When children begin the acquisition of a second language (L2), whether in the home or at school, their cognitive resources clearly play a central role in the rapidity and ultimate success with which that language is acquired. Other individual factors, such as motivation, and contextual factors that determine amount and type of exposure to the second language, are also central to the acquisition process and in most situations are likely to interact with cognitive factors. I argue in this chapter that the process of second-language acquisition can be clarified by distinguishing between two dimensions of proficiency that relate in specific ways to determinants of the acquisition process, namely, attribute-based and input-based aspects of proficiency.

Attribute-based aspects of proficiency refer to those dimensions of proficiency whose acquisition is strongly influenced by relatively stable attributes of the individual learner, for example, cognitive and personality variables. Input-based aspects of proficiency, on the other hand, are considerably less related to stable attributes of the individual than they are to the quality and quantity of L2 input received from the environment. In the initial stages of acquiring the L2 the distinction between attribute- and input-based dimensions of proficiency may not be apparent but, over time, the distinctive influences of attributes and input will result in differentiation between these dimensions.