The Relationship Between Degree of Bilingualism and Cognitive Ability: A Critical Discussion and Some New Longitudinal Data 320 HAKUTA AND DIAZ ! Kenji Hakuta Yale University ! I Rafael M.
Diaz University of N e w Mexlco II I ate bilingual saniples to a n extent such that the prototype of subsequent studies o n bilingualism became group comparisons of balanced bilinguals 10 monolingual counterparts matched o n appropriate variables. Second, the results served to allay cornnionly held fears concerning the products of bilingual education. amely, that it would produce retarded. poorly educated, anomie individuals without affiliation to either ethnolinguistic group and incapable of functioning in either language (Tucker & d’Anglejan, 1971). Bilingual education would not create, the study assured, a social or cognitive Frankenstein. In this chapter, we provide a brief review of research prior to Peal a n d Lambert’s study and more recent studies o n bilingualism and intelligence (for a n earlier review with a linguistic focus, see Lindholm, 1980; for a n expanded and detailed review of the first 6 decades of research, see Diaz, 1983).In the course o f t h e review.
we point out both theoretical and methodologica~ weaknesses inherent in the typical bilingual-monolingual comparison. In addition, we stress the paucity o f longitudinal investigations that allow for the assessment of statements concerning the cause-effect relations between bilingualism and cognitive abilities. Then, we report preliminary results from our own study. which attempts to correct for these weaknesses. We conclude with some theoretical speculations regarding the nature of the relationship between bilingualism and thought. . ! I I I ! .
. . I . I . . 1. i 1 !In 1962, Elizabeth Peal and Wallace Lambert of McGill University published a monograph entitled “The Relation of Bilingualism to Intelligence.
”The research, conducted in Montreal with 10-year-old children, compared the performance of monolinguals t o that of bilingual, French/English-speaking subjects on a variety of standard tests of intelligence. In contrast to previous research on bilingualism and intelligence, Peal and Lambert (1962) discovered that their bilingual sample showed superior performance o n measures of verbal intelligence and o n nonverbal tests “involving concept-formation o r symbolic flexibility Ip. 41. ” What differentiated the study from its ancestral relatives was the care with which Peal and Lambert exercised control over sample selection. They drew a distinction between true, “balanced bilinguals” who are proficient in both their first (11) and second language (L2) and “pseudo-bilinguals” who, for various reasons, have not yet attained ageappropriate abilities in their second language. According to Peal and Lambert(l962): “The pseudo-bilingual knows one language much better than the other, and does not use his second language in communication.T h e true (or balanced) bilingual masters both at a n early age and has facility with both as means of communication [p.
61. ” Into their sample of bilinguals, only those considered “balanced” were admitted. Peal and Lambert’s study had substantial impact on two fronts. First, it raised the consciousness of researchers on the problem of selecting appropri- ! THE FIRST 4 DECADES OF RESEARCH Psychological studies of the relation between bilingualism and cognitive abilities began in the early 1920s out of concern raised by the flourishing of psychometric tests o f intelligence.The concern was that bilingual children would suffer from some linguistic disadvantages, which could, in turn, prevent [air assessment o f their intellectual abilities and potential. The fact that the measurement of intelligence is heavily dependent on verbal abilities made psycholog,ists and educators deeply concerned (and rightfully so! ) about the validity o f such tests for bilingual children. As expected, the majority of studies prior to Peal and Lambert’s (1962) study found that bilinguals were linguistically deficient in comparison to their monolingual counterparts.
Among other things, bilinguals were shown t o have deficient articulation (Carrow, 1957). lower standards in written composition, more grammatical errors (Harris, 1948; Saer, 1924). and a considerably reduced vocabulary (Barke & Williams, 1938; Grabo, 1931; Saer, 1924). Theconsistcnt finding was that bilinguals suffered from a so-called “language handicap” (see review5 b i Arsenian. 1937; Darcy, 1953, 1963; Macnamara, 1966). Unfortunately, consistent findings about bi1inguals’“lanpuagc handicap” led too quickly to statcincnts regarding the negative elfccts of bilingualism ! , I . .
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… 10. BILINGUALISM AND COGNITIVE ABILITY 321 322 HAKUTA AND OlAZ . rather than to a serious questioning of the validity of psychometric testsof intelligence for this population. Negative, and rather harsh, statements condemning bilingualism as a social plague (Epstein, 1905) or as “a hardship devoid of apparent advantage” (Yoshioka, 1929) were common in the early literature.
In short, the measured language handicap of bilinguals was interpreted as a linguistic confusion that deeply affected children’s intellectual development and academic performance up to the college years (Saer, 1940). The majority of early studies in the area, however, suffered from a wide range of methodological problems, and as a result, most current investigatorsin the field regard the findings of early studies as totally unreliable (see Cummins, 1976). A good number of early studies, for example, failed to control for group differences in socioeconomic status between bilingual and monolingual samples.As early as 1930, McCarthy pointed out that bilingualism in the US was seriously confounded with low socioeconomic status. She found that more than half of the bilingual school children could be classified as belonging t o families from the unskilled labor occupational group. Along the same lines, Fukuda (1925) alerted researchers to the fact that high-scoring English-speaking subjects were mostly in the occupational and executive classes; he reported a correlation of 3 3 between the Whiltier (socioeconomic) Scale and the Binet IQ in his sample.Nevertheless, prior to the early 1960s.
most studies investigating the effects of bilingualism on children’s intelligence did not take into account group differences in socioeconomic status. A second major methodological flaw of early studies is that it was often questionable whether the “bilingual” subjects were in fact fluent in both languages. Brunner (l929), for example, assumed that children’s bilingualism would be estimated by the foreignness of their parents.Brunner divided his bilingual sample into three categories: (1) both parents born in this country; (2) one parent born here and the other abroad; and (3) both parents born abroad. The classification was simply assumed to represent various degrees of children’s bilingualism. In other studies, the sample’s bilingualism was assessed through family names or even place of resldence (see Darcy, 1953, for a review). As present investigators have repeatedly stated, it is impossible to ascertain whether the bilingual subjects of many studies were indeed bilingual or just monolingual of a minority language.
Toward the end of the 1950s, research o n the effects of bilingualism showed the following consistent findings. Monolinguals performed significantly higher than bilinguals on measures of verbal intelligence. On measures of nonverbal ability, some studies showed that monolinguals were also at an advantage, but group differences on this variable were not consistent across studies. However, because of the serious methodological flaws just mentioned, none of the results, even if they were statistically significant in either direction, can be easily interpreted.Peal and Lambert’s (1962) study, de- scribed briefly in the introduction, took steps to assure the selection O f true, balanced bilinguals. In addition, it recruited wisdom from previous studies in controlling for socioeconomic level. ! PEAL AND LAMBERT (1962): THE PUNCTUATION POINT IN RESEARCH – I Both bilingual and monolingual samples for the Peal and Lambert study were selected from the same school system in Montreal.
All I0-year-old children in the system were included in the initial screening by four measures, the composite of which was used to determine whether the child should be considered monolingual or balanced bilingual. The measures were: ( I ) the relative frequency of words provided in a word association task in LI and L2; (2) the relative frequency of words in LI and L2 detected in a series of letters; (3) the frequency of words recognized in L2 (English) from a subset chosen from the Peabody Picture Vocabulary Test; and (4) subjective self-ratings& ability in speaking, understanding, reading, and writing in L2.Children who fell in the extremes of these scales were determined to be monolingual or balanced bilingual. The final sample consisted of 75 monolinguals and 89 bilinguals; all children were administered a modified version of the LavoieLarendeau Group Test of General Intelligence (Lavoie & Laurendeau, 1960). lhe Raven’s Coloured Progressive Matrices (Raven, 1956; Raven, Court, & Raven. 1976). and a French version of selected subtests of the Thurstone Primary Mental Abilities Test (Thurstone & Thurstone, 1954).
Contrary to the findings of earlier studies, the results of the Peal and Lamberi study showed that bilinguals performed significantly better than monolinguals o n most of the cognitive tests and subtests, even when group differences in sex, age. and socioeconomic status were appropriately controlled. Bilingual children performed significantly higher than monolinguals on tests of both verbal and nonverbal abilities; the bilinguals’ superiority in nonverbal tests was more clearly evident in those subtests that required mental manipulation and reorganization of visual stimuli, rather than mere perceptual abilities.A factor analysis of test scores indicated that bilinguals were superior to monolinguals in concept formation and in tasks that required a certain mental or symbolic flexibility. Overall, bilinguals were found to have a more diversified pattern of abilities than their monolingual peers. . Peal and Lambert’s (1962) findings must be considered with a certain degree of caution, however.
First, as Macnamara (1966) has pointed out, the process of subject selection might have introduced a bias in favor of the bilingual sample.Peal and Lambert’s bilingual sample included only children who scored above a certain determined level in the English Peabody Picture Vocabulary Test, a test commonly used to measure intelligence in monolinguals. I 1 I I I ! , ! i 1 10. BILINGUALISM AND COGNITIVE ABILITY 323 324 HAKUTA AND OlAZ It is possible that the intelligence of French-Canadian children might be rcflected in a measure of English (the second language) vocabulary.Second, the bilingual sample, on the average, belonged to a higher grade than the monolingual sample; perhaps the superiority observed in bilinguals was the result of their having longer exposure to formal education. And third, the frequency distribution of the Raven’s test scores was very different for the two groups of children; it, was negatively skewed for bilinguals, whereas the opposite was true for monolinguals. In short, the cognitive advantages observed in Peal and Lambert’s balanced bilingual sample could have been inflated by several artifacts in their subject selection procedures.
As Peal and Lambert (1962) admitted: A partial explanation of this [the results] may lie in our method of choosing the bilingual sample. Those suffering from a language handicap may unintentionally have been eliminated. We attempted to select bilinguals who were balanced, that is. equally fluent in both languages. However, when the balance measures did not give a clear indication of whether or not a given child was bilingual, more weight was attached to his score on the English vocabulary tests. Thus some bilinguals who might be balanced, but whose vocabulary in English and French might be small, would be omitted from our sample.The less intelligent bilinguals, those who have not acquired as large an English vocabulary, would not be considered bilingual enough for our study [p.
IS]. Nevertheless. Peal and Lambert’s (1962) empirical distinction between bilinguals and pseudobilinguals made a significant (and much needed) methodological contribution to the field. Their distinction has forced recent investigators to select their bilingual sample with greater care and measure the sample’s actual knowledge of the two languages. RECENT STUDIESSince Peal and Lambert (1962). a variety of studies have been reported in o which monolingual children are compared t balanced bilingual children. In most of these studies, balanced bilinguals have shown advantages in several cognitive abilities, such as concept formation (Liedtke & Nelson, 1968) and melalinguistic awareness (Cummins, 1978).
In addition, many studies suggest that balanced bilinguals demonstrate a greater flexibility than monolinguals in their performance on different cognitive tasks (Balkan, 1970).Above all, recent research not only has replicated Peal and Lambert’s positive findings regarding balanced bilingualism, but also has given empirical support for linguists’ statements regarding the cognitive and linguistic advantages of raising a child bilingually. In contrast to early psychological studies of childhood bilingualism, individual case studies by linguists (Leopold, 1939, 1947, 1949a, 1949b; Ronjat, 1913) had concluded that early bilingualism was advantageous to children’s cognitive and linguistic development.In particular, Leopold (l96l), based on observat ions of his bilingually raised daughter, suggested that bilingualism promoted an early separation of the word sound from the word meaning: “a noticeable looseness of the link between the phonetic word and its meaning (p. 3581. ” Furthermore, Leopold postulated a fascinating connection between the semantic and cognitive development of bilingual children, namely that the separation of sound and meaning leads to an early awareness of the conveniionality of words and the arbitrariness of language.This awareness could promote, in turn, more abstract levels of thinking.
Vygotsky (1935/1975) saw the cognitive advantages of bilingualism along the same lines; in his own words (as cited in Cummins, 1976). bilingualism frees the mind “from the prison of concrete language and phenoniena [p. 341. ” Leopold’s observations have been tested empiricaly by lanco-Worrall (1972) in a remarkable well-designed and controlled study of English. Afrikaans bilingual children in South Africa. The bilingual sample consisted of nursery school children who had been raised in a one person-one language environment. imilar to the situation of Leopold’s daughter Hildegard.
The sample’s degree of bilingualism was determined by several measures, including detailed interviews with parents and teachers as well as a direct test of the children’s vocabulary in both languages. Two comparable monolingual samples, one English and one Afrikaans. were included in the study. in a first experiment, children were administered a semantic-phonetic preferences test. The test consisted of eight sets of three words each. A typical set contained the words cup, con, and har.Children were asked questions such as: “Which word is more like cup, can or hal? ”Choosing the word can or /tal indicated the child’s phonetic preference or semantic preference, respectively, in analyzing word similarities.
The capacity to compare words on the basis of a semantic dimension is regarded as more advanced developmentally than comparing words along a phonetic dimension. The results of this experiment showed not only that semantic preferences increased with age, but also that bilinguals outranked monolinguals in choosing words along a semantic rather than a phonetic dimension.As lancoWorrall(l972) reported: “of the young 4-6 year old bilinguals, 54% consistently chose to interpret similarity $between words in terms of the semantic dimension. Of the unilingual groups of the same age, not one Afrikaans speaker and only one English speaker showed similar choice behavior (p. l3YSl. ” lanco-Worrall concluded that bilingual children raised in a one person-one language environment reach a stage of senlatitic development 2-3 years earlier than monolingual children. .
. ’ ’ . 10. BILINGUALISM AND COGNITIVE ABILITY 325 326 HAKUTA AND DlAZ &In a second experiment, using Vygotsky’s (1962) interviewing techniques, lanco-Worrall (1972) asked her subjects to explain the names of different things (e. g. , why is a dog called “dog”? ). She also asked children whether or not the names of things could be arbitrarily interchanged.
Children’s responses to the first question were assigned to different categories such as perceptible attributes, functional attributes, social convention, and so forth. The results of this second experiment showed no reliable differences between bilingual and monolingual children in the types of explanations offered.For the second question, however, the differences favored the bilingual children; bilinguals replied that names of objects could in principle be changed, whereas the opposite was true for monolingual children. Lindholm (1980) reports a study by Sandoval (1976) in which a replication of lanco-Worrall’s results was attempted with a Spanish/Enlish. sample of kindergartners and third graders. Although there was a trend favoring the bilinguals, it did not reach statistical significance. However, it is not clear whether the bilingual subjects were truly balanced or simply enrolled in bilingual schools.
More recently, Bain and Yu (1980) investigated the cognitive consequences of raising children according to Ronjat’s (1913) one person-one language principle. Bain and Yu studied German-French. English-French, and Chinese-English bilinguals with monolinguals from the respective languages in three different geographical locations. The dependent cognitive variables concerned children’s developing awareness of the functions of language as postulated by Luria and examined in tasks such as the ability to follow a set of increasingly complex directions in hide-and-seek situations.In particular, Bain and Yu hypothesized that bilingualism would promote the use of language as a self-directive tool in cognitive tasks. Using a modified version of Luria’s (1961) experimental procedures, the results showed that, at about age 4, children raised bilingually in one person-one language environment were better able to use both overt and covert language as a guide and control in their cognitive functioning. The data also favored younger bilingual children, but this trend failed to reach statistical significance.
More importantly, the findings were consistent across different language groups and geographical situations.Several investigators have explored the effects of bilingualism on the development of metalinguistic awareness. Metalinguistic awareness refers to the ability to analyze linguistic output objectively; that is, according to Cummins (1978). “to look of language rather than lhrough it to the intended meaning [p. 1271. ’’ Indeed, as children develop, they become more capable of looking at language as an objective set of rules, as an objective tool for communication. Because bilingualism induces an early separation of word and referent, it is possible that bilingual children also develop an early capacity to focus on ‘ , and analyze the structural properties of language.
Vygotsky( 1935/1962)suggested that, because bilinguals could express the same thought in different. languages, a bilingual child would tend to“see his language as one particular system among many, to view its phenomena undet more general categories, and this leads to an awareness of his linguistic operations [ 1962, p. I IO]. ” More recently, Ben-Zeev (I 977b) hypothesized that bilinguals dtvelop an analytic strategy toward language in order to fight interference between their t w o languages.Lambert and Tucker (1972) noted that children in the St. Lambert bilingual experiment engaged in some sort of “contrastive linguistics” by comparing similarities and differences between their two languages. Cunmins (1978) investigated the metalinguistic development of third- and sixth-grade Irish-English bilinguals.
Children in the sample came from homes where both Irish and English were spoken; all children received formal school instruction in Irish. An appropriate monolingual comparison group was selected that was equivalent to the bilingual‘group on measures of IQ and socioeconomic status.A first task investigated children’s awareness of the arbitrariness of language. Similar to the measure used by lancoWorrall(1972). children were asked whether names of objects could be interchanged; they were then asked to explain or justify their responses. The results indicated that, at both third- and sixth-grade levels, bilinguals showed a greater awareness of the arbitrary ‘nature of linguistic reference. In a second task, children were presented with several contradictory and tautological sentences about some poker chips that were either visible or hidden.
The sentences varied in two additional dimensions: true versus false and empirical versus nonempirical. According to Cummins (1978), nonempirical statements refer to sentences that “are true or false by virtue of their linguistic form rather than deriving their truth value from any extralinguistic state of affairs [p. 129j. ” The task was chosen as a measure of metalinguistic awareness because previous researchers had shown that, in order to evaluate contradictions and tautologies correctly, it is necessary to examine language in,an objective manner.Although the performance on this evaluation task was not clear-cut in favor of the bilinguals, sixth-grade bilingual children showed a marked superiority in correctly evaluating hidden nonempirical sentences. But Cummins (1978) notes that the monolinguals “analyzed linguistic input less closely, being more content to give the obvious‘can’t tell’ response to the hidden nonempirical items [p. 1331.
” In a second experiment with balanced Ukranian-English bilinguals, Cumniins (1978) investigated children’s metalinguistic awareness usiiig a wide variety of measures, including analysis of ambiguous sentences and a class inclusion task.Contrary to previous findings, the bilinguals in this study did not show advantages on the Seniantic-Phonetic Preference Test or on an arbitrariness of language rask. However, Cummins (1978) reports that “the results of the Class Inclusion and Ambiguities tasks are consistent with previ- iI – I I I !! ! .. . .i.
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, , , . io. BILINGUALISM AND COGNITIVE ABILITY 327 328 HAKUTA AND DlAZ ous findings in that they suggest that bilingualism promotes an analytic orientation t o linguistic input [p. 1351. In the studies that have been described, bilinguals exceeded monolinguals in a wide range of linguistic and melalinguistic abilities. Bilinguals showed a greater capacity to analyze the similarity of words along semantic rather than acoustic dimensions, a greater use of language as a self-directing tool in cognitive tasks, and a greater overall awareness of the conventional nature of words and language. Also, in a different study (Feldman & Shen, 1971).
bilingual 5-year-olds were better than their monolingual peers at relabeling objects and expressing relations between objects in simple sentences.This awareness or flexibility with respect to the use of language seems to play an important role in the cognitive functioning of bilingual children. In fact, several investigators have claimed that bilinguals are more “cognitively flexible” than monolinguals when performing on both verbal and nonverbal tasks. Although the notion of cognitive flexibility has never been adequately explained (see Cummins, 1976)’ it is possible that bilinguals’ unusual use and awareness of language contributes to a greater flexibility in the manipulation of both verbal and nonverbal symbols.One of the most frequently cited studies of bilinguals’ cognitive flexibility was conducted in Switzerland by Balkan (1970). Balkan administered several tests of nonverbal abilities that purportedly measured cognitive flexibility. The bilingual group, as expected, performed significantly higher than the control monolingual group in two of these measures.
One task, Figures Cachees, involved the ability to reorganize a perceptual situation, similar to the familiar Embedded Figures Test. The other task, Histoires, involved sensitivity to the different meanings of a word.Interestingly enough, the positive effects of bilingualsm on these measures were much stronger for children who had become bilingual before the age of 4. The differences between monolinguals and children who had become bilingual at a later age were in favor of the latter but did not reach statistical significance. ’ Balkan’s study implies, as earlier individual case studies by linguists had suggested, that bilingualism might have the most beneficial cognitive effects for those children who learn their two languages simultaneously. Because balanced bilinguals have two different words for most referents, it s not surprising that they show a greater sensitivity than monolinguals to the possible different meanings of a single word, as shown in the Histoires task. On the other hand, Balkan’s study offers no clue as to how or why bilingualism should contribute to a greater ability to reorganize and reconstruct perceptual arrays, as shown in the Figures Cachees task.
As recent research suggests, the clue might be in bilinguals’greater awareness and flexibility with respect to the use of language, as well as in their greater use of both overt and covert speech in the monitoring of their cognitive functioning (Bain & Yu. 980). Further evidence of bilinguals’ so-called cognitive flexibility has been offered by Ben-Zeev’s (1977) study with Hebrew-English bilingual children. When compared to monolinguals, the bilingual children in the study showed a marked superiority in symbol substitution and verbal transformation tasks. The symbol substitution task involved children’s ability to substitute words in a sentence according to the experimenter’s instructions. In a typical instance, children were asked to replace the word I with the word spughetli.Children were given correct scores when they were able to say sentences like“Spaghetti om cold,” rather than “Spaghetti iscold” or a similar sentence that, although grammatically correct, violated the rules of the game.
The bilinguals’ higher performance on the verbal transformation task involved better detection of changes in a spoken stimulus that is repeated continuously by means of a tape loop. Warren and Warren (1966) have reported that when a spoken stiinulus is presented in such a way, subjects older than 6 years report hearing frequent changes in what the taped voice says.The authors attributed this illusion to the development of a reorganization mechanism that aids in the perception of ongoing speech. The bilingual children in Ben-Zeev’s study also outperformed the monolingual group on certain aspects of a matrix transposition task. Bilinguals were better at isolating and specifying the underlying dimensions of the matrix, but no group differences were found on the rearrangement of figures in the matrix. The two comparison groups also performed similarly on Raven’s progressive matrices.I t should be noted that the bilinguals in Ben-Zeev’s study showed cognitive advantages only on measures that were directly related to linguistic ability and on the verbal aspects of the matrix transformation task.
Ben-Zeev( 1977) noted that throughout the study bilingual children seemed to approach the cognitive tasks in a truly analytic way. They also seemed more attentive to both the structure and details of the tasks administered, as well as more sensitive to feedback from the tasks and the expcrimenter.BenZeev explained these improved abilities in terms of bilinguals’ confrontation with their t w o languages. She argued that, in order to avoid linguistic interference, bilinguals must develop a keen awareness of the structural similarities and differences between their two languages as well as a special sensitivity to linguistic feedback from the environment. Supposedly, this more developed analytic strategy toward linguistic structures is transferred to other structures and patterns associated with different cognitive tasks.Ben-Zeev (1977) summarized her results as follows: Two strategies characterized the thinking patterns of the bilinguals in relation to verbal material: readiness to impute structure and readiness to reorganize. I j i I I The patterns they seek are primarily linguistic, but this process also operates with visual patterns, as in their aptness at isolating the diniensions of a matrix.
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,. . 10.BILINGUALISM AND COGNITIVE ABILITY 329 . 330 HAKUTA AND DlAZ With visual material the spatial reorganizational skill did not appear, however [p. 1017). In conclusion, the nature o r meaning of cognitive flexibility is far from being fully understood; however, the studies just reviewed suggest that the flexibility noted in bilinguals could stem from language-related abilities, such as the use of language in monitoring cognitive functioning or an early awareness of the conventionality and structural properties of language.
OBSERVATIONS ON THE STATE OF THE ART The recent studies suggest that if one compares bilinguals who are approximately equivalent in their abilities in L l and L2 with a monolingual group matched for age, socioeconomic level, and other relevant variables and administers a measure of cognitive flexibility to both groups, the bilinguals will do better. Now, consider an ideal (from the viewpoint of good experimental science) research design. You would begin by taking a random sample of individuals and assigning them randomly to either an experimental group or control group, thereby controlling for any background error in sampling.The experimental group is placed in an environment that fosiers bilingualism, whereas the control group remains in a monolingual environment. Once the treatment has had time to take effect (Le. , once the subjects in the condition have become balanced bilinguals), you administer your dependent measure, making sure that the person who administers the dependent measure is blind to whether the subject being tested is in the treatment or control condition. And, lo and behold, you find a difference in favor of bilinguals.
Under this ideal situation, one can reasonably conclude that bilingualism causes cognitive flexibility, o r whatever cognitive advantage this flexibility stands for. You could also go on to speculate about why this result came about and construe various other experimental conditions to test your hypotheses. In what ways is the ideal research design unlike the circumstances under which current studies of bilingualism are conducted? We would point out at least two. First, in the real world, there is n o such thing as random assignment to a bilingual o r monolingual group.Most often, bilingualism or monolingualism is determined by sociolinguistic facts that are, as would be true of most sociolinguistic facts, related to a wide range of social variables. What this really means is that there will be a large number of variables that differentiate the bilingual from the monolingual other than simply that the bilingual speaks two languages and the monolingual one. It is possible.
of course, to match the two groups with respect to some variables (e. g. , ethnicity) or to control them statistically (e. g. , by partialing out the effect
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