Individual differences in resource availability, and the patterns of cognitive abilities they contribute to, are important to:• explaining variation between learners in the effectiveness of second language (L2) instructional treatments;• describing differences in implicit, incidental and explicit L2 learning processes; and• explaining child-adult differences in acquisition processes, and therefore to any general theory of second language acquisition (SLA).In this article I describe a framework for research into the effects of cognitive abilities on SLA which is based on four interlocking hypotheses. These hypotheses are drawn from research in psychology, education and SLA and, where possible, I present evidence to support each of them. The hypotheses are:1) the Aptitude Complex Hypothesis;2) the Ability Differentiation Hypothesis;3) the Fundamental Difference Hypothesis; and4) the Fundamental Similarity Hypothesis.The hypotheses, and the relationships between them, define an Aptitude Complex/Ability Differentiation framework for further examining the influence of individual differences in cognitive abilities on SLA, and for developing a theoretically motivated measure of language learning aptitude. I argue that such research should adopt the interactionist approach described by Snow (1994) to identifying individual difference/learning condition interactions at a number of levels. I illustrate some of these interactions.
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