Critical period hypothesis

The critical period hypothesis^[1] is a theory within the field of linguistics and second language acquisition that claims a person can only achieve native-like fluency^[2] in a language before a certain age. It is the subject of a long-standing debate in linguistics^[3] and language acquisition over the extent to which the ability to acquire language is biologically linked to developmental stages of the brain.^[4] The critical period hypothesis was first proposed by Montreal neurologist Wilder Penfield and co-author Lamar Roberts in their 1959 book Speech and Brain Mechanisms,^[5] and was popularized by Eric Lenneberg in 1967 with Biological Foundations of Language.^[6]

The critical period hypothesis states that the first few years of life is the crucial time in which an individual can acquire a first language if presented with adequate stimuli, and that first-language acquisition relies on neuroplasticity of the brain. If language input does not occur until after this time, the individual will never achieve a full command of language.^[6] There is much debate over the timing of the critical period with respect to second-language acquisition (SLA), with estimates ranging between 2 and 13 years of age.^[7]

The critical period hypothesis is derived from the concept of a critical period in the biological sciences, which refers to a set period in which an organism must acquire a skill or ability, or said organism will not be able to acquire it later in life. Strictly speaking, the experimentally verified critical period relates to a time span during which damage to the development of the visual system can occur, for example if animals are deprived of the necessary binocular input for developing stereopsis.

Preliminary research into the critical period hypothesis investigated brain lateralization as a possible neurological cause;^[8] however, this theoretical cause was largely discredited since lateralization does not necessarily increase with age, and no definitive link between language learning ability and lateralization was ever determined. A more general hypothesis holds that the critical period for language acquisition is linked to the interaction of the prolonged development of the human brain after birth and rearing in a socio-linguistic environment.^[9] Based on studies of the critical period for development of the visual system,^[10] this hypothesis holds that language-specific neural networks in the brain are constructed by the functional validation of synapses that are specifically activated by exposure to a linguistic environment early in life. Humans are uniquely capable of language due to the genetically determined size and complexity of the brain and the long period of postnatal development, during which the environment can select neuronal circuits that facilitate language.^[9] Recently, it has been suggested that if a critical period does exist, it may be due at least partially to the delayed development of the prefrontal cortex in human children. Researchers have suggested that delayed development of the prefrontal cortex and an associated delay in the development of cognitive control may facilitate convention learning, allowing young children to learn language far more easily than cognitively mature adults and older children. This pattern of prefrontal development is unique to humans among similar mammalian (and primate) species, and may explain why humans—and not chimpanzees—are so adept at learning language.^[11]