Cognitive advantages of bilingualism
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A bilingual person can traditionally be defined as an individual who uses (understands and produces) two (or more) languages on a regular basis. A bilingual person's initial exposure to both languages may have started in early childhood, e.g. before age 3, but exposure may also begin later in life. While some people assume that bilinguals must be equally proficient in their languages, perfectly equal proficiency is rarely attested, and proficiency typically varies by domain. For example, a bilingual person may have greater proficiency for work-related terms in one language, and family-related terms in another language.
Being bilingual has been linked to a number of cognitive benefits. Research has studied how a bilingual individual's first language (L1) and second language (L2) interact, and it has been shown that both languages have an influence on the function of one another, and on cognitive function outside of language. Research on executive functions such as working memory, perception, and attentional and inhibitory control, has suggested that bilinguals can benefit from significant cognitive advantages over monolingual peers in various settings. There are also age-related benefits, which seem to help older adults on the battle against cognitive decline.
Throughout the history of research into the cognitive advantages of bilingualism, views have shifted from a subtractive to an additive perspective; that is from believing that being bilingual detracts from one's abilities, to believing that being bilingual adds to an individual's abilities.
There is, however, strong disagreement over how findings on this subject should be interpreted. Systematic reviews and meta-analyses of studies in the field have found no evidence for cognitive advantages in healthy adults and that reporting is subject to publication bias, which has therefore given a distorted view of the evidence.
- 1 History
- 2 Executive functions
- 3 Benefits in older age
- 4 Age of acquisition
- 5 Language
- 6 See also
- 7 References
Over the course of the past few years, the prevalence of bilinguals in the United States has increased dramatically. While the United States Census Bureau does not directly poll for bilingualism, they do poll for what languages are used in an individual's home, and if it is a language other than English, they then poll for how well that same individual speaks English. In 2012, François Grosjean, a professor of Linguistics from the University of Neuchâtel, interpreted the results from the Census Bureau as follows: 11% of the population was bilingual in 1980, 14% in 1990, and 20% in 2012. This positive increase matches the shift to more positive beliefs about the cognitive advantages of bilingualism.
Before the 1960s, research on bilingual individuals was varied. There was a specific pattern of conclusions, namely that being bilingual was detrimental to a child's linguistic and cognitive development, and that it put the individual at a disadvantage compared to monolingual peers. The general opinion was that bilinguals would have smaller vocabularies, stunted cognitive abilities and that children learning two languages from a young age would be spending too much of their energy differentiating and building the two languages to become competent in either one. Studies referred to the topic as the "problem of bilingualism" or the "handicapping influence of bilingualism" and reported that bilinguals performed worse in IQ tests and suffered in most aspects of language development.
These studies suffered from several methodological problems that undermined the soundness of their conclusions. They employed unstandardized and subjective definitions of bilingualism and of a bilingual individual (e.g., labeling a person as bilingual or monolingual through assumptions based on the national origin of that person's parents or even based on that person's family name), raising the concern that there is no way of determining whether their samples were truly representative of a bilingual population. They also did not control for socioeconomic status (SES) and many of them administered verbal-intelligence tests to non-proficient speakers of a second language in that second language.
In 1962, Peal and Lambert published a study highlighting the importance of controlling for such factors as age, sex, and SES, as well as of having a standardized measure for bilingualism when selecting a sample of bilinguals to be studied. In their study they carefully matched bilingual to monolingual participants, and they found that the bilinguals showed significant advantages over the monolinguals in both verbal and non-verbal tests, especially in non-verbal tests that required more mental flexibility.
Since then, the literature has consistently found advantages for bilinguals over matched monolingual peers, in particular in relation to cognitive areas of aptitude such as perception and executive functioning.
Executive function is the domain of high-level cognitive processes that assist in goal-oriented tasks, such as problem solving, mental flexibility, attentional control, inhibitory control, and task switching. A great deal of research has been committed to investigating a potential connection between bilingualism and enhanced executive function. Many researchers have asserted that bilinguals show better executive control than monolinguals matched in age and other background factors (e.g. socio-economic status), suggesting an interaction between being bilingual and demonstrating honed executive functions. Moreover, the ability to better attend to or inhibit irrelevant information has been found to persist into adulthood with bilingual adults (that have been bilingual since childhood) who show better controlled processing than monolingual peers, and has even been linked to slowing age-related cognitive decline (see age-related effects section below).
In support of a connection
Ellen Bialystok has done extensive research into the cognitive advantages of bilingualism. In several studies, she has shown that bilinguals outperformed their peers in tasks measuring executive function, suggesting that being bilingual gives the individual an advantage of better control of attention and therefore facilitates processing and functioning in several cognitive tasks.
Bialystok makes a distinction between two types of processing that aid children in language development: analysis, which involves the ability to represent and understand abstract information, and control, which involves the ability to selectively attend to specific aspects of structures whilst ignoring irrelevant information. The literature concludes that it is in the aspect of control that bilinguals have been found to have an advantage over their monolingual peers when it comes to cognitive abilities.
In one study, Bialystok administered a non-linguistic card-sorting task to her participants that required flexibility in problem solving, inhibiting irrelevant information, as well as recognizing the constancy of certain variables in the face of changes in the rules. She found that bilingual children significantly outperformed their monolingual peers in this task, suggesting early development of inhibitory function that aids solving problems which require the ability to selectively focus attention.
In a following study, Bialystok and Martin aimed to determine what gave bilinguals an advantage in solving the card-sorting task (and generally an advantage in problem solving situations). Though the groups were equivalent in their ability to represent the stimuli (reflecting Worrall's findings, described below), and both were equally able to inhibit learned motor responses, bilinguals showed a firm advantage in the task requiring conceptual inhibition; the ability to inhibit previous associations and create new mental representations of the stimulus according to task changes.
Building on Bialystok's research, Carlson and Meltzoff studied Spanish-English bilingual children. There were three groups: native bilinguals, English monolinguals, and English monolinguals enrolled in an immersion program. The bilingual children's scores were on par with the other groups, but when the two groups were adjusted for age, parent income and education, and verbal scores, the bilinguals outperformed the monolinguals on "conflict tasks," or tasks that required resolving multiple attention demands.
Finally, Bogulski, Rakoczy, Goodman, and Bialystok investigated how "lapsed bilinguals" (participants who used to be bilingual but are now monolingual) compared to monolinguals and fluent bilinguals in executive function tasks. The lapsed bilinguals tested better than monolinguals but worse than their fluent counterparts. 
David Green offered an explanation for this phenomenon with his "inhibitory control model." Proposed in 1998, this model references a bilingual's constant need to suppress one language while using another. Because this task requires suppressing a source of distraction, this kind of control is then applied to other tasks. This assertion was bolstered by a study of unimodal bilinguals (bilinguals who communicated with two spoken languages) and bimodal bilinguals (bilinguals who used one spoken language and sign language). Because bimodal bilinguals can express themselves in both languages at the same time, they may require less inhibition. This idea was supported by the results of the study; only unimodal bilinguals were found to have an advantage, as measured by the flanker task (a cognitive task that measures attentional focus and inhibition). Bimodal bilinguals also switch languages less frequently, because they are more likely to use both languages at once than to completely switch from one to the other. For this reason, the researchers of this study hypothesized that it may be the switching between languages that gives unimodal bilinguals the advantage. Prior and Gollan conducted a study investigating this idea, and found that bilinguals who switched languages often had an advantage in task shifting over bilinguals who did not frequently switch languages. However, this study did not control for similarity between the languages (languages that are more similar might require more attention to keep straight). When Verreyt, Woumans, Vandelanotte, Szmalec, and Duyck ran a similar study but with all participants having the same languages, they replicated the results of Prior and Gallan. Additionally, because their study looked at tasks measuring inhibition even though language switching should directly affect switching tasks, they argued that the effects of language-switching carry over multiple facets of executive control.
Bialystok and others have echoed this idea that the greater ability of bilinguals to selectively attend to important conceptual attributes of a stimulus may stem from the bilinguals' constant need to inhibit competing labels in their two languages for one object according to the currently relevant language. Bilinguals have different representations in each language for similar concepts and therefore need to constantly be aware of which language they are using and what the appropriate word is to be used in that context. This culminates in an advantage of cognitive control, since the ability to switch between languages and select the appropriate word for use is directly linked to the ability to better attend to relevant, or inhibit irrelevant, information. A further explanation refers to bilinguals' unique experience with using two languages in the same modality (spoken), differentiating them from monolingual peers, and requiring them to make the decision about how best to respond to a situation, as well as have better control over what they select.
Is bilingualism or executive control the causative force?
Hakuta and Diaz, addressed the chicken and egg question concerning bilinguals and their enhanced cognitive abilities; do children with greater cognitive abilities tend to learn more than one language, or is it knowing more than one language that enhances cognitive ability? They administered a set of non-verbal tests that are designed to measure cognitive ability (Raven's Progressive Matrices) to a bilingual sample of children. They found a high correlation with the degree of bilingualism (how proficient the individuals were in each of their languages) of their sample and scores on the test, as well as bilingualism did in fact predict performance (and therefore cognitive ability). However, an important point to note, is that most native bilinguals haven't learnt a second language because they are more intelligent. In most cases, they have grown up in a family where use of the two languages is necessary and therefore it is unlikely that the child's intelligence will allow them to learn the second language.
Against a connection
Not all researchers agree that bilingualism contributes to enhanced executive function.
Some take issue with methodology. Virginia Valian asserts that the correlations between bilingualism and executive function are inconsistent, largely due to the fact that executive function is not uniformly defined and that different tasks contribute to executive function. Because some of these tasks are available to monolinguals and bilinguals may similarly participate in these tasks to varying degrees, she argues that bilinguals cannot be assumed superior to monolinguals in executive function. She also notes that bilinguals are not consistently better at all executive function tasks. Ramesh Kumar Mishra builds upon Valian's suggestions by arguing that research studies should shift to comparing bilinguals of different proficiencies instead of bilinguals to monolinguals. She argues that things like exercising and video game playing can affect executive function, and since they are unrelated to language, they must be controlled for. Kaushanskaya and Prior respond to Valian that it is not only the lack of uniformity in defining executive function, but also the difficulty in defining bilingualism which make it problematic to draw strong conclusions about the effects of bilingualism on executive function.
Some researchers have also simply found results suggesting there is not a connection. A meta-analysis of more than 150 studies comparing the performance of monolinguals and bilinguals on different cognitive tasks found no evidence of a bilingual advantage. 
A study by Kalia, Wilbourn, and Ghio found that monolinguals and early bilinguals performed similarly on executive function tasks, while late bilinguals performed slightly worse. This study may not have controlled for the same variables (such as parental education and socio-economic status), which could account for some of the disparity between results.
Finally, Paap and Greenberg assert that bilinguals are not necessarily superior at executive processing. They assess their sample as similar in confounding variables and found that not only was there not evidence supporting an advantage for bilinguals, but that, if anything, the evidence would argue against this.
Parallel activation of both languages
It has been found that a bilingual's two languages are simultaneously active, both phonologically and semantically, during language use. This activation is indicated by electrophysiological measures of performance. Not only is a person's dominant language (L1) active when using the less dominant language (L2), but their L2 is also activated when using L1. This happens once the individual is adequately proficient in the L2. They are both active when listening to speech, reading words in either language or even planning speech in either language. Also, both languages are activated even when only one language is needed by the user.
Bilingualism studies have mostly looked at Spanish-English or Dutch-English bilinguals. These languages share the Roman alphabet, and there are many cognates (words which have the same linguistic derivation e.g. 'piano' is the same in all 3 languages). Cross-language activation therefore seems less surprising. However, cross-language activation has also been reported in bilinguals whose two languages have different scripts (writing systems) and lexical forms (e.g. Japanese and English). A study by Hoshino & Kroll (2008) demonstrated that Japanese-English and Spanish-English bilinguals performed similarly in picture naming tasks even though the cognates for Spanish-English bilinguals shared phonological and orthographic (sound and spelling) information whereas the Japanese cognates were only phonologically similar (sound). Although the words were spelt and presented differently for Japanese-English bilinguals, this did not affect the simultaneous activation of both their languages.
In 2011, Wu and Thierry conducted a study where Chinese-English bilinguals were shown picture pairs. Participants were asked to name the second picture in the pair when it was shown and then were asked to judge whether the word pairs corresponding to the pictured objects rhymed or not. Word pairs were designed so that they either rhymed in both L1 and L2 or only in one of the two languages. Electrophysiological measures (see Event-related potential) of the effect (priming) of the sound repetition induced by the rhyming of the word pairs showed that even though the participants were performing the task in their L2, they showed a priming effect (albeit delayed) when those L2 words rhymed with words in the L1. This suggests that in regards to language use, both L1 and L2 are accessed and compete for selection during L2 production.
In 2012, Hoshino and Thierry conducted a study where Spanish-English bilingual participants were shown word pairs in English, their L2, and asked to judge whether the word pairs were related. Sometimes, things presented would be "interlingual homographs," or words that sound the same in both languages but have a different meaning in each. These pairs would be primed by things relating to one of the meanings or to neither, and the effects of this priming were measured electrophysiologically. Participants judged whether the words in the pairs were related, and electrophysiological results revealed that semantic priming (facilitation of processing of the words) occurred when the words in the pairs were related to each other whether the meaning was interpreted in English or Spanish.
The two immediately preceding studies conclude that both languages of an individual are constantly unconsciously active and interfering with one another. The results, in regard to word processing, can help demonstrate how bilinguals have advantages over their monolingual peers when it comes to this area of study.
The fact that both languages are constantly activated means that they potentially compete for cognitive resources; bilinguals need to acquire a way to control or regulate the competition, so as to not use the wrong language at the wrong time. Inhibition refers to being able to ignore irrelevant information and therefore not be distracted by non-target stimuli. For example, a test that is widely used to assess this executive function is the Stroop task, where the word for a colour is printed in a different colour than the name (e.g. the word 'red' printed in blue ink). This causes interference and distraction; reaction times are measured to see how distracted the individual is by the incongruent word and colour. Bilinguals compared to monolinguals have shown an advantage at this task, suggesting that bilinguals have a more developed inhibition process, potentially due to the constant inhibition of their non-target language.
Inhibition has been suggested as the executive control system that allows successful linguistic selection even when both languages are co-activated in bilinguals. De Groot & Christofells (2006) proposed a distinction between two types of inhibition that may occur; global inhibition and local inhibition. Global inhibition refers to suppression of an entire language system, e.g. inhibiting Spanish when speaking English, and local inhibition refers to inhibition of a more specific competing vocabulary, e.g. the translation of the same word or phrase. Local inhibition mostly affects linguistic performance whereas global inhibition affects both linguistic and cognitive performance. Despite the apparent advantages for bilinguals in terms of non-linguistic cognitive processing, there seem to be some drawbacks for bilinguals in terms of linguistic cognitive processing: bilinguals have been shown to exhibit reduced speech fluency and speed of lexical access compared to monolinguals.
Bilingual individuals have also shown superiority in metalinguistic ability. This additional advantage seems closely tied to executive function. Metalinguistic awareness is the understanding of the separation between language's structure and its meaning. For example, being able to judge the grammaticality of a sentence regardless of whether it is sensical, or being able to separate the set of sounds comprising a word from the word's meaning. The ability to suppress distracting information, such as semantics, is an act of inhibition, meaning that it falls into executive function. This ability could also be exercised by being bilingual, given that a bilingual individual has to suppress their knowledge of another language system when operating in one of their languages.
Bialystok also studied metalinguistic abilities in bilinguals versus monolinguals by having subjects judge whether a sentence was grammatical, regardless of its logical sense. Bilinguals outperformed monolinguals in judging that a nonsensical sentence was correct. Additionally, when brainwaves of bilingual adults were observed during the task, they showed less of a reaction indicative of processing conflict as reflected on the P600 waveform.
Studies by Leopold and Worrall investigated how being bilingual affected children's awareness of the fact that the connection between words and meaning is arbitrary. See language section below.
Benefits in older age
There has been a surge in interest in the benefits of bilingualism against age-related cognitive decline. Klein & Viswanathan found that the normal decrease in attention control observed in older adults was reduced in bilinguals, suggesting that bilingualism may be protective against the effects of cognitive aging. Elderly bilinguals have also been shown to be better at switching between tasks, ignoring irrelevant information and resolving conflicting cognitive alternatives. Bilingualism may be one of the environmental factors which contributes to 'cognitive reserve'. Cognitive reserve is the idea that engagement in stimulating physical or mental activity can act to maintain cognitive functioning in healthy aging and postpone the onset of symptoms in those suffering from dementia. Factors that contribute to this also include education, occupational status, higher socioeconomic class, and the continuing involvement in physical, intellectual and social activities.
To test the protection of bilingualism against Alzheimer's disease (AD), Bialystok et al. (2007) examined the hospital records of monolingual and bilingual patients who had been diagnosed with various types of dementia. After controlling for various cognitive and other factors, the researchers found that bilinguals experienced the onset of symptoms and were diagnosed approximately 3–4 years later on average than monolinguals. This was replicated with patients all diagnosed with AD. It is important to stress, however, that the studies did not show that bilingualism directly prevents one from having AD, but rather enables functional cognition for a longer period of time; it delays the onset of symptoms for those with the disease. This was confirmed by the finding that when monolingual and bilingual Alzheimer's patients' brains were scanned, bilinguals actually had more pathology (signs of disease) and damage than the monolingual patients. This suggests that active use of the two languages protects against the symptoms of the disease; areas of the brain that enable cognitive control may have benefited from the bilingual experience and so improve cognitive function in older age.
The finding that bilingualism contributes to cognitive reserve has also been replicated by several other studies For example, Abutalebi et al. (2015) tested 19 bilinguals (8 Cantonese-Mandarin and 11 Cantonese-English, age range 55-75) and 19 monolinguals (Italian speakers, age range 49-75) who had been matched for education level, performance on the Flanker Task (a cognitive response test,) and socioeconomic status. It is important to remember that this is a relatively small sample size; however, the results did confirm previous studies. According to the research, the bilinguals outperformed the monolinguals on all experimental tasks, and the researchers found that monolinguals' neural imaging showed higher signs of age-related effects on performance of tasks and decreased gray matter density. Meanwhile, the bilinguals' neural imaging showed higher levels of gray matter along the anterior cingulate cortex. Because of these results, the investigators concluded that bilingualism aids in protection against cognitive decline.
The bilingual advantage in cognitive function has been demonstrated especially in children and older adults, while the advantage in young adults has been rather variable. Suggestions for this finding may be that young adults are at their peak cognitive function, so it may be difficult to show any bilingual advantages beyond that peak level, especially in simple executive function tasks. It is thought that the benefits may be particularly beneficial to individuals at points in their lives when they are more vulnerable, for example in early development and later in life, when ordinary cognitive processes decline.
Age of acquisition
A debate within the linguistic community is whether the age of acquiring one's L2 has effects on the cognitive advantages. A study on native bilingual vs late bilingual vs monolingual children in the USA revealed an overall bilingual advantage. Furthermore, native bilingual children demonstrated better performance on a selection of executive function tasks compared with their late bilingual and monolingual counterparts. Participants were controlled for age, verbal ability, and socioeconomic status (indicated by parent education level). However, there are various methodological factors which may call into question the validity of these results. Firstly, a small sample size was used, with only 12 children in the bilingual group, 21 in the late bilingual group, and 17 in the monolingual group. 'Late bilingual' in this study was classified as a monolingual child who had been in a bilingual school for 6 months (where half the lessons were in English and half in Spanish or Japanese). This may be a poor representation of 'late bilinguals', as 6 months may not be enough time for cognitive changes and adaptations to the brain to have taken place, and these children will unlikely already be 'proficient' in the L2, therefore this may not an appropriate group sample to support the claims being made. In addition, the effect sizes on all the individual executive function tests were all small to moderate effect sizes (ƞ2= 0.01 to 0.2). In combination with the lack of power due to small sample size, strong conclusions cannot be drawn from this data.
Another study, Kapa and Columbo (2013) investigated the attentional control of monolingual children, Spanish-English bilingual children who had learned both languages before the age of 3, and Spanish-English children who had learned English after age 3. Attentional control is a cognitive skill in which one can ignore unnecessary or impertinent information to the task at hand. Children were tested using an Attention Network Test. Although all groups obtained the same accuracy rates, the researchers found that early L2 learners (those who learned both languages before the age of 3) had the fastest reaction time. The late learners and monolinguals did not significantly differ in response time, illustrating that early L2 acquisition could be a decisive factor in executive control levels.
As one of the pioneers to the study of child language and bilingualism, Werner F. Leopold often used his daughter, Hildegard, to record his observations on this subject. In his studies, he observed that Hildegard had "loose connections" between the (phonetic) structure of words and their semantics (meaning) because of her frequent substitutions of English words with German words and vice versa. This was noted in her everyday speech and well-rehearsed songs or rhymes. He noted that she had a greater flexibility in the use of language that was unobserved in monolingual children of her age. Leopold considered that perhaps this loose connection between the meaning and form of a word could result in more abstract thinking or greater mental flexibility for bilingual children. Following this study, several others were formed to test similar things and find out more about the mental abilities of bilinguals with relation to their languages.
Anita Ianco-Worrall, author of Bilingualism and Cognitive Development, designed a study to test Leopold's observations and was able to replicate them. She tested two groups of monolingual and bilingual children at ages 4–6 and 6–9. These participants were given tasks to assess whether they showed a semantic or phonetic preference when categorizing words. An example of one task given in the study was to decide which of the two words, either can or hat, was more similar to the word cap. The semantic choice would be hat while the phonetic choice would be can. Other tasks were designed to provide a choice between semantic and phonetic interpretation of objects. For instance, in a hypothetical situation, could you call a cow a dog and if you did, would this dog bark?
The results of Ianco-Worrall's study showed that although both monolingual and bilingual children had no differences in the way they understood the words used, 54% of the younger bilingual children consistently showed a semantic preference in contrast to their monolingual peers. In monolingual children, semantic preference increased with age, suggesting that bilingual children reach a stage of semantic development 2–3 years earlier than their monolingual peers. This finding is in stark contrast to the early research and claims about bilingualism, which warned that bilingualism stunts children's linguistic development.
Language structure and awareness
In their book In Other Words, Ellen Bialystok and Kenji Hakuta, both professors studying bilingualism, examined the idea that "the knowledge of two languages is greater than the sum of its parts." They argued that the linguistic benefits of being bilingual are more than simply being able to speak two languages. For instance, if a child is learning two languages whose structures and rules are significantly different from each other, this would require the child to think in more complicated ways. Take for example the arbitrariness of labels for objects, or distinguishing between and using two different grammatical or syntactical structures. These areas would be quite difficult for a child to learn, but would increase the understanding of the structure of language and help gain a greater awareness of meaning. This greater awareness of meaning for bilinguals is what is referred to as metalinguistic awareness (see metalinguistics abilities section above).
Bilinguals have also been found to outperform monolinguals in reading ability, as seen in another study by Bialystok. To analyze this area of bilingualism, Bialystok discussed the representational principle, which refers to the symbolic representation of spoken language, or the connection between spoken and written language systems. Understanding this principle would help one with acquiring literacy. For the testing of this principle, she gave children a "Moving Word Task" where the child had to appropriately match the written word to the object on a card. If they could correctly match the two after some rearranging of the cards, it was agreed that they could understand written words as representations of specific words whose meanings cannot change. The study was taken further in order to see when bilinguals grasped this principle in comparison to monolinguals. The results showed that bilingual children were correct on their "Moving Word Task" over 80% of the time, which is a percentage equal to that of monolinguals who were one year older than the bilinguals being tested. Overall, the bilinguals seemed to understand the representational principle earlier than monolinguals, meaning they were earlier prepared for literacy acquisition.
In another study done by Durgunoglu, Nagy, and Hancin-Bhatt, this same concept for bilinguals' reading abilities was also studied. For this specific study, native Spanish speaking children who were learning to read English were tested. The researchers observed these bilinguals to find that their levels of phonological awareness and word recognition in Spanish could predict how well they would be able to recognize words in English. The results showed that the phonological awareness skills established in one language could be transferred to the reading ability in another language. Again, bilinguals seem to be more advanced than monolinguals when it comes to reading ability.
It is a well-replicated finding that bilinguals have a smaller vocabulary size than their monolinguals counterparts. Given that bilinguals accumulate vocabulary from both their languages, when taking both languages into account, they have a much larger vocabulary than monolinguals. However, within each language bilinguals have a smaller vocabulary size and take longer to name pictures as seen in standardized vocabulary tests, such as the Peabody Picture Vocabulary Test and Boston Naming task. A possible explanation may be that the frequency of use of words is related to increased lexical accessibility, meaning that words that are used more frequently are accessed more quickly. Therefore, bilinguals may be 'less proficient' relative to monolinguals, purely because they use one sole language less frequently than monolinguals, who use the same language all the time. In addition, the need to select the appropriate language system makes ordinary linguistic processing more effortful. The simple act of retrieving a common word is more effortful for bilinguals than monolinguals due to the competition of the two languages.
Other things to consider in this area of a bilingual's language were pointed out in Bialystok, Luk, Peets, and Yang's study from 2010. They note that certain vocabulary tests could yield artificially low scores for bilingual children according to the domain from which the test words are taken. For example, this group of researchers found that monolingual and bilingual 6-year-olds in their study had similar scores on English words that were associated with schooling. However, when the children were tested on English words that were associated with the home, the scores were significantly lower for the bilingual (English-Spanish) children. The researchers interpret this result as reflecting an asymmetry in vocabulary domains and language exposure: monolingual and bilingual children were equally exposed to the school context in the same language (English), but English was not commonly used in the home environments of the bilingual children. Therefore, one cannot conclude that the bilingual children exhibited a true deficit in vocabulary ability.
Effects on L1 from prolonged exposure to L2
It has been suggested that prolonged naturalistic exposure to L2 affects how L2 is processed, but it may also affect how the L1 is processed. For example, in immersion contexts, the individual experiences reduced access to L1 and extensive contact with L2, which affects and facilitates processing of L2. However, this may also consequently affect processing of their L1, such as with increased difficulty in naming objects and phonology.
To test this hypothesis, Dussias & Sagarra (2007) investigated how individuals interpreted temporarily ambiguous phrases. For example, 'Alguien disparó al hijo de la actriz que estaba en el balcón' = 'Someone shot the son of the actress who was on the balcony'. When asked the question, '¿Quien estaba en el balcón?' = 'Who was on the balcony?', monolingual Spanish speakers will typically answer 'el hijo' = 'the son' as they have a high attachment preference, meaning they attach the modifier to the "higher" verb phrase [shot the son]. This differs from monolingual English speakers who will typically answer 'the actress' as they have a low attachment preference, meaning they attach the modifier to the "lower" verb phrase [the actress who was on the balcony]. The researchers found that Spanish-English bilinguals in a Spanish-speaking environment showed preference for the typical Spanish high-attachment strategy. However, Spanish-English bilinguals in an English-speaking environment showed preference for the typical English low-attachment strategy, even when reading the phrase in Spanish, their dominant language. This may be because they have more exposure to English constructions, making it more available to them. But altogether, this supports the idea that the L2, English in this case, is affecting the way the native Spanish speakers use their L1.
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