Despite scoring at the level of a one-year-old upon her initial assessment, Genie quickly began adding new words to her vocabulary. She started by learning single words and eventually began putting two words together much the way young children do. Curtiss began to feel that Genie would be fully capable of acquiring language.
After a year of treatment, she even started putting three words together occasionally. In children going through normal language development, this stage is followed by what is known as a language explosion. Children rapidly acquire new words and begin putting them together in novel ways. Unfortunately, this never happened for Genie. Her language abilities remained stuck at this stage and she appeared unable to apply grammatical rules and use language in a meaningful way. At this point, her progress leveled off and her acquisition of new language halted.
While Genie was able to learn some language after puberty, her inability to use grammar (which Chomsky suggests is what separates human language from animal communication) offers evidence for the critical period hypothesis.
Of course, Genie's case is not so simple. Not only did she miss the critical period for learning language, she was also horrifically abused. She was malnourished and deprived of cognitive stimulation for most of her childhood. Researchers were also never able to fully determine if Genie suffered from pre-existing cognitive deficits. As an infant, a pediatrician had identified her as having some type of mental delay. So researchers were left to wonder whether Genie had suffered from cognitive deficits caused by her years of abuse or if she had been born with some degree of mental retardation.