children their language anyway? Not necessarily. In the short run, a parent is usually more concerned with communication than with instruction. Accordingly, if we found it easier to imitate our children's sounds, that's what we'd do. But if parents were inclined and able to learn to speak the ways their children do, those children would lose both incentive and opportunity to learn to speak like adults, and — if every child acquired a different set of language sounds — no common, public language would ever have evolved in the first place! If this is right, puberty-linked genes for suppressing speech-sound learning may have formed fairly early in the evolution of human languages. No one knows when that occurred, but if biologists could find and date the genes for this, we could obtain a clue about the time of language's unknown origin, perhaps within the last half million years. sh\