Introduction
Given the growing global prevalence and significant impact of depression and anxiety, which increased by approximately 50% between 1990 and 2013 (World Health Organization, 2016) and further surged by an estimated 25% during the first year of the COVID-19 pandemic (World Health Organization, 2022a), understanding the underlying mechanisms that contribute to the development and persistence of emotional disorders has become increasingly essential. The frequent comorbidity between these two pathologies, approximately 50% (Mineka et al., Reference Mineka, Watson and Clark1998), has led to the inclusion of these diagnostic categories under the broader classification of emotional disorders.
Emotional disorders significantly impact individuals’ quality of life and well-being, affecting various areas, including personal, familial, occupational, and social domains. Numerous studies have highlighted deterioration in social functioning and the profound consequences these disorders have on affected individuals, their families, and the broader community (Defar et al., Reference Defar, Abraham, Reta, Deribe, Jisso, Yeheyis and Kebede2023; Kessler et al., Reference Kessler, Walters and Forthofer1998; Kvarstein et al., Reference Kvarstein, Langsrud, Hummelen and Pedersen2023). Complementing this research is the economic analysis presented in a study, which underscores the substantial financial benefit of investing in the treatment of these pathologies. Specifically, it was found that every dollar invested in the treatment of depression and anxiety yields a return of four dollars in improved health and work capacity. This figure is particularly significant when considering that depression and anxiety collectively cost the global economy approximately one trillion dollars annually (Chisholm et al., Reference Chisholm, Sweeny, Sheehan, Rasmussen, Smit, Cuijpers and Saxena2016). In this context, expanding our understanding of the variables involved in the onset and maintenance of anxiety and depressive symptoms in emotional disorders is of crucial importance.
Among the variables most frequently and consistently associated with emotional disorders is experiential avoidance (Akbari et al., Reference Akbari, Seydavi, Hosseini, Krafft and Levin2022; Eustis et al., Reference Eustis, Cardona, Nauphal, Sauer-Zavala, Rosellini, Farchione and Barlow2020; Hayes et al., Reference Hayes, Luoma, Bond, Masuda and Lillis2006; Spinhoven et al., Reference Spinhoven, Drost, de Rooij, van Hemert and Penninx2016). It has been defined as “the phenomenon that occurs when a person does not want to remain in contact with particular private experiences (e.g., bodily sensations, emotions, thoughts, memories, images, behavioral predispositions) and takes steps to alter the form or frequency of those experiences or the contexts that produce them” (Hayes et al., Reference Hayes, Wilson, Gifford, Follette and Strosahl1996, p. 1154).
In this regard, various studies have supported the relationship between higher levels of experiential avoidance and negative affect (Levin et al., Reference Levin, Krafft, Pierce and Potts2018), depression (Akbari et al., Reference Akbari, Seydavi, Hosseini, Krafft and Levin2022; Chawla & Ostafin, Reference Chawla and Ostafin2007; Cheung & Ng, Reference Cheung and Ng2019; Cookson et al., Reference Cookson, Luzon, Newland and Kingston2020; Wagener et al., Reference Wagener, Baeyens and Blairy2016), and anxiety (Berghoff et al., Reference Berghoff, Tull, DiLillo, Messman-Moore and Gratz2017; Eustis et al., Reference Eustis, Cardona, Nauphal, Sauer-Zavala, Rosellini, Farchione and Barlow2020; Spinhoven et al., Reference Spinhoven, Drost, de Rooij, van Hemert and Penninx2016). In anxiety disorders, experiential avoidance has been specifically linked to panic symptoms and panic disorder (Eifert & Heffner, Reference Eifert and Heffner2003; Karekla et al., Reference Karekla, Forsyth and Kelly2004). It has also been associated with symptoms of post-traumatic stress disorder (Lewis, Reference Lewis2012) and generalized anxiety disorder (Lee et al., Reference Lee, Orsillo, Roemer and Allen2010).
On the other hand, self-esteem has been another transdiagnostic construct of great relevance in research, consistently linked to emotional disorders. It has been associated with various psychopathological processes (Obeid et al., Reference Obeid, Lahoud, Haddad, Sacre, Fares, Akel and Hallit2020; Rosenberg, 1965/Reference Rosenberg1973; Silverstone & Salsali, Reference Silverstone and Salsali2003). Specifically, low self-esteem has been more frequently associated with emotional disorders (Acar et al., Reference Acar, Avcılar, Yazıcı and Bostancı2022; Michalak et al., Reference Michalak, Teismann, Heidenreich, Ströhle and Vocks2011; Orth & Robins, Reference Orth and Robins2014; Zeigler-Hill, Reference Zeigler-Hill2011).
Some studies have indirectly suggested that experiential avoidance could be related to self-esteem. While these studies have not directly examined both constructs together, they have explored related aspects. For instance, an association between behavior and self-esteem has been proposed, indicating that self-esteem depends on how a person behaves in relation to life challenges: confronting versus avoiding (Mruk, Reference Mruk1998). This association between behavior and self-esteem is established on the premise that the outcomes of managing everyday difficulties provide feedback about oneself, which is subsequently linked to one’s level of self-esteem. Specifically, it has been suggested that an avoidant coping style in response to stress is not effective in the long term and, therefore, negatively impacts self-esteem. This avoidant style has been considered a specific form of experiential avoidance, with the latter being a broader construct that also encompasses other forms of avoidance (Hayes et al., Reference Hayes, Strosahl, Wilson, Bissett, Pistorello, Toarmino, Polusny, Dykstra, Batten, Bergan, Stewart, Zvolensky, Eifert, Bond, Forsyth, Karekla and McCurry2004).
Additionally, it has been suggested that individuals who score high in experiential avoidance may perceive themselves as less capable of regulating their emotional responses (Karekla & Panayiotou, Reference Karekla and Panayiotou2011). This self-perception of reduced capability could be related to lower self-esteem. Furthermore, experiential avoidance has been reported to hinder the pursuit of actions aligned with personal values (Fledderus et al., Reference Fledderus, Bohlmeijer and Pieterse2010), indicating that it can obstruct taking steps toward what is important in life. Assuming responsibility for acting in accordance with one’s values and desired goals has been identified as crucial for the development of healthy self-esteem (Branden, Reference Branden1995). Based on these findings, it could be hypothesized that experiential avoidance, by limiting the achievement of personally significant aspects, may be associated with lower self-esteem.
Moreover, the interest in studying the relationship between experiential avoidance and self-esteem has been highlighted by a small number of authors who have directly examined the association between these variables and their implications in various psychopathological processes. However, these studies have been limited and have not produced conclusive results. Some found an association between the two variables, while others identified independent effects in their relationship with different pathologies. In some studies, the relationship between experiential avoidance and self-esteem was bidirectional, with both variables mediating the relationship with the psychopathological symptoms studied, particularly in the case of paranoia (Udachina et al., Reference Udachina, Thewissen, Myin-Germeys, Fitzpatrick, O’kane and Bentall2009, Reference Udachina, Varese, Myin-Germeys and Bentall2014). In others, a moderating effect of acceptance, which is associated with low experiential avoidance, was found in the relationship between self-esteem and psychopathology (Michalak et al., Reference Michalak, Teismann, Heidenreich, Ströhle and Vocks2011). Conversely, other studies observed that the effects of both variables were independent in their association with psychopathology and were not related to each other (Al-Jabari, Reference Al-Jabari2012; Moroz & Dunkley, Reference Moroz and Dunkley2015).
Furthermore, the samples used in the previously mentioned studies were limited to students, non-pathological adults, or very specific clinical populations (schizophrenic, delusional, or schizoaffective disorders). To the knowledge of the present authors, no studies have examined the relationship between experiential avoidance, self-esteem, and anxious-depressive symptomatology in samples of individuals with emotional disorders, one of the most prevalent pathologies today. Additionally, none of the studies utilized the Brief Experiential Avoidance Questionnaire (BEAQ), which is recognized as the most appropriate measure for assessing experiential avoidance (Wolgast, Reference Wolgast2014) and addresses the limitations of previously developed instruments (Gámez et al., Reference Gámez, Chmielewski, Kotov, Ruggero, Suzuki and Watson2014). Consequently, expanding our understanding of how experiential avoidance and self-esteem interrelate and how both variables are associated with one of the most prevalent and significant pathologies in our population—emotional disorders—may shed light on the complex and still unclear mechanisms that lead to psychopathology. Specifically, the present research aims to explore the potential mediation of self-esteem in the relationship between experiential avoidance and depressive symptoms on the one hand and anxious symptoms on the other.
Method
Participants
This study involved 174 patients treated at a community mental health unit within the Andalusian Health Service. Of these participants, 123 were women (70.7%) and 51 were men (29.3%), with a mean age of 46.7 years and an age range of 18 to 73 years (SD = 11.98). Additional sociodemographic and clinical data are presented in Table 1. The inclusion criteria were as follows: meeting the diagnostic criteria for one of the various emotional disorders according to the ICD-10 (World Health Organization [WHO], 1992), being an adult (≥18 years), consenting to participate in the study, and having no cognitive impairment or other physical limitations that would prevent completion of the questionnaire.
Table 1. Sociodemographic characteristics of the participants (N = 174)
Instruments
Brief Experiential Avoidance Questionnaire (BEAQ; Gámez et al., Reference Gámez, Chmielewski, Kotov, Ruggero, Suzuki and Watson2014)
The BEAQ is a self-report questionnaire with 15 items that measure the degree of experiential avoidance. In its original English version, it shows the association with measures of avoidance, psychopathology, and quality of life. It shows a strong convergence with each of the six dimensions of the Multidimensional Experiential Avoidance Questionnaire (MEAQ; Gámez et al., Reference Gámez, Chmielewski, Kotov, Ruggero and Watson2011), the original scale from which the items are extracted for this short version. The items are rated on a six-point Likert scale (1 = strongly disagree to 6 = strongly agree). The scores range from 15 to 90 points, with higher scores showing greater experiential avoidance. The original questionnaire presented good reliability (α = .80–.89). The Spanish version validated by Vázquez-Morejón et al. (Reference Vázquez-Morejón, León Rubio, Martín Rodríguez and Vázquez Morejón2019), which reported a Cronbach’s alpha of .82, was used in this study. In the current sample, the internal consistency was adequate (α = .78).
Rosenberg Self-Esteem Scale (RSES; Rosenberg, 1965/Reference Rosenberg1973)
This scale is designed to assess overall self-esteem. It consists of 10 items with content referring to feelings of respect and acceptance of oneself. It is scored on a Likert scale with a rank of four points (1 = strongly agree to 4 = strongly disagree). The result is an indicator that oscillates between 10 and 40 points, with higher scores showing higher self-esteem. The Spanish adaptation used in this study (Vazquez et al., Reference Vazquez Morejon, Jimenez García-Bóveda and Vazquez-Morejon2004) obtained a Cronbach’s alpha coefficient of .84, indicating adequate internal consistency. In the present study, the scale also showed good reliability, with a Cronbach’s alpha of .85.
Patient Health Questionnaire-9 (PHQ-9; Kroenke et al., Reference Kroenke, Spitzer and Williams2001; Spitzer et al., Reference Spitzer, Kroenke and Williams1999)
This is a self-report questionnaire developed for evaluating the intensity and severity of depressive symptoms. It contains nine items covering the nine DSM-IV depression criteria scored from 0 (not at all) to 3 (almost every day). The total score ranges from 0 to 27. In addition, intervals have been proposed for interpreting the total score: 5–9 (minimal depression symptoms), 10–14 (minor depression), 15–19 (moderately severe depression), and > 20 (severe depression). In this study, we used the Spanish version validated by Vázquez et al. (Reference Vázquez, Castillo, Segura and Salas2014). In the present sample, the scale demonstrated excellent internal consistency, with a Cronbach’s alpha of .90.
Generalized Anxiety Disorder-7 Scale (GAD-7; Spitzer et al., Reference Spitzer, Kroenke, Williams and Löwe2006)
This is a self-report questionnaire that detects both the severity of generalized anxiety and other types of anxiety disorders. It has seven items scored on a scale of 0 (never) to 3 (almost every day). The total score can range from 0 to 21, with higher scores indicating a higher intensity of anxiety symptoms. Cutoff points have been proposed to interpret the results: 5 (mild anxiety), 11 (moderate anxiety), and 16 (severe anxiety). The Spanish adaptation used in this study presented good psychometric properties, with a Cronbach’s alpha of .87 reported in the original validation (García-Campayo et al., Reference García-Campayo, Zamorano, Ruiz, Pardo, Pérez-Páramo, López-Gómez, Freire and Rejas2010). In the present sample, the scale demonstrated high internal consistency, with a Cronbach’s alpha of .89.
Procedure
This research was approved by the Andalusian Biomedical Research Ethics Committee (Code 0869-N-16). Patients who were diagnosed with emotional disorders according to ICD-10 criteria and treated in a community mental health unit in Seville were invited to participate in the study. Diagnoses were established through clinical interviews conducted by experienced mental health professionals, in accordance with standard procedures at the community mental health center. In the session with the professional, they were informed about the aims of the study and that their participation was voluntary. After signing their informed consent, those who agreed to participate were given the questionnaires and instructions for their completion during the session. When they had finished, the questionnaires were checked to see that they had been filled out correctly, and the data were entered in the database for later analysis. This was an ex post facto cross-sectional study. The non-random sample was collected by non-probability sampling on the basis of accessibility.
Statistical Analysis
First, descriptive analyses of the scores on the scales in the study were performed using measures of central tendency and dispersion. Scores on the scales were also checked using the Kolmogorov–Smirnov goodness-of-fit test to see whether they followed a normal distribution. Correlations between scale scores were examined using Pearson’s r.
Various statistical analyses were conducted to examine the differences in scores based on age and gender. Pearson’s r correlation was used to determine whether age was correlated with experiential avoidance, with self-esteem, or with anxiety and depression symptoms. Moreover, Student’s t test was used to analyze the study variables for any significant gender differences.
To explore the influence of experiential avoidance on emotional disorders, a simple mediation analysis was performed (Preacher & Hayes, Reference Preacher and Hayes2004) using a 95% confidence interval and 5000 bootstrap samples to calculate the coefficient of the indirect effect.
First, this analysis was performed with experiential avoidance as an independent variable, self-esteem as a mediator, and depressive symptoms as a dependent variable. Then, the same analysis was carried out, but with anxiety symptoms as the dependent variable. The analyses were performed with the SPSS macro PROCESS (Hayes, Reference Hayes2013). Confidence intervals that did not contain zero would show a significant indirect effect, thus confirming the mediation of the variable studied. The influence of age was controlled for in these analyses. All data analyses were carried out using SPSS v.21.
Results
Descriptive and Correlational Analyses
The Kolmogorov–Smirnov test for normality indicated that the score distributions of the questionnaires used followed a normal distribution (p > .05). Means, standard deviations, and correlations between the different scales are presented in Table 2. Regarding the correlation between age and the scores obtained on the different scales, a significant correlation was found only with the variable experiential avoidance (r = .16, p < .05). In contrast, the correlations between age and the RSES (r = .02, p = .80), PHQ-9 (r = .09, p = .23), and GAD-7 (r = .00, p = .99) scales were not significant.
Table 2. Means, standard deviations, and correlations between study variables (N = 174)
Note: *p < .01 (bilateral).
In terms of the gender variable, no significant differences were observed between women and men in the scale scores used. Both groups showed no significant differences in scores on the BEAQ (t (163) = −.60, p = .55), RSES (t (163) = 1.40, p = .16), PHQ-9 (t (159) = −1.56, p = .12), and GAD-7 (t (161) = −1.66, p = .10) scales.
Mediation Analysis
To examine the potential mediation of self-esteem in the relationship between the independent variable experiential avoidance and the dependent variable depressive symptomatology, a mediation analysis was conducted while controlling for age. The results indicated that experiential avoidance indirectly influenced depressive symptomatology through a decrease in self-esteem.
As shown in Table 3, individuals with higher experiential avoidance had lower self-esteem (a = −0.185), and those with lower self-esteem exhibited greater depressive symptomatology (b = −0.879). The 95% bootstrap confidence interval for the indirect effect (ab = 0.163), based on 5000 bootstrap samples, did not cross zero (0.115 to 0.218), confirming the indirect effect of experiential avoidance on depressive symptoms via self-esteem. The effect size, indicated by kappa-squared ( κ 2), was 0.364, suggesting that the observed indirect effect (ab = 0.163) represents 36% of the maximum possible value. Additionally, there was no evidence that experiential avoidance directly influenced depressive symptoms beyond the effect through self-esteem (c’ = −0.050, p = .11).
Table 3. Coefficients of the mediation of self-esteem in the relationship between experiential avoidance and depressive symptoms, controlling for age
These findings support the hypothesis of self-esteem mediating the relationship between experiential avoidance and depressive symptoms, while controlling for age.
Regarding the mediation of self-esteem in the relationship between experiential avoidance and anxiety symptoms, Table 4 presents the major results. It was observed that participants with higher experiential avoidance had lower self-esteem (a = −0.185), and those with lower self-esteem exhibited higher anxiety symptomatology (b = −0.468). The indirect effect (ab = 0.087), calculated with a 95% confidence interval based on 5000 bootstrap samples, was above zero (0.056 to 0.125), confirming the indirect effect of experiential avoidance on anxiety symptoms through self-esteem. The effect size, indicated by kappa-squared ( κ 2), was 0.232, suggesting that the observed indirect effect (ab = 0.087) represents 23% of the maximum possible value. As for the direct effect of experiential avoidance on anxiety symptoms, it was not significant (c’ = 0.025, p = .37). These findings support the hypothesis that self-esteem mediates the relationship between experiential avoidance and anxiety symptoms, controlling for age.
Table 4. Coefficients of the mediation of self-esteem in the relationship between experiential avoidance and anxiety symptoms, controlling for age
Discussion
Overall, the results highlight the importance of experiential avoidance and self-esteem in emotional disorders. Specifically, this study finds a mediating effect of self-esteem in the relationship between experiential avoidance and anxious-depressive symptoms. These findings underscore the significance of addressing both experiential avoidance and self-esteem, identifying them as key processes in psychological interventions aimed at preventing or reducing emotional disorders.
In light of the data obtained, it can be proposed that to the extent that individuals allow themselves to engage with both positive and negative internal experiences, this may lead to higher self-esteem and personal approval, which in turn could improve mood. In this context, some studies provide a possible explanation, suggesting that greater acceptance of internal experiences contributes to reduced worry and rumination over self-critical thoughts. Simultaneously, these critical thoughts are increasingly viewed as mental processes rather than actual facts, and by offering less resistance and attention to them, high self-esteem is promoted (Pepping et al., Reference Pepping, O’Donovan and Davis2013; Randal et al., Reference Randal, Pratt and Bucci2015).
Regarding anxiety symptoms, the results indicated that lower experiential avoidance contributes to higher self-esteem, which in turn leads to reduced anxiety symptoms, regardless of age. This suggests that engaging with negative internal experiences, without rigidly and consistently avoiding them, contributes to a higher self-assessment, which is in turn associated with lower anxiety symptoms. It is also worth noting that the direct effect of experiential avoidance on anxiety symptoms was not significant, indicating that in the present sample, the effect of experiential avoidance on anxiety symptoms occurs indirectly through self-esteem.
The relationship among these four variables (experiential avoidance, self-esteem, and anxiety/depression) found in the present results can also be understood within the explanatory framework of self-determination theory (Deci & Ryan, Reference Deci and Ryan1985). Specifically, this theory suggests that openness and contact with oneself (which could be conceptualized as low experiential avoidance) facilitate the satisfaction of what are considered the three basic psychological needs of human beings: autonomy, competence, and relatedness. These, in turn, promote high self-esteem. This type of self-esteem is associated with greater well-being and health (Ng et al., Reference Ng, Ntoumanis, Thøgersen-Ntoumani, Deci, Ryan, Duda and Williams2012; Ryan, Reference Ryan2017).
According to this theoretical framework, when behavior is oriented toward satisfying the intrinsic basic needs of human beings, it is more frequently associated with a type of self-esteem linked to well-being. The fulfillment of the needs for competence, autonomy, and social relatedness, in turn, facilitates social integration and individual well-being. In this context, one of the strategies proposed to achieve this type of self-esteem is to enhance self-awareness and autonomous behavior congruent with true internal needs. Specifically, one approach from this perspective to increase self-esteem is through the practice of self-awareness techniques, such as mindfulness (Brown & Ryan, Reference Brown and Ryan2003; Ryan & Brown, Reference Ryan, Brown and Kernis2006), which, among other benefits, effectively reduce experiential avoidance.
As noted, self-determination theory connects self-esteem with the motivations underlying human behavior. This suggests that experiential avoidance, as it involves actions aimed at avoiding discomfort, may represent extrinsically motivated behaviors, which are, therefore, associated with lower and more unstable self-esteem and increased psychopathological issues (Akbari et al., Reference Akbari, Seydavi, Hosseini, Krafft and Levin2022; Williams et al., Reference Williams, Hedberg, Cox and Deci2000). Conversely, low experiential avoidance (or high acceptance) would contribute to the satisfaction of the three basic innate needs proposed by the model, which are linked to well-being. For instance, when an individual engages in low experiential avoidance, they are more aware of their needs and are more likely to act according to their values and what is important to them. This likely satisfies their need for autonomy, as the individual feels they are the origin of their own behaviors, acting in alignment with their personal values rather than conforming to others’ expectations.
Furthermore, experiential avoidance may also impact the other two basic needs: competence and relatedness. When an individual exhibits high experiential avoidance, they may feel insincere with themselves and others, as noted in research analyzing the effects of emotional suppression (John & Gross, Reference John and Gross2004). Specifically, this study indicated that suppression—a form of experiential avoidance—creates a sense of inauthenticity, leading to negative self-perception and hindering the development of closer social relationships. According to these findings, we can infer that low experiential avoidance would foster closer, more authentic social relationships and a stronger sense of personal competence.
For these reasons, we believe that self-determination theory could serve as an appropriate and timely explanatory framework for the relationship found between experiential avoidance, self-esteem, and emotional disorders.
The findings of the present study underscore the importance of considering both experiential avoidance and self-esteem in interventions aimed at preventing or mitigating anxiety and depressive symptoms. These results suggest that it is crucial not only to address self-esteem in terms of self-worth but also to consider an individual’s relationship with their internal experiences to enhance psychological well-being. Greater self-acceptance and contact with one’s own emotions promote more adaptive responses and contribute to improved social integration. The present research contributes to the ongoing discourse on self-esteem by highlighting that interventions focused solely on boosting self-esteem as a means to personal well-being and social adjustment may not yield the desired outcomes. Such approaches, particularly those that enhance self-esteem without considering behavioral aspects, may lead to unrealistic expectations, narcissistic behaviors, and even antisocial tendencies. Therefore, this study identifies experiential avoidance as a key variable for interventions that seek to foster a healthier form of self-esteem, as opposed to a more narcissistic or pathological self-esteem. Furthermore, this study aligns with suggestions that indirect approaches, such as reducing experiential avoidance, may more effectively enhance self-esteem related to mental health, rather than targeting self-esteem directly. Given the increasing prevalence of emotional disorders, both in general and particularly within the workplace (WHO, 2022b), these findings provide valuable insights into the mechanisms that contribute to their development and suggest potential strategies for psychological interventions aimed at reducing emotional symptoms through decreasing experiential avoidance and fostering a healthier form of self-esteem.
Regarding the limitations of this study, it is important to note the cross-sectional nature of the research, which necessitates caution when interpreting causal relationships between variables. Future studies would benefit from longitudinal data to examine and confirm the proposed mediation models. Additionally, participant selection was based on non-probabilistic convenience sampling, which suggests that confirming these results in randomized samples would be valuable. The exclusive use of self-report scales for data collection also presents limitations inherent to a single source of information. Future research should consider incorporating additional assessment tools from diverse sources (clinical assessments or other informants) to enhance the validity of the findings. Moreover, it would be worthwhile to explore the relationships identified in this study among individuals with other psychopathological disorders as well as in the general population. Further analysis of this model could include investigating new relationships between the same variables, such as whether self-esteem also predicts experiential avoidance, and examining the impact of emotional disorders on both experiential avoidance and self-esteem through more complex structural equation modeling. Lastly, it would be interesting to analyze the association between experiential avoidance, self-esteem, and other variables such as quality of life and emotional well-being.
Data availability statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.
Author contribution
R.V.-M.: conceptualization (conceptualized the study), formal analysis (performed and interpreted the statistical analyses), project administration (coordinated the research process), and writing—original draft (wrote the first draft of the manuscript). J.M.L.R.: conceptualization (contributed to the development of the theoretical framework) and writing—reviewing and editing (critically revised the manuscript). A.M.-R.: methodology (supervised the methodological design) and writing—reviewing and editing (contributed to the discussion section). A.J.V.M.: investigation (contributed to data collection), conceptualization (contributed to the development of the theoretical framework), data curation (participated in the interpretation of the results), and writing—reviewing and editing (revised the manuscript). All authors have read and approved the final manuscript.
Funding statement
This research received no specific grant from any funding agency, commercial or not-for-profit sectors.
Competing interests
The authors declare none.
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