Critical Period

Critical period hypothesis

The Critical Period Hypothesis (CPH) holds that there is a critical time period for a person to learn a new language to a native proficiency. This critical period typically starts at around age two and ends before puberty¹. The hypothesis implies that acquiring a new language after this critical window will be more difficult and less successful.

Critical period in Psychology

The critical period is a key concept within the subject of Psychology. Psychology often has close links with English Language and Linguistics with a key area of study being Language Acquisition.

Critical period Psychology definition

In developmental psychology, the critical period is the maturing stage of a person, where their nervous system is primed and sensitive to environmental experiences. If a person doesn't get the right environmental stimuli during this period, their ability to learn new skills will weaken, affecting many social functions in adult life. If a child passes through a critical period without learning a language, it will be very unlikely for them to gain native fluency in their first language².

Graph of the ease of language acquisition.

During the critical period, a person is primed to acquire new skills because of the brains' neuroplasticity. The connections in the brain, called synapses, are highly receptive to new experiences since they can form new pathways. The developing brain has a high degree of plasticity and gradually becomes less 'plastic' in adulthood.

Critical and sensitive periods

Similar to the critical period, researchers use another term called the 'sensitive period' or 'weak critical 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.

First language acquisition in the critical period

It was Eric Lenneberg in his book Biological Foundations of Language (1967), who first introduced the Critical Period Hypothesis concerning language acquisition. He proposed that learning a language with high-level proficiency can only happen within this period. Language acquisition outside of this period is more challenging, making it less likely to achieve native proficiency.

He proposed this hypothesis based on evidence from children with certain childhood experiences that affected their first language ability. More specifically, the evidence was based on these cases:

• Deaf children that didn't develop native proficiency in verbal language after puberty.

• Children that experienced brain injury had better recovery prospects than adults. It is more likely for children with aphasia to learn a language than it is for adults with aphasia.

• Children who were victims of child abuse during early childhood had more difficulties learning the language since they were not exposed to it during the critical period.

Critical period example

An example of the critical period is Genie. Genie, the so-called 'feral child', is a key case study in regard to the critical period and language acquisition.

As a child, Genie was a victim of domestic abuse and social isolation. This took place 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. This meant that she wasn't able to develop adequate 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 because 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.

The case of Genie supports Lenneberg's theory to an extent. However, academics and researchers still argue about this topic. Some scientists claim that Genie's development was disrupted because of the inhumane and traumatic treatment she suffered as a child, which caused her inability to learn a language.

Second language acquisition in the critical period

The Critical Period Hypothesis can be applied in the context of second language acquisition. It applies to adults or children who have fluency in their first language and try to learn a second language.

The main point of evidence given for the CPH for second language acquisition is assessing older learners' ability to grasp a second language compared to children and adolescents. A general trend that can be observed is that younger learners grasp a complete command over the language compared to their older counterparts³.

Although there may be examples where adults achieve very good proficiency in a new language, they usually retain a foreign accent which isn't common with younger learners. Retaining a foreign accent is usually because of the function that the neuromuscular system plays in the pronunciation of speech.

Adults are unlikely to attain a native accent since they are beyond the critical period to learn new neuromuscular functions. With all this being said, there are special cases of adults who achieve near-native proficiency in all aspects of a second language. For this reason, researchers have found it tricky to distinguish between correlation and causation.

Some have argued that the critical period doesn't apply to second language acquisition. Instead of age being the main factor, other elements such as the effort put in, the learning environment, and time spent learning have a more significant influence on the learner's success.

1. Kenji Hakuta et al, Critical Evidence: A Test of the Critical-Period Hypothesis for Second-Language Acquisition, 2003.

2. Angela D. Friederici et al, Brain signatures of artificial language processing: Evidence challenging the critical period hypothesis, 2002.

3. Birdsong D., Second Language Acquisition and the Critical Period Hypothesis. Routledge, 1999.