Cognitive effects of multilingualism


A bilingual individual is traditionally defined as someone who understands and produces two or more languages on a regular basis. A bilingual individual's initial exposure to both languages may start in early childhood, e.g. before age 3, but exposure may also begin later in life. Equal proficiency in a bilingual individuals's languages is rarely attested as it typically varies by domain. For example, a bilingual individual 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 and second language interact, and it has been shown that both languages have an influence not only on the function of one another, but also 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 confer protective effects against cognitive decline in older adults.
Throughout the history of research into the cognitive advantages of bilingualism, views have shifted from a subtractive to an additive perspective: it is now believed that being bilingual adds to an individual's abilities rather than subtracting from it.
There is, however, strong disagreement over how findings on this subject should be interpreted. Systematic reviews and meta-analyses of studies assessing executive functioning have failed to find compelling evidence for cognitive advantages in healthy adults or in participants across a broader age range. Moreover, the distribution of effect sizes in the meta-analyses suggest that the reporting of effects of bilingualism on executive functioning has been subject to publication bias, which has therefore given a distorted view of the evidence.

History

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 raised a question into the depth of cognitive activity in bilinguals and whether or not there were benefits in multilingualism to be found.
According to the Singapore Management University School of Social Sciences, research before the 1960s on bilingual individuals was varied, but commonly supporting the idea that there were disadvantages to bilingualism. A 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. This information alluded to the idea that being bilingual was detrimental to a child's linguistic and cognitive development. According to a journal called "The Journal of Genetic Psychology" various reported studies at the time that held these perceptions held a similar view on bilingualism, and it was referred to as a "problem of bilingualism" or the "handicapping influence of bilingualism." Following studies reported that bilinguals performed worse in IQ tests and suffered in most aspects of language development. Journalist Rafael M. Diaz, from Yale University mentions that perspectives like these were in part believed to have been influenced by variables that had impacted or changed how a society functioned.
In 1977, the published an influential study which discussed bilingualism as it relates to education and what effects there are on a child's performance maintaining with the class. This study, along with other research of its time, played a large role in our understanding of multilingualism and the effects that it has on the brain. While historically relevant and necessary, in context of meeting today's modern methodological standards, these studies in particular are argued to be missing necessary pieces of data that create controversy over their credibility. With current research and data suggesting benefits to bilingualism, the soundness of the former studies conclusions are in brought into question. According to "The Journal of Genetic Psychology," the many of these studies employed unstandardized and subjective definitions of bilingualism and of a bilingual individual, raising the concern that there is no way of determining whether their samples were truly representative of a bilingual population. Another element that contrasted with more modern researching techniques was the lack of a control for socioeconomic status 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 found that the bilinguals appeared to have significant advantages to that of their monolingual peers outperforming in both verbal and non-verbal tests, more specifically in the non-verbal tests. In continuation of this study, and studies alike, the literature after this point began to undergo a shift of focus erring more particularly into areas of cognitive development and aptitude such as: perception and executive functioning.

Cognitive advantages

Executive function

is the domain of high-level cognitive processes that assists in goal-oriented tasks, such as problem solving, mental flexibility, attentional control, inhibitory control, and task switching. Much of the current research on cognitive effects of bilingualism investigate a correlation between bilingualism and executive function. A study titled "Bilingualism, aging, and cognitive control" amongst various similarly conducted studies released data suggesting that monolinguals and bilinguals were found having varying ability in the executive function part of the brain. From these tests, bilinguals showed a higher executive control than their monolingual peers. When matched in age and other background factors, indications of a possible correlation between the demands on the brain that learning multiple languages requires and higher executive functioning skills. Executive function may also have effects for older adults. Inhibitory processes of adults who learned a second language at a young age show better controlled processing than monolingual adults. Reported links to slowing age-related cognitive decline such as dementia have been found. Results of a bilingual advantage in executive functioning have been mixed, with many studies and meta-analyses coming to opposite conclusions.

Clarification: Possible connection

The modern day approach to researching multilingualism is suggesting that there are cognitive advantages to becoming bilingual. One of the more frequently tested aspects to language development has been in relation to a possible link between bilingualism and higher executive functioning skills. In many studies bilingual groups outperformed monolingual control groups in executive function tasks. These findings suggest bilingualism is correlated with better control of attention and facilitates processing and functioning in several cognitive tasks.
There are 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 aspect of control is linked to the bilingual effect on cognitive abilities.
In one study, researchers administered a non-linguistic card-sorting task to 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. 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, researchers aimed to determine what gave bilinguals an advantage in solving the card-sorting task. Groups were equivalent in their ability to represent the stimuli, and both were equally able to inhibit learned motor responses. Bilinguals performed better on the task to measure conceptual inhibition; the ability to inhibit previous associations and create new mental representations of the stimulus according to task changes.
Another study used three language groups: native bilinguals, English monolinguals, and English speakers enrolled in an immersion program. The bilingual children's scores were similar to the other groups despite lower parent socioeconomic and education level and lower verbal scores. When the two groups were adjusted for age, parent income and education, and verbal scores, the bilingual children outperformed monolinguals on conflict tasks that required resolving multiple attention demands.
Bogulski, Rakoczy, Goodman, and Bialystok investigated how "lapsed bilinguals" compared to monolinguals and fluent bilinguals in executive function tasks. The lapsed bilinguals tested better than monolinguals but worse than their fluent counterparts.
Some researchers demonstrate a contextual effect of sociocultural aspects of bilingualism. Others find these effects across various sociolinguistic settings such as comparison groups with bilingual children speaking a second regional and second migrant language or bilingual children of low-income immigrant families and monolingual children of low-income non-immigrant families.
Analyses
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 and bimodal bilinguals. 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. 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. 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, 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.
Bilingualism vs. executive control
Hakuta and Diaz, addressed the chicken and egg question concerning bilinguals and their reported enhanced cognitive abilities; do children with greater cognitive abilities tend to learn more than one language, or could knowing more than one language contribute to enhanced cognitive ability? They administered a set of non-verbal tests that are designed to measure cognitive ability to a bilingual sample of children. From these tests, the results showed a higher correlation with the degree of bilingualism of their sample and scores on the test, as well as bilingualism did in fact predict performance. 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.

Clarification: In opposition

The idea that bilingualism enhances executive function is not universally accepted among researchers. Mixed experimental results have led some researchers, such as Paap, to question the very existence of this advantage.
The methodology has been disputed. Virginia Valian finds correlations between bilingualism and executive function to be inconsistent. Executive function is not uniformly defined and 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 bilinguals cannot be assumed superior to monolinguals in executive function. She also notes 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 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 found results with no 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. Another notable study with a large sample size of 252 Basque-Spanish bilingual children and 252 monolingual Spanish children compared inhibitory skills. Researchers used two Stroop tasks in which the participants had to ignore distracting information to complete the task. They found no differences or bilingual effects between groups.
Paap and Greenberg assert bilinguals are not 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 the evidence would argue against this.
Claims of publication bias dispute validity of findings supporting bilingualism and cognitive benefits of executive control. For example, a study examining abstracts of research on bilingualism and executive control between 1999 and 2012 found the research results in support of an advantage were published more often and results that did not support the theory were published least often. Lack of difference or dispute of sample size, measure, and statistical power suggests not error but publication bias.

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 an individual's dominant language active when using the less dominant language, 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. Cross-language activation therefore seems less surprising. However, cross-language activation has also been reported in bilinguals whose two languages have different scripts and lexical forms. A study by Hoshino & Kroll 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 information whereas the Japanese cognates were only phonologically similar. 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 of the effect 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 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 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. 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 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.

Other advantages

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.
Improved theory of mind, which is the ability to understand someone else's frame of reference, has also been implicated in bilingualism. A meta-analysis in 2018 found that bilingual children performed better on a theory of mind test compared to monolingual children.

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, Bialystok et al. 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 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. tested 19 bilinguals and 19 monolinguals who had been matched for education level, performance on the Flanker Task 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. 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. 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. 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 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 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.

Language

Language Use

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 structure of words and their semantics 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.

Semantic development

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 there are linguistic benefits to being bilingual and that they are more than simply being able to speak two languages. A child learning two languages whose structures and rules are significantly different from each other requires the child to think in cognitively demanding ways. An example of this cognitive demand in action would be in the arbitrariness of labels for objects, or distinguishing between and using two different grammatical or syntactical structures. These areas are quite arduous for a child to learn, but have been shown to increase the understanding the structure of language and introduce a greater awareness of meaning. Bilinguals who have worked through these strenuous stages of development obtain what is referred to as metalinguistic awareness.

Reading ability

As an attempt to further analyze bilingualism as it relates to reading capability, there was another study done by Bialystok. This study was conducted by testing a group of children ages 4 to 5, equally familiar in secondary language as with their native tongues. The assessment was made using what Bialystok discussed as a representational principle; which refers to a symbolic representation of spoken language or the connection between the spoken and written language systems. For the testing of this principle, she gave children a "Moving Word Task" where a child would have to appropriately match a written word to an object on a card. If they could correctly matched the two after some rearranging of the cards, it suggested that they could understand the 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.

Vocabulary

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 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, 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 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 . 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 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.