The apparent specialization of the left hemisphere for language is usually described in terms of lateral dominance or lateralization (one-sidedness). Since the human child does not emerge from the womb as a fully articulate language-user, it is generally thought that the lateralization process begins in early childhood. It coincides with the period during which language acquisition takes place. During childhood, there is a period when the human brain is most ready to receive input and learn a particular language. This is sometimes called the “sensitive period” for language acquisition, but is more generally known as the critical period.

Though some think it may start earlier, the general view is that the critical period for first language acquisition lasts from birth until puberty. If a child does not acquire language during this period, for any one of a number of reasons, then he or she will find it almost impossible to learn language later on. In one unfortunate but well documented case, we have gained some insight into what happens when the critical period passes without adequate linguistic input.