Critical Period Hypothesis

Many of us have been exposed to language since birth and appear to have picked it up without even thinking about it. But what if we were deprived of communication from birth? Would we still learn to speak? There is a theory that states we will not be able to develop language fluency if we are not exposed to it during our first few years of life. It's known as the Critical Period Hypothesis.

According to this hypothesis, there is a critical time period for a person to learn a new language to native level. This stage usually begins around the age of two and ends before puberty. This implies that learning a new language after this critical window will be more difficult and less successful.

The Critical Period Hypothesis concerning language acquisition was first introduced by Eric Lenneberg in his book Biological Foundations of Language (1967). He proposed that learning a language to a high level of proficiency could only take place during this time period. Language acquisition outside of this time period is more difficult, making native proficiency less likely.

Similar to the critical period, researchers use another term called the 'sensitive period'. The sensitive period is similar to the critical period since it's characterized as a time in which the brain has a high level of neuroplasticity and is quick to form new synapses. The main difference is that the sensitive period is considered to last for a longer time beyond puberty, but the boundaries are not strictly set.

Here's a video that explains it a bit more deeply: 

There's a key case study on this hypothesis. A girl named Genie was a victim of domestic abuse and social isolation from the age of 20 months until 13 years old. During this period, she didn't speak to anyone and rarely had any interaction with other people. Therefore, she wasn't able to develop proper language skills. 

When authorities discovered her, she could not speak. Over a few months, she acquired some language skills with direct teaching but the process was quite slow. Although her vocabulary grew over time, she had difficulty learning basic grammar and maintaining conversations.

The scientists that worked with her concluded that since she wasn't able to learn a language during the critical period, she wouldn't be able to achieve full competency in language for the rest of her life. Although she made clear improvements in her ability to speak, her speech still had a lot of abnormalities, and she had difficulty with social interaction.

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