Language learning is known as a complex process that requires different cognitive and linguistic skills and competencies to successfully occur (Larsen-Freeman and Cameron, 2008). When it is combined with psychology, emotions, and communication skills, this complexity is multiplied in human beings. Many of the underlying factors of language learning and learner's affect are in close relationships with other variables which are constantly interacting with each other (Xie and Derakhshan, 2021). The story gets more complicated when most of the factors and constructs involved in the language learning process develop through time and vary across contexts (MacIntyre et al., 2020). As a result, researching language learning, in general, and constructs related to learner-teacher psychology, in particular, are a demanding task that is impossible via traditional and simplistic approaches. To capture the dynamism of many variables in second language acquisition (SLA) and language learning psychology (LLP),scholars are recommended to use Complexity Dynamic System Theory (CDST) which grew out of Larsen-Freeman's (1997) seminal work.

The propositions of CDST have been widely applied in different fields such as humanities, social sciences, physics, natural sciences, psychology, and communication (Fogel, 2006; Wang et al., 2021). In contrast, SLA and applied linguistics have mainly used CDST as a theoretical lens instead of an empirical research basis (MacIntyre et al., 2017). As pinpointed by Larsen-Freeman (2017), the reason behind this gap is that in these two fields, running empirical studies on CDST has long been perceived as meta-theoretical in essence. That is to say, CDST is not a language learning theory, but instead an approach to develop a theory. Nevertheless, with the groundbreaking works of a number of SLA researchers (e.g., De Bot et al., 2007; Dörnyei et al., 2014; Larsen-Freeman, 2017; Hiver et al., 2021, among others), the applicability of this meta-theory in the field of SLA has been scientifically substantiated. Language education, owing to its inherent dynamism, variation, and stability, is a fertile ground to spread the seeds of CDST on. It goes well with CDST in that language learning and development is not solely a matter of language competence occurring in a vacuum, yet the offshoot of the interaction of numerous intervening factors which are continuously influencing each other as well as the language learning process.

Drawing on these conceptualizations, recently, SLA scholars have examined CDST in relation to the different learner and teacher psychology constructs including anxiety, motivation, willingness to communicate (WTC), agency, self-efficacy, demotivation, and enjoyment (Almutlaq and Etherington, 2018; Boudreau et al., 2018; Hiver and Papi, 2019; Larsen-Freeman, 2019; Syed and Kuzborska, 2020). In a similar manner, the impact of CDST on L2 students' listening (Dong, 2016), speaking (Dou et al., 2021), and writing skills (Fogal and Verspoor, 2020) has caught duly attention among researchers in this domain. However, most of the conducted studies in this area have collected their data without referring to the perspectives of CDST which can generate novel interpretations and methodological suggestions. Against this lack, the current study aimed to explain the research benefits of applying CDST to learner/teacher psychology and summons researchers for a revision in choosing their data collection methods.

Over the last decades, numerous studies have been conducted on different variables related to teacher-learner psychology mainly using one-shot and quantitative research designs (MacIntyre et al., 2020). Among many constructs in L2 learning, anxiety, motivation, stress, efficacy, WTC, interpersonal communication skills, resilience, stroke, enjoyment, engagement etc. have been substantially explored in different educational contexts (Dewaele and MacIntyre, 2014; Almutlaq and Etherington, 2018; Boudreau et al., 2018; Derakhshan et al., 2019; Derakhshan, 2021; Pishghadam et al., 2021). However, these studies have mostly been correlational and based on quantitative data. In scrutinizing LLP, some qualitative methods such as interviews, observations, and journals have also been employed to examine the process of change in these variables, but normally they have not captured the dynamism of change in action which is the heart of CDST.

It is evident that many of these constructs related to emotions and interpersonal communication skills (e.g., clarity, credibility, immediacy, rapport, and care) are unlikely to be measured via simple quantitative, one-shot, and pre/post-test designs. The nature of these variables in SLA is extremely complicated which takes a long period of time to be (re)constructed in a learner. Hence, the use of traditional quantitative-qualitative research designs falls short of scientific standards in this scholarly domain. That is why, when the same variables are examined by CDST, contradictory results may be obtained when the researcher scrutinizes the nature of a variable and its interacting sub-systems.

Despite its existing logistic challenges and the generalizability of findings, CDST has different applications to research in SLA and LLP. As a case in point, researchers can investigate constructs related to teacher-learner psychology focusing on different timescales to see if the observed patterns of behavior occur at other levels. Similarly, CDST provides fresh insights for running idiodynamic studies to explore the nature of dynamic, stable, and variable changes in a construct (Boudreau et al., 2018). Moreover, this theory urges avid scholars to focus on unusual, unpredictable, unique, and outlier cases. They can also integrate social and cognitive dimensions of SLA using CDST (Larsen-Freeman, 2010). Additionally, CDST opposes a priori determination and definition of constructs and their measurements and stresses that LLP constructs must be put in motion to see how they interact and go together leaving a space for the potential influences from other sources on their development. Methodologically, CDST has introduced novel quantitative and qualitative research methods to applied linguistics including panel designs, latent growth curve modeling, multilevel modeling, idiodynamic method, qualitative comparative analysis, process tracing, retrodictive qualitative modeling, agent-based modeling, and socialnetwork analysis (Hiver and Al-Hoorie, 2020). Each of these methods can significantly capture the complexity and dynamism of LLP variables. Nevertheless, CDST is still a nascent approach to research the dynamics of teacher-learner psychology which requires complementary studies at various levels to have substantial contributions. Otherwise, it fades away as time passes, and if nothing valuable is offered by the theory.

In this study, it was concluded that language learning psychology is a complicated phenomenon that is not measurable via one-shot research instruments. To capture its complexity and dynamism, CDST approach was introduced to SLA and LLP research to detect the developmental process of many social, cognitive, and psychological teacher-learner variables. Although this theory seems technical, complicated, and with logistic problems, its application to L2 education is advantageous in that it can increase the awareness of L2 teachers, students, materials developers, and researchers. Teachers can use the findings to improve students' language learning by using teaching methods and classroom activities that are in tune with the principles of CDST. Similarly, students would find this study of value in that they can realize that language learning is the result of an interplay of numerous interacting components and takes a logical time to develop. Materials developers can develop textbooks and activities which reflect the dynamism and non-linearity of language competence. Finally, researchers can use the ideas of this study to run comparable studies in other fields and contexts as CDST is context-specific. Moreover, running cross-cultural studies is a fresh topic for research to see the applicability of CDST in different cultures. Additionally, future studies can be conducted on teacher-related and interpersonal communication variables from the perspective of CDST. Likewise, emotions in language education can be explored longitudinally through the lens of CDST. Finally, CDST research designs can be employed by future scholars to drive the field forward regarding the dynamics of SLA.

JY independently drafted the first manuscript and revised meticulously before it was submitted to this special issue for Frontiers in Psychology.

The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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