"Berko 1958 found clear evidence that English-learning children had acquired productive morphological rules by as early as 4 years of age." Discuss. 

Suggested readings

• Berko, Jean. 1958. The child's learning of English morphology. (pdf)

• Clark, Eve ch. 19, Morphology and language acquisition (pdf)

• Derwing, Bruce. 1980. English pluralization: A testing ground for rule evaluation (pdf) 

• Derwing, Bruce and William Baker. 1980. Rule learning and the English inflections (with special emphasis on the plural) (pdf)

• Embick, David and Alec Marantz 2005, Cognitive neuroscience and the English past tense (pdf)

• Ettlinger, Marc and Jennifer Zapf. 2011. The Role of Phonology in Children's Acquisition of the Plural (pdf)

• Köpcke, Klaus-Michael. 1998. The acquisition of plural marking in English and German revisited: schemata versus rules (pdf)

• Marcus, Gary. 1995. The acquisition of the English past tense in children and multilayered connectionist networks (pdf)

• McClelland, J. 1988. Connectionist models and psychological evidence. Journal of Memory and Language 27:107-123. (pdf)

• Meech, Sandra. 1974. Pluralizing behavior of preschool children (pdf)

• Pinker, Steven. 2006.  Whatever happened to the past tense debate? (pdf)

• Pinker, Steven and Alan Prince. 1988. On language and connectionism: Analysis of a parallel distributed processing model of language acquisition (pdf)

• Plunkett, Kim and Patrick Juola. 1999. A Connectionist Model of English Past Tense and Plural Morphology (pdf)

• Rumelhart, D. and J. McClelland. 1986. On learning the past tenses of English verbs. Implicit rules or parallel distributed processing? In J. McClelland, D. Rumelhart and the PDP Research Group (Eds.), Parallel distributed processing: Explorations in the microstructure of cognition. Cambridge, MA: MIT Press. (pdf)

• Yang, Charles. 2005. On productivity (pdf)

• Yang, Charles. 2017. How to wake up irregular (and speechless) (pdf)