Human capacity for language acquisition is not strictly compartmentalized, with one acquisitional mechanism for the native language and others totally unrelated to it; rather, it consists of a unified mechanism flexible enough to handle various differences in external settings. This learning system operates on the formal properties of the linguistic devices used in natural languages. The human language system contains cognitive capacities especially geared to acquisition of language structures, and it also has some of the general properties of other learning systems, such as developmental stages, reliance on prior knowledge, and the decomposition of complex phenomena for learning purposes. Aspects of second language acquisition such as language contact or conflict, loyalty or shift, intellectual advantages or disadvantages, attitudes, interference, borrowing, pidginization, and de-creolization can be studied profoundly only when general language acquisitional factors are considered. (MSE)