The treatment of exceptions has long been a controversial topic in linguistics and cognitive science; see, among others, the past tense debate. While traditional approaches draw a categorical distinction between general rules and exceptions (e.g., “all grammars leak”), recent work has emphasized a gradient continuum of productivity across linguistics processes.

We review cross linguistic evidence that children are remarkably good at recognizing and learning productive regularities in the grammar while shelving aside lexically and contextually restricted exceptions for memorization and storage. This bodes well for the classic view on productivity but in turn raises important questions for language learning: how do children identify productive and exceptional patterns in language and acquire them according?

We show that exceptions increase the processing complexity of language, which leads to a threshold function for productivity. Such a cost-benefit approach to language learning can be applied to a broad range of empirical cases including phonology, morphology, syntax, and language change.