The Research Trends of Multilingualism in Applied Linguistics and Education (2000–2019): A Bibliometric Analysis

: This study explored the state of the arts of bilingualism or multilingualism research in the past two decades. In particular, it employed a bibliometric method to examine the publication trend, the main publication venues, the most inﬂuential articles, and the important themes in the area of bilingualism or multilingualism. The main ﬁndings are summarised as follows. First, a signiﬁcant increase of publications in the area was found in the past two decades. Second, the main publication venues and the most inﬂuential articles were reported. The results seemingly indicated that the research in the area focused largely on two broad categories, that is, (1) bilingualism or multilingualism from the perspective of psycholinguistics and cognition research and (2) how second / additional languages are learned and taught. Last, the important themes, including the hot and cold themes, were identiﬁed. Results showed that researchers prefer to study bilingualism or multilingualism more from deeper cognition levels such as metalinguistic awareness , phonological awareness , and executive control . Also, they may become more interested in the issue from multilingual perspectives rather than from the traditional bilingual view. In addition, the theme emergent bilinguals , a term closely related to translanguaging , has recently gained its popularity, which seemingly indicates a recent advocate for heteroglossic language ideologies .


Introduction
Bilingualism or multilingualism refers to the use of more than one language by individuals or a group of people. Researchers have a different understanding and categorisation of bilinguals/bilingualism and multilinguals/multilingualism (see [1] for a detailed discussion). In the present study, we do not distinguish bilinguals/bilingualism and multilinguals/multilingualism and use the terms interchangeably. In most cases, we use "bilingualism or multilingualism" to refer to the use of more than one language, which includes both bilingualism (the use of two languages) and multilingualism (the use of three or more languages). It was estimated that more than 60% of children grow up in a bilingual environment and have gradually gained competence in both languages [2]. In addition, approximately more than one third of the world's population regularly speak two or more languages and even more people occasionally use a language other than their mother tongue [1]. Due to the pervasive presence of multilingualism and its close relation to the cognitive development of the human being, the issue has long attracted attention from a wide range of areas in academia, such as applied linguistics, education, and psychology, or the overlapping areas of the preceding areas such as psycholinguistics and educational psychology.
Studies of multilingualism in the areas of applied linguistics and education is broadly categorized into several lines of research such as multilingualism and cognition, multilingualism and second/additional language acquisition, and multilingualism and language policy. First, researchers are interested in the relationship between multilingualism and cognition. In the first half of the 20th century, bilingualism or multilingualism were considered as a cognitive disadvantage [3], since bilingual or multilingual children might score lower on verbal tests of cognitive capabilities [4]. Based on such findings, bilingualism or multilingualism was considered as "the problem of the bilingual child" [5] and "one of the chief factors in producing mental retardation" [6]. However, recent studies prefer to view bilingualism or multilingualism as an advantage since it has been found that bilingual or multilingual people may come out with cognitive advantages [7] such as increased attentional control [8], metalinguistic awareness [9,10], working memory [11,12], and problem solving capabilities [13]. Of course, researchers may not always agree with the point of bilingual advantages and studies have found no significant difference between bilinguals and monolinguals such as [14], [15], and [16] in terms of the previous mentioned points such as cognitive advantages and working memory (see [17] for a recent review on this topic).
Last, researchers have examined bilingualism or multilingualism from the perspective of language policy. For example, numerous studies have explored the bilingual or multilingual policy in various countries and regions such as France [34], Cameroon [35], and the Arabian Peninsula [36]. Other topics in this line of research include bilingualism or multilingualism policy and politics [37,38], bilingual language policy in classrooms [39,40], bilingual language policy in universities [41,42], and bilingual language policy in family [43][44][45].
Since a large number of studies in the area have been performed, it would be of interest to examine the research trends in bilingualism or multilingualism. Hence, the present study aims to explore the landscape or the state of the arts of bilingualism or multilingualism research in the past two decades with a bibliometric method. In particular, the following research questions are to be addressed in the present study.

1.
What is the publication trend in the area of bilingualism or multilingualism? 2.
What are the main publication venues in the area of bilingualism or multilingualism? 3.
What are the most influential articles in the area of bilingualism or multilingualism? 4.
What are the important research themes, including hot and cold themes, in the area of bilingualism or multilingualism?

Methods
The bibliometric approach [46][47][48] was used in the present study and the methods for data retrieval and processing are described as follows. The bibliometric approach, also known as informetrics [49] or scientometrics [50], refers to the quantitative analysis of academic literature based on relevant bibliometric information such as the authors, the publication venues, and citation counts [51]. First, the database of Web of Science Core Collection (indexed in SSCI and A&HCI) was retrieved on January 20, 2020. The database was used for the reason that they included high-impact journal publications as well as their corresponding bibliometric information such as the authors, the journals, the abstracts, and the article citation counts that were to be used for the bibliometric analyses in this study. The queries presented in Table 1 were used to extract the bibliometric information that was necessary for the follow-up analyses. To be specific, the purpose of the retrieval was to search the articles of bilingualism or multilingualism in the areas of linguistics and education that were published in English from 2000 and 2019. We set 2000 as the starting year of the queries since the database in our university library starts at 2000. In addition, we retrieved articles in the areas of linguistics and education since (1) most articles pertinent to bilingualism or multilingualism were published in journals in the two areas, and (2) such an option not only matches our research expertise but also fits well the theme of the special issue. Furthermore, the combined query "(TS = bilingual* OR TS = multilingual*)" was used to retrieve all publications in the database that contained key words such as "bilingual*" or "multilingual*" (i.e., "bilingual", "bilinguals", "bilingualism", "multilingual", "multilinguals", and "multilingualism") in the titles or abstracts. At this step, a total of 7216 entries were found by the queries. Second, the detailed bibliometric information of the entries was then downloaded. The bibliometric information of the entries included article titles, authors, journal titles, publishing years, abstracts, citation counts of each article, etc. The entries without information such as the publishing year, the journal title, or an abstract were excluded, and the remaining 6909 entries were employed for the follow-up analyses.
Third, the number of articles published in each year of the examined span was calculated for the analysis of the publication trend. Also, the information of journal titles was used and counted for the analysis of the main publication venues.
Fourth, the information of citation counts of each article was used for the analysis of the most influential articles. The number of citations that an article receives is closely related with the number of years it has published. That is, the longer an article has been published, the more citations it may have received. Hence, the citation counts of each article were normalised with the following formula based on the common practice of previous bibliometric research such as [48].
Last, the important themes as well as the hot and cold themes in bilingualism or multilingualism were extracted based on the methods used in [52]. That is, all abstracts were first parsed for their sentence-level syntactic dependency relations with spaCy with homemade Python scripts. Then, the noun phrases were extracted out of the dependency parsing results. In the present study, we, following [53], defined a topic or a theme as a lexical noun phrase (the structure criterion) that occurs frequently across a wide range of texts (the repetition criterion). It should be noted that the specification of the thresholds for frequency and range of a theme is contingent. That is, it depends on many factors such as the data size used in the study and the research purpose. Also, the structure and repetition criteria may not guarantee that a frequent lexical noun phrase is a topic or a theme, and a manual check by the researchers or professionals (in case the researchers are not in the examined area) is needed. Next, the important themes were filtered in out of the candidate noun phrases with their frequency and range. Several rounds of experiments were performed in order to find the optimal criteria of frequency and range. The final criteria were set at 30 for both frequency and range, that is, a noun phrase may be considered as an important research theme in bilingualism or multilingualism if it occurs at least 30 times in at least 30 abstracts. The two researchers first individually judged if the candidate noun phrases were themes in bilingualism or multilingualism. They then discussed together and agreed that all the extracted candidates noun phrases were themes in the area. Finally, the hot and cold themes in the research themes were identified across the examined two decades with the normalised frequency of each research theme. The normalised frequency of each research theme was calculated with the following formula and the hot and cold themes were identified with a first-order autoregressive model. The first-order autoregressive model is a linear regression model that examines the trend of a group of values on a time series. It has been widely used to detect the research trend of themes such as in [52,54]. In the present study, the hot themes are those that have been detected a significant increase in use in the examined span by the autoregressive model, while the cold themes are those that have significantly decreased in use in the examined period. The first-order autoregressive model was fit with the packages forecast and lmtest in the R language [55].
Normalised frequency of each year = Raw f requenncy Number o f abstracts in each year (2)

Results and Discussion
In this section, major findings of the present study are reported.

Main Publication Venues
The top 20 journals in terms of the number of publications are presented in Table 3. The journals are the main publication venues of articles in bilingualism or multilingualism. It is obvious that approximately a half of the journals (nine journals) are in the areas of psycholinguistics, neurolinguistics, and cognition studies (i.e.  Journal of Neurolinguistics 88

Main Publication Venues
The top 20 journals in terms of the number of publications are presented in Table 3.
The two most recent articles of the top 20 highly cited ones are [30,67], both of which discuss the issue of translanguaging. Translanguaging has recently attracted much attention in the area of bilingualism or multilingualism. It was originally used to refer to the pedagogical practice of using more than one language (such as the target second language and the learner's mother tongue) in classroom teaching [73]. Such an understanding of the term has been widely discussed and advocated in works such as [30,74,75]. Li [67], however, extends translanguaging to a practical theory of language which takes language as a resource for people to think and to communicate. Hence, translanguaging, as [67] argues, goes much beyond the pedagogical practice and may serve as a transdisciplinary theory across the overlapping research areas such as linguistics, education, and psychology.

Important Research Themes of Bilingualism or Multilingualism
A total of 82 research themes met the threshold of frequency and range and were hence detected and extracted. The important research themes in bilingualism or multilingualism are presented in Table 5. We fit first-order autoregression models to detect the hot and cold themes in bilingualism and or multilingualism. A total of 12 hot themes, those that had gained significantly more attention in the examined period, were detected. Meanwhile, two cold themes, those that had received significantly less attention, were identified. See Table 6 for the details. Vocabulary knowledge has received significantly less attention, which suggests that this topic/theme has not garnered extensive attention in bilingualism or multilingualism research. Rather, they prefer to examine the issue more from deeper cognition levels such as metalinguistic awareness, phonological awareness, and executive control.
In addition, it is of interest to find that the theme of bilingual students has also lost its popularity in the past two decades. In contrast, researchers may become more interested in the issue from multilingual and more diverse perspectives, which is evidenced by the occurrence of such hot themes as multiple languages, multilingual students, multilingual children, and linguistic diversity. Researchers may also have attempted to address the concerns from the perspective of special groups of learners (deaf children) and socioeconomic views (socioeconomic status).
Last, the theme emergent bilinguals, which is closely related to translanguaging as discussed in the previous section, has recently gained its popularity in academia. Following [76], researchers such as [77][78][79] emergent bilinguals to refer to immigrant children who may learn the second/additional language at school while they still speak their mother tongue at home. Such a term, different from previously used popular terms such as bilingual students/learners, shows a recent advocate for heteroglossic language ideologies from monoglossic language ideologies [79].

Conclusions
In this study, we examined the research trends of bilingualism or multilingualism in the past two decades. The main findings are summarised as follows. First, a significant increase of publications in the area was found in the past two decades. Second, the main publication venues and the most influential articles were reported. The results seemingly indicated that the research in the area focused largely on two broad categories, specifically, (1) bilingualism or multilingualism from the perspective of psycholinguistics and cognition research and (2) how second/additional languages are learned and taught. Last, the important themes, including the hot and cold themes were identified. Researchers prefer to examine bilingualism and or multilingualism more from deeper cognition levels such as metalinguistic awareness, phonological awareness, and executive control. Also, they may become more interested in the issue from multilingual perspectives rather than the traditional bilingual view. The theme emergent bilinguals, a term closely related to translanguaging, has recently gained its popularity in academia, which seemingly indicates a recent advocate for heteroglossic language ideologies [79]. While the issue of bilingualism or multilingualism has attracted more attention in academia as the findings of the present study suggest, it may not only serve as a language issue or a concept in academia. In fact, it is closely related to social factors such as politics, culture, and economy [1]. For example, hot themes identified in the study such as emergent bilinguals and translanguaging are pertinent to the social or cultural identities of language learners. Hence, the issue of bilingualism or multilingualism may also play a role in the sustainable development, particularly when bilingualism or multilingualism is pervasive in the world [2].
Although the study has obtained findings that may illuminate the research area, it is limited in the type and discipline of the data. Only journal articles in linguistics and education science were used for the analyses. Future research may extend the study to other data types such as theses, dissertations, edited books, and monographs in linguistics and education, as well as other disciplines such as psychology and sociology in order to map a fuller picture of the state of the arts and research trends in the area of bilingualism or multilingualism.

Acknowledgments:
The authors would like to thank the editor and the anonymous reviewers for their insightful comments and suggestions.

Conflicts of Interest:
The authors declare no conflict of interest.