In this short article review, I set out to provide evidence for a null hypothesis, that the critical period hypothesis (CPH) is not applicable for second language (L2) acquisition. Research on the critical period in L2 acquisition is theoretically important since insight on the topic has far-reaching implications into many fields: early childhood development, linguistics, cognitive psychology, neurology & evolutionary theory (DeKeyser & Larson-Hall, 2005). It also holds practical importance because the findings can be used in teaching and learning strategies for language.

Evidence against the CPH first peaked when the feral child, Genie, grew to puberty with minimal human contact, yet she was still able to acquire limited language skills (Fromkin, 1974). It was unclear whether Genie’s inability to develop language was due to her environment or brain deficits (Kenneally, 1998). Because of the scarcity of feral children, first language (L1) acquisition is near impossible to study for the CPH. Research has since focused on L2 acquisition. Genie’s case study neither proved nor disproved the CPH; only plunging it deeper into discrepancy. More debate has arisen around the existence of a critical period for L2 acquisition because of the many assumptions placed on its definition. Research previously assumed that all aspects of the CPH applied to first and second language learning (Penfield & Roberts, 1959). The CPH also assumed that child L2 acquisition vastly differs from that of adult L2 acquisition (Lenneberg, 1967). However, the current state of the research posits that the CPH does not extend into L2 acquisition, and that child and adult L2 acquisition are not very disparate. Perhaps most significant is the assumption of nature’s greater influence over nurture; that as the brain matures, it becomes less plastic which impedes L2 learning (Flege, Yeni-Komshian & Liu, 1999). But do children learn language better because of the brain’s plasticity at that age or because of the intensity of language learning present in childhood otherwise absent in adulthood (Johnson & Newport, 1989)? A slew of research in the last 5 years further rejects these assumptions of the CPH in L2 acquisition (eg.: Brice & Brice, 2008; DeKeyser, Alfi-Shabtay & Ravid 2010; Jarvis, 2008; Jia & Fuse, 2007). In examining field research, we will accept or reject the null hypothesis that the CPH does not apply to L2 learning.

The critical period hypothesis (CPH) is a term coined by Wilder Penfield (1960) and popularized by Lenneberg (1967), which states that language acquisition is innately tied to age. If language learning does not occur before the start of puberty (when brain lateralization is completed) an individual will not achieve a full command of L1 or L2. The Conrad Phenomenon occurs when late L2 learners, having a near-perfect or perfect command of the L2 (vocabulary, morphology, and syntax), do not prove the CPH for L2 acquisition since they cannot pass as native speakers due to their distinct foreign accent, which relates to an imperfect phonology. A proficient speaker must have full mastery of morphology, phonology and syntax. (Scovel, 1969). Operational terms frequently used in the field of psycholinguistics are Age of Arrival (AoAr) meaning time for first exposure to a L2, a morpheme which is the smallest meaningful unit of a language, morphology which is the study of the structure and content of words, phonology which is the organized study of systems of sounds in languages, and syntax which refers to the study of the principles and processes by which sentences are constructed (Galotti, 2009). A Grammaticality Judgement Test (GJT) is a 204-item test to assess proficiency in English as a Second Language (ESL) speakers.

Among the first to disprove the existence of the critical period hypothesis in L2 acquisition are Snow & Hoefnagel-Höhle (1978). The pair claimed that the CPH would yield similar results with L2 acquisition as with L1 acquisition. The independent variable was the age of the English speaker and if they were beginner or advanced L2 Dutch speakers, while the dependent variable was the subject’s’ grasp of Dutch. Monolingual Dutch speakers were used as a control group. The subjects (51 English speakers) were tested three times during their first year in Holland on morphology, syntax, vocabulary and comprehension. The English speakers were first divided by their level of Dutch (advanced and beginner) then further into five age categories. The results showed that 8-10 and 12-15-year-olds had achieved the best control of Dutch after the first year while the 3-5-year-olds scored lowest on all tests. Disproving the CPH for L2 learning is the finding that the 12-15 age-group and adults made the fastest progress during the first few months of learning Dutch.

In another study, Hakuta, Bialystok & Wiley (2003) hypothesized that the slope of the relationship between L2 acquisition and the age of arrival to a new country would be markedly different on either side of the critical age point. English proficiency was measured against AoAr, level of education, and age pertaining to a critical period. Census data was collected using responses from 2.3 million immigrants to the U.S.A who had a Chinese or Spanish first language. The participants were asked to report their English ability on a likert scale and another census study was used to validate scores. Researchers used both 15 and 20 years as cut off points for the end of the critical period. The results disproved their hypothesis and showed a gradual negative correlation decline in L2 acquisition across the lifespan. This finding indicates that the discontinuity in L2 acquisition, essential to the CPH, is nonexistent.

Further disproving the CPH, an experiment by Jia & Fuse (2007) was conducted to predict how age-related differences, age of arrival, and language environment account for the difference in performance between tense (e.g. -ed) and non-tense-related morphemes (e.g. -s), error patterns in morpheme use, and the acquisition of regular (e.g. burn+(ed) = burned) versus irregular (e.g. see+(n) = seen) morphemes. 10 native Mandarin-speaking children and adolescents from ages 5 to 16 were subdivided into two groups: six early arrivals (under 9 years old) and four late arrivals (over 12 years old). A 5 year longitudinal study began 3 months after the participants were immersed into English-speaking schools with sessions consisting of 5 activities: language tasks, child interview, parental interview, language background questionnaire, and observations of child’s language use. Each participant was tested on performance of each morpheme. A growth curve analysis was conducted to compare AoAr and language environment (either at home or at school) showing that early arrivals had higher averages for L2 acquisition than late arrivals. This empirical study suggests that environmental factors contribute to age-related differences in L2 acquisition, failing to support that the CPH is innately tied to age.

Further challenging the truth of the critical period hypothesis and adhering to the Conrad Phenomenon, Abu-Rabia & Kehat (2004) examined 10 case studies to elucidate the possible variables responsible for successful late L2 learners. The variable measured was the severity of a foreign accent when speaking Hebrew. This single-blind case study used 10 female immigrants whose L2 is Hebrew. The women moved to Israel at different ages (7 to 40 years old) from seven various countries. Three native speakers were included for control. Along with an interview, a series of reading tests were empirically gathered. Based on the subjects’ proficiency and phonology, 5 judges ranked the data on a 5-point scale. The variables that account for the success in phonology appear to be the length of L2 exposure, type of input, learning styles, self-esteem, motivations, attitudes and most importantly the amount of L1 and L2 use; all environmental influences. Despite being past the “critical period”, results found that most of the non-native speakers’ were indistinguishable from native Hebrew-speakers, some even scoring better than native speakers.

A further analysis shows two parallel studies conducted by DeKeyser, Alfi-Shabtay & Ravid, (2005) which measured the length of residence, age of acquisition, and age at the time of testing to identify if there is a general decline of L2 acquisition due to age of arrival. The study used 150 Russian participants above the age of 18 who acquired either English in North America or Hebrew in Israel over a span of eight years and tested grammar of the L2. Both groups were asked to complete a background questionnaire, a grammar test, and a verbal aptitude test. The procedures used GJT scores to find the correlation between each group. The means of each results for Russians learning English and for Russians learning Hebrew showed that each group had similar results in L2 acquisition, but found a decline of grammar acquisition after the age of 18 and a decline in language acquisition after the age of 40. This finding provides evidence that there is a general decline of language learning ability due to AoAr, thus interpreted as evidence for a critical period in L2 acquisition.

In 1989, Johnson & Newport examined whether the critical period hypothesis extends to L2 acquisition by comparing the effect of age of arrival on L2 acquisition measures. The subjects were 46 Chinese and Korean ESL speakers, ranging from 3 to 39 years old for AoAr to the USA. Half of the participants were early arrivals (before 15 years) and the other half were late arrivals (after 17 years). A control group of 23 native English speakers was also tested. The subjects were asked to judge the grammatical correctness of 276 spoken English sentences to determine their knowledge of syntax and morphology. Results of this non-blind empirical study found that the control group performed significantly better than all subjects, except for those arriving before age 7, who were able to attain a native-English level of performance. For adults, later AoAr determined that one would not become native or near-native in the L2. On average, late arrivals performed worse than early arrivals, showing that children are better than adults in L2 acquisition. This results in an apparent negative correlation between age of arrival and L2 acquisition measures.

Recent studies support the claim that a critical period is non-existent for L2 acquisition. Snow & Hoefnagel-Höhle’s (1978) results show that teens and adults made the fastest progress during the first months of L2 acquisition. Hakuta, Bialystok & Wiley (2003) found that there is a gradual decline in L2 acquisition along age. Jia & Fuse (2007) emphasized environmental over biological influences. Despite being past the hypothesized critical period, Abu-Rabia & Kehat (2003) found that non-native speakers’ were indistinguishable from native speakers, again emphasizing environmental factors. All these findings fail to support the CPH in L2 acquisition. Contrary to contradicting articles, overwhelming evidence points to the rejection of the CPH for L2 learning. Although findings in contradicting studies are similar, they are interpreted quite differently. DeKeyser, Alfi-Shabtay & Ravid (2005) purport a general rather than an abrupt decline in L2 learning in relation to age of arrival. Johnson & Newport (1989) also find that children appear to have a special capacity for learning L1 and L2 with a slow decline with age. Unlike Hakuta, Bialystok & Wiley (2003) who also found a slow decline in L2 acquisition with age, DeKeyser, Alfi-Shabtay & Ravid (2005), and Johnson & Newport (1989) interpret this same data as proof for the CPH. However, the CPH remains difficult to prove when all declines in age on L2 acquisition appear with no steep drop-offs. In conclusion, research seems to reject the CPH extending into L2 acquisition. Consequently, because of the influence of environmental factors and an apparent gradual rather than abrupt decline in age-related differences of L2 learning, the critical period hypothesis needs be revisited and redefined for second language acquisition.

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