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.