Cognitive advantages of bilingualism
Being bilingual has been linked to a number of cognitive benefits. Research has studied how a bilingual individual's L1 first language (L1) and second language (L2) interact, and has shown that both languages have an influence on the function of one another, and also on cognitive function outside of language. Research on the cognitive advantages to linguistic development, perception, and attentional and inhibitory control has shown that bilinguals can benefit from significant cognitive advantages over monolingual peers in various settings.
During history of research into the cognitive advantages of bilingualism, the view has 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.
A bilingual can be defined as an individual that is exposed to two languages simultaneously from a young age (under 4), although the definition may vary slightly depending on the studies being presented and their sample selection processes. Several definitions have been given in the literature for bilingualism, for example, either individuals that are learners of another language irrespective of proficiency, or individuals that are equally proficient in both languages.
Before the 1960s research on bilingual individuals was varied but with a specific pattern of conclusions, namely that being bilingual was detrimental to a child's linguistic and cognitive development, and put the individual at a disadvantage compared to monolingual peers. The general opinion was that bilinguals would have smaller vocabularies, stunted general 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 “problem of bilingualism” the “handicapping influence of bilingualism”  and reported that bilinguals performed worse in IQ tests, had smaller vocabularies, and suffered in most aspects of language development, as revealed mostly through verbal IQ tests.
However, these studies suffered from several methodological problems that undermined the soundness of their conclusions: These studies 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 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 which brought to light the importance of controlling for such factors as age, sex, and SES, as well as of having a standardised measure for bilingualism, when selecting a sample of bilinguals to be studied. In their study where they carefully matched their bilingual to their monolingual participants, 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 of bilinguals over matched monolingual peers in several aspects of language development and ability, as well as in more general areas of aptitude such as perception and executive functioning.
Leopold, in his record of his daughter's bilingual development of language  observed that she had “loose connections” between the (phonetic) structure of words and their semantics (meaning), demonstrated by her frequent substitution of English words with German and vice versa, in every day speech and even well rehearsed songs and rhymes. He notes that this gave her a greater flexibility in the use, and a way of use of language, that was unobserved in monolingual children of her age, and that perhaps this loose connection between the meaning and form of a word could result in more abstract thinking or greater mental flexibility.
In 1972, Worrall  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, and presented them with tests to assess whether they showed a semantic or phonetic preference when categorising words. An example of one task given in the paper was to decide which of the two words can and hat were more similar to the word cap. The semantic choice would be hat, versus the phonetic which would be can. Other tests were designed to provide a choice between semantic and phonetic interpretation of objects (e.g. in a hypothetical situation could you call a cow a dog and if you did would this dog bark?).
She found that, although both monolingual and bilingual children showed 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.
In their book In Other Words, Ellen Bialystok and Kenji Hakuta examine the idea that "the knowledge of two languages is greater than the sum of its parts." They argue that the linguistic benefits of being bilingual are more than simply being able to speak two languages. 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. An example of this is the understanding of the arbitrariness of labels for objects discussed in the above paragraph, but also being able to distinguish between and use two different grammatical or syntactical structures. This enables the child to increase their understanding of the structure of language and gain a greater awareness of meaning; an increase of metalinguistic awareness.
Bialystok argues that metalinguistic awareness also increases bilinguals' control of linguistic processes, such as having a greater ability to detect grammatical or syntactical errors, and recognize words in continuous speech. Bilinguals have also been found to outperform monolinguals in reading ability (an effect modulated by the relationship of the two languages), and better reading skills in L1 were demonstrated with as little as an hour a week of L2 learning, suggesting that being bilingual is also advantageous in the development of reading as well as spoken language.
Metalinguistic awareness has therefore been shown to benefit the individual in the acquisition and use of language, giving bilingual individuals (who acquire metalinguistic awareness earlier in life than their monolingual peers) a firm advantage. However, it has also been proposed that metalinguistic awareness could result in advantages in other cognitive abilities besides language, giving individuals a generalised cognitive advantage over monolinguals (see below).
Finally, studies have shown that regardless of the language in use at a present time, both languages are constantly active both phonologically and semantically in bilingual individuals, as indicated by electrophysiological measures of performance, even though behavioural measures such as reaction times often do not indicate such an interference.
In 2011, Wu and Thierry  conducted a study where bilinguals were shown picture pairs and 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 L2, they showed a priming effect (albeit delayed) when those L2 words rhymed in L1.
In 2012, Hoshino and Thierry  conducted a study where participants were shown word pairs in L2 that sometimes contained words that were cognates to L1 words. 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 or not the meaning was interpreted in L1 or L2.
Both these studies conclude that both languages of an individual are constantly unconsciously active and interfering with one another with facilitatory results regarding the processing of words, explaining bilinguals' advantages over their monolingual peers when it comes to linguistic processing.
Executive functions are those cognitive processes such as problem solving, mental flexibility, attentional control, inhibitory control, and task switching. Bilingual individuals have been shown over a number of different tasks and situations to be better at such processes; suggesting an interaction between being bilingual and executive functions.
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).
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 that 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 back to Worrall's findings ), 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.
An explanation offered by Bialystok  and others for this greater ability of bilinguals to selectively attend to important conceptual attributes of a stimulus, is that it 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 which 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.
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. 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. Studies have correlated bilingualism with the delayed onset of dementia in Alzheimer's disease (AD) for as long as 5 years  and a recent study  provided neurological support for these findings by analyzing CT scans of patients diagnosed with possible AD. Certain functional abilities, that correlate with environmental factors remain in individuals with AD, enabling them to function somewhat normally even in advanced stages of the disease process; these have been termed Cognitive Reserves. This study hypothesized that bilingualism may be a contributing factor to cognitive reserves in AD, measurable by the amount of AD-related atrophy in the brain of bilingual individuals with the disease, matched to monolinguals. As predicted, bilinguals showed a higher level of atrophy than the monolinguals even though their cognitive functioning remained similar, suggesting strongly that bilingualism could be a cognitive reserve that delays the onset of signs of dementia in AD.
A review of the literature suggests that bilingualism has an additive effect on an individual's creativity, by enhancing their mental flexibility, their ability to solve problems, and to perceive situations in different ways and the ability to maintain or manipulate these perceptions to suit the task at hand, all in ways that matched monolingual peers do not exhibit.
One study addressed a less explored field of cognitive advantages bilingual children may exhibit, in the use of creativity to solve of mathematical problems. Participants were presented with problems that were either mathematical in nature (arranging two sets of bottle caps to be equal according to instruction) or non-mathematical (a common household problem represented in pictures) and were asked to provide solutions, while being rated on scales of creativity, flexibility and originality. The results of the study confirmed that the bilingual children were more creative in their problem solving than their monolingual peers. One attribution for this trait could be bilinguals' increased metalinguistic awareness, which creates a form of thinking that is more open and objective, resulting in increased awareness and flexibility.
This enhanced mental flexibility that develops in bilinguals influences more than their problem solving or linguistic skills. Language appears to change the way the world is perceived between individuals that speak different languages, and it has been shown to influence the perception of color  as well as the categorisation of objects.
Thierry et al. studied how having different words for different colors in one language might affect the perception of that color as compared to a language that does not discriminate between those colors. In Greek, "light blue" is distinguished from "blue", not simply as a different shade but as a whole different category of color. In this study, bilingual and monolingual Greek/English participants were shown different shades of blue and light blue as well as green and light green (for which a distinction is not made in Greek) and ERPs were recorded. Electrophysiological measures showed a distinct pattern for the bilinguals indicating that they were perceiving the two colors as completely separate.
Cook et al. explored the fact that Japanese speakers are likelier than English ones to categorise objects according to their material as opposed to their shape. In their study  they found that the preferences of Japanese monolinguals learning English changed; the more proficient they became in English, the more their object categorization results matched those of English monolinguals.
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