Every healthy person is born with the biological capacity to think and communicate by a semiotic system and to learn any language. It also enables one to acquire a set of individual languages, during primary language acquisition or later (s. Polyglossie).
Secondary language acquisition
Once primary language acquisition is completed, one's human language faculty has been molded in terms of one's native language. Non-first languages may be acquired under different conditions, which lead to different forms of competence and proficiency. Important factors include the age of acquisition during or after the critical period and acquisition by participating in communication or by learning in a controlled environment. And while individuals differ in their native-language competence, they apparently differ even more in their capacity of learning additional languages. Anyway, the following are frequent and well-known courses:
time
mode ╲ | during critical period | after critical period |
| uncontrolled | A young immigrant picking up the local language “on the street” acquires full competence and proficiency which is then indistinguishable from competence in his first language. | An adult immigrant acquiring the local language at work or in other social contexts learns to communicate fluently, but keeps an accent and permanent errors. |
|---|---|---|
| controlled | A child learning a foreign language at school attains moderate proficiency and keeps an accent. | An adult taking a foreign-language course attains a low degree of proficiency both in production and perception. |
The first language learnt fixes certain cognitive and communicative categories and operations in terms of the system of that language. These fixations are automatized, so it becomes hard to overcome them and assimilate new forms of categories and operations. While this is true for the whole language system, it applies a fortiori to prosody and speech melody, as these are already entrenched in the prenatal phase. This is the main reason why most adults who acquire an additional language keep their phonetic accent even if they have an impeccable competence otherwise.
If one learns more than one language during the critical phase, one preserves part of the flexibility and openness with which the infant starts language acquisition. Persons who have learnt additional languages during the critical period experience comparatively less difficulties in learning another language during adulthood, while persons who have been monolingual up to adulthood often have considerable difficulties in learning a second language.
Knowledge and abilities related to language are for the most part stored in the dominant hemisphere, in Broca's and Wernicke's areas. When a second language is learnt, it does not oust the first language in the brain. The brain region where non-first languages are stored differs according to the age of acquisition (Kim et al. 1997):
- Several languages, no matter when acquired, occupy overlapping subareas in Wernicke's area.
- Languages learnt during the critical period are stored together in Broca's area.
- Languages learnt later are stored in regions adjacent to Broca's area.
After learning, the latter circumstance conditions a difference in processing (Newman et al. 2012).
Bilingual primary language acquisition
Most persons on earth are born into a monolingual society and may remain monolingual for the rest of their life. Most speech communities on the globe are multilingual. These are mostly small communities all of whose members are in steady contact with one or more neighboring and equally small communities. Many of these, e.g. Amerindian societies in North America or Papua societies in New Guinea, obey a law of exogamy. As a consequence, one's parents speak different languages; and the same may be true of one's grandparents, so that primary language acquisition for a child usually means multilingual acquisition.
In predominantly monolingual societies, it nevertheless does happen that a child's parents have different native languages. Each parent may then speak his own language with the child, so that the child can grow up bilingual. Not seldom, persons grown up like this are reported to speak one or even both languages imperfectly and are then adduced in arguments against bilingual education. As a matter of fact, most such cases are the outcome of unwise behavior on the part of the caretakers. No child has a problem with a rule that requires him to speak in one way to one person and in another way to another person. This is just normal linguistic variation that all of us are exposed to in one or another form. Thus, if both parents are consistent in each one using exclusively one language in communicating with their child, this will have no problem in learning both languages. However, many parents spoil the game by inconsistency. If the same person speaks one language occasionally and another language on other occasions, the infant has no chance to know that he is being confronted with two different languages, and consequently will have learning problems.