A critical period in development is a limited span of time during which an organism is sensitive to external stimuli and capable of acquiring certain skills. Apparently, there is an ideal time "window" in which a language is learned. After that time window, learning the language becomes much more difficult. This is called the critical period hypothesis. The critical period hypothesis states that the first few years of life is when an individual can acquire a first language. Genie, a feral child's, case was one of the first to put the critical period theory to the test. Genie was the victim of severe abuse, neglect and isolation from her father. Genie never learned to speak, and she was isolated from the world for the first thirteen years of her life, so she never learned a first language in her critical period. Genie learned how to use the toilet and how to dress herself rapidly, but she still couldn't pick up on learning a language. If given a much more stable learning environment, could Genie overcome her deprived childhood and learn a first language? Could she learn a first language after the critical period? If she could, it would suggest that the critical period hypothesis is wrong, and if she couldn't, it would suggest that the critical period hypothesis is correct.
Although Genie scored at the level of a one-year-old on her initial assessment and spoke in one-word snippets, she started to enhance her vocabulary. She was saying words for colors, the numbers one through five, and words like "store," "pail," and "bucket." She could also say "mama," and some peoples' names. Genie followed some of the normal patterns of young children learning a first language, but researchers noticed many differences with her linguistic development. Finally, after a year of treatment, she started putting a few words together the way children do.