*5.4. Test of Mediating Effects of the Commitment to Learning Sustainability*

To provide stronger evidence to determine whether commitment could play a role in Model C, an investigation of its mediating effect was conducted [44]. In light of the findings reported in the previous sections, the mediating role of commitment was examined. A two-step approach was used in this study [25]. The first step involved evaluating the three conditions given by Baron and Kenny [45]. If any of these conditions are not met, the mediating effect would be negligible [46]. Once these conditions are met, another mediation test (i.e., the second step) would be performed. Given the results shown in Tables 3–5, the three conditions were met. First, attitudes, subjective norms, and perceived behavioral control were significantly associated with intentions. Second, attitudes, subjective norms, and perceived behavioral control were significantly associated with commitments. Third, commitment and intention were significantly related after controlling for attitudes, subjective norms, and perceived behavioral control.

The second step is to perform the Sobel's product of coefficients test, which is probably the most popular method to test for mediation effects [47]. This test relies on standard errors and is suitable for nonlinear multivariate analyses, including WarpPLS [24]. The Sobel's z-value and the significance level of each mediating effect were computed according to the formula provided by Preacher and Hayes [48]. The results show that the commitment to learning sustainability significantly mediated the relationships of attitudes (z = 6.808; *p* < 0.001), subjective norms (z = 4.116; *p* < 0.001), and perceived behavioral control (z = 2.351; *p* < 0.01) to the intention to learn sustainability.

#### **6. Discussion**

This study has found that Model C was the best-fit model. The results show that the extended TPB is suitable for explaining the intent of learning sustainability. The main contribution of this study is twofold. First, the study has found that attitudes, subjective norms, and perceived behavioral control could explain the commitment to learning sustainability. This is a new discovery that alters our understanding of the consequence of these three exogenous variables, which were originally thought to be directly related to behavioral intentions. Perhaps, adding the variable "commitment" to the TPB offers a more comprehensive model for understanding the phenomenon. Future research in this area is therefore strongly recommended. Among these three exogenous variables, students who had a more positive attitude toward learning sustainability were found to be more committed to learning sustainability. For example, school gardening may help students develop their intrinsic values in conserving flora and fauna [49]. Teachers may also increase students' awareness of local environmental issues, with a particular focus on how knowledge of sustainability can improve the situation. Moreover, students who were followers of their referent others were found to be more committed to learning sustainability if such referent others would like them to do so. Thus, it becomes important to find who their referent others are. Their encouragement and support for students should not be ignored. On the other hand, students who were more confident in learning sustainability were found to be more committed to learning it. In order to help students to improve their self-efficacy of learning sustainability, teachers should plan effective learning activities to stimulate students' interest in learning [50]. With the aid of modern computer technology and geography teaching equipment, teachers can help students overcome the difficulties in studying junior middle school geography that is usually involved in understanding SD issues with scientific methods. As pointed out by Cai [51], geography teachers should provide students with rich and joyful geography lessons while learning sustainable geography knowledge.

Second, this research has found that commitment was positively related to the intention to learn sustainability. This is another new discovery and is consistent with the

existing literature that organizational commitment is the cause of the intention to quit or stay in the organization, especially empirical studies like Lee and Jeong [16] who found the mediating role of organizational commitment between job insecurity and turnover intention. This enhances the understanding of the TPB, which has not yet accounted for the commitment-intention pair that has long been adopted in the study of employee turnover. Since environmental awareness and consciousness are the common goals of learning both geography and SD [52], teachers should help students develop an interest in environmental protection in order to strengthen their willingness to learn sustainability, especially focusing on certain consumption behaviors as the underlying causes of corresponding environmental consequences, thereby encouraging greater participation in learning sustainable resource spending [53].

#### **7. Conclusions**

This study, perhaps the first of its kind, supports an extended TPB model that adds the variable "commitment" as a mediator regulating the influences of attitudes, subjective norms, and perceived behavioral control on sustainability learning intentions. It also demonstrates how to compare the three theoretical models using the robust factor-based PLS-SEM through the structural model fit indicators. Besides its empirical contributions, this study has limitations. First, the sample was students from a junior secondary school in China; thus, the results may not be generalized to a larger population, such as all schools in China, let alone schools in other countries. The results should be interpreted with care. Having said that, this study still provides some useful information for those who are interested in this research topic or teachers who need to teach sustainability in a subject. Furthermore, this research used a cross-sectional questionnaire survey, which had the problem of common method biases. Yet, this has been resolved by checking FCVIF. The method bias effect was trivial in this study. All in all, this may be the first study to consider the inclusion of commitment into a TPB model. It is recommended to continue examining the value of commitment in explaining behavioral intentions. Among other theories, the three-dimensional model of organizational commitment proposed by Allen and Meyer [15] has been widely used to study employee turnover. Future research may explore the role of these dimensions (i.e., affective, continuance, and normative) of other commitment types in explaining behavioral intentions, such as comparing the effects of these dimensions of commitment to learning sustainability on the intention to learn sustainability. Other compelling insights may then emerge.

**Author Contributions:** Conceptualization, H.H. and E.W.L.C.; methodology, H.H. and E.W.L.C.; software, H.H. and E.W.L.C.; validation, E.W.L.C.; formal analysis, H.H. and E.W.L.C.; investigation, H.H. and E.W.L.C.; resources, E.W.L.C.; data curation, H.H.; writing—original draft, H.H. and E.W.L.C.; writing—review and editing, E.W.L.C.; visualization, H.H.; supervision, E.W.L.C.; project administration, H.H. and E.W.L.C.; funding acquisition, E.W.L.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by The Education University of Hong Kong, grant number R6403.

**Institutional Review Board Statement:** The study was conducted in accordance with the Declaration of Helsinki, and approved by the Institutional Review Board (or Ethics Committee) of The Education University of Hong Kong (with approval code: 2021-2022-0168 and approved on 29 December 2021).

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** The data for this study are held by the first author. For any access to the data, please contact the first author.

**Acknowledgments:** We would like to thank the reviewers for their constructive comments given to us.

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

#### **Appendix A**

Table A1 lists the items for measuring each variable.

#### **Table A1.** Corresponding items for each variable.

