If first language is rational in the sense that acquisition produces an end-state model of language that is a proper reflection of input and that optimally prepares speakers for comprehension and production, second language is usually not. This paper considers the apparent irrationalities of L2 acquisition, that is the shortcomings where input fails to become intake. It describes how ‘learned attention’, a key concept in contemporary associative and connectionist theories of animal and human learning, explains these effects. The fragile features of L2 acquisition are those which, however available as a result of frequency, recency, or context, fall short of intake because of one of the factors of contingency, cue competition, salience, interference, overshadowing, blocking, or perceptual learning, which are all shaped by the L1. Each phenomenon is explained within associative learning theory and exemplified in language learning. Paradoxically, the successes of L1 acquisition and the limitations of L2 acquisition both derive from the same basic learning principles.

This paper considers how fluent language users are rational in their language processing, their unconscious language representation systems optimally prepared for comprehension and production, how language learners are intuitive statisticians, and how acquisition can be understood as contingency learning. But there are important aspects of second language acquisition that do not appear to be rational, where input fails to become intake. The paper describes the types of situation where cognition deviates from rationality and it introduces how the apparent irrationalities of L2 acquisition result from standard phenomena of associative learning as encapsulated in the models of Rescorla and Wagner (1972) and Cheng and Holyoak (1995), which describe how cue salience, outcome importance, and the history of learning from multiple probabilistic cues affect the development of ‘learned selective attention’ and transfer. This article considers how fluent language users are rational in their language processing, rational in the sense that their unconscious language representation