The relationship between stress and smartphone addiction among adolescents: the mediating effect of grit

The purpose of this study was to investigate the mediating effect of grit on the relationship between stress and smartphone addiction among adolescents. Participants were 605 Korean students from age 12 to 16 (mean age = 13.97 years). Stress was assessed using the Daily Hassles Scales for Children in Korea developed by Han and Yoo (Journal of the Korean Home Economics Association, 33(4), 49-64, 1995). Grit was measured by the Korean translated version of the Original Grit Scale (Duckworth et al., Journal of Personality and Social Psychology, 92(6), 1087-1101, 2007; Park et al., Current Psychology, 39(2), 413-418, 2020), and smartphone addiction was measured by using the Smartphone Addiction Proneness Scale for Youth developed by the National Information Society Agency (2011). The mediating effect was analyzed by using PROCESS macro version 3.5, and bootstrapping was conducted to test the significance of the mediating effect. The results showed that adolescent's stress and grit significantly influenced smartphone addiction. Also, grit partially mediated the relationship between stress and smartphone addiction. This means that high levels of stress reduced grit, which in turn increased smartphone addiction proneness among adolescents. In addition, two factors of grit (consistency of interest and perseverance of effort) both mediated the association between stress and smartphone addiction. The current study is meaningful in that it is the first study to empirically investigate adolescent's grit in relation to stress and smartphone addiction. Moreover, this study can provide useful information about prevention and intervention strategies for smartphone addiction.


Introduction
Smartphone usage has notably increased over the past decade. Although smartphones provide convenience to everyday life, excessive use and over-dependence on them can lead to smartphone addiction. Smartphone addiction is characterized by repetitive failure to resist the impulse of using smartphone, withdrawal, excessive time spent on smartphone usage, and recurrent physical or psychological problem caused by excessive smartphone use (Lin et al., 2016). Recently, smartphone addiction has become common for adolescents. In fact, according to a recent systematic review (Sohn et al., 2019), approximately one in every four children and young people had smartphone addictions. Adolescents can easily be addicted to smartphones than adults due to their high impulsivity and novelty-seeking tendencies (Chambers et al., 2003;Jo et al., 2018). Furthermore, according to Cha and Seo (2018), the prevalence of smartphone addiction in Korean adolescents was 30.9%, which is higher than other European and Asian countries. Therefore, it is important to examine smartphone addiction focusing on adolescents in Korea.
Smartphone addiction has many negative consequences such as negative physical health (Kim et al., 2015), lower productivity (Duke & Montag, 2017), and poor mental health (depression, anxiety, suicidal ideation) (Thomée, 2018). However, relatively few studies investigated how adolescents become addicted to smartphones. Thus, further work is needed to examine the predictive factors smartphone addiction in adolescents. Among various factors that predict smartphone addiction, stress is one of the major factors that leads to problematic smartphone usage (Liu et al., 2018;Wang et al., 2015). Individuals with high levels of stress tended to excessively use smartphones to escape from life problems and alleviate stress (Wang et al., 2015). Also, Panova and Lleras (2016) discovered that people who had access to their mobile phones were more likely to show increased resilience in stressful situations. Researchers related this effect to "security blanket effect" in that it is similar to how children hold on to blankets or other objects to obtain a sense of comfort and safety in a threatening environment. However, depending on mobile phones was detrimental to longterm psychological health, indicating that it is a maladaptive coping mechanism (Panova & Lleras, 2016).
Adolescents in Korea grow in a competitive society, where children's academic performance is highly valued. Thus, Korean adolescents are more likely to experience greater academic stress than adolescents from other countries. This might have led them to use more smartphone addiction in order to escape from academic stress (Cha & Seo, 2018). In fact, many studies reported that students with high academic stress tended to become more addicted to smartphones (Kim & Shin, 2016;Xu et al., 2019). Adolescents who excessively depend on smartphones used them for various reasons such as "pleasure", "games", "not to be left out", "stress relief", and "online chat" (Lee et al., 2017). However, limited studies explored the psychological mechanism of the relationship between stress and smartphone addiction. Thus, the current study focused on how stress influences smartphone addiction among adolescents.
According to previous studies, factors such as resilience (Park & Kwon, 2019), self-control (Liu et al., 2018), and grit (Yoo & Choi, 2019) had significant mediating effects on stress and smartphone addiction. This study will be focusing on grit which is a relatively new psychological concept defined as "perseverance and passion for long-term goals" (Duckworth et al., 2007(Duckworth et al., , p.1087. Grit was first introduced as a non-cognitive skill that leads to success (Duckworth et al., 2007). Grit is characterized by the combination of consistency of interest and perseverance of effort. First, consistency of interest refers to the ability to continuously pursue focused interest over a long period of time without frequently changing goals. Second, perseverance of effort refers to the tendency to sustain effort in accomplishing long-term goals even in the face of adversity and challenges (Duckworth et al., 2007). Gritty people sustain effort and pursue consistent interest to accomplish long-term goals even in the face of failure and adversity.
Although majority of grit studies investigated the predictive power of grit in academic success (Duckworth et al., 2011;Lee & Sohn, 2013), recent studies expanded the function of grit and associated it with mental health. Some studies suggested that grit could serve as a buffer against negative outcomes of stress (Blalock et al., 2015;Jung et al., 2020;Marie et al., 2019). For instance, Blalock et al. (2015) argued that individuals high in grit were less likely to have suicidal ideation in negative situations than individuals low in grit. This may be because gritty people tend to focus more on long-term goals and minimize their attention to stress. Also, grit might enable people to reframe negative situations more positively and use more effective problem-solving strategies during highly stressful situations (Blalock et al., 2015). Moreover, studies revealed that grit is also related to addictive behaviors (Borzikowsky & Bernhardt, 2018;Griffin et al., 2016;Yoo & Choi, 2019). For example, according to Borzikowsky and Bernhardt (2018), grittier people were less likely to become addicted to online games. This indicates that grit could be negatively associated with overall addiction and could operate as a protective factor for addiction. Another study (Yoo & Choi, 2019) in South Korea discovered that grit was negatively associated with smartphone addiction. The researchers explained that this might be because smartphone addiction is related to lack of patience, and the concept of grit encompasses the notion of perseverance (Yoo & Choi, 2019). However, studies about grit and addiction are still limited and therefore, more research in this area is necessary to fully understand how grit is associated with addictive behaviors (Borzikowsky & Bernhardt, 2018). Based on these previous findings, we hypothesized that grit would be negatively associated with smartphone addiction and mediate the relationship between stress and smartphone addiction.
In addition, while the majority of grit studies used the total score of grit in the past (Eskreis-Winkler et al., 2014;Robertson-Kraft & Duckworth, 2014), recent studies argued that two components of grit can cause different consequences (Bowman et al., 2015;Disabato et al., 2019). For instance, Datu et al. (2016) discovered that the correlation between two factors of grit was very weak, and the perseverance of effort predicted academic engagement, life satisfaction, and positive affect more strongly than consistency of interest. Also, Bowman et al. (2015) revealed that perseverance predicted greater academic adjustment, GPA, and sense of belonging than consistency among college students. On the other hand, Knauft et al. (2019) demonstrated that consistency of interest, not perseverance of effort, was related to lower bulimia and body dissatisfaction scores. The study could not identify the specific mechanism but explained that individuals with continued interest in enduring goals would focus on long-term goals even when immediate rewards are absent, which could lead to lower eating disorder-related attitudes and behaviors (Knauft et al., 2019). These results indicate that two components of grit should be analyzed independently to better understand the functions of grit. However, most of the studies used the total score of grit in the past (Eskreis-Winkler et al., 2014;Robertson-Kraft & Duckworth, 2014). Thus, we planned to analyze the two facets of grit separately and complement the previous findings.

Research questions
As reviewed above, stress appeared to be related to smartphone addiction, and grit may influence the relationship between stress and smartphone addiction. Based on the previous findings, the current study aimed to examine the relationship between stress, grit, and smartphone addiction among Korean adolescents. In particular, this study focused on exploring the mediating effect of grit on the relationship between stress and smartphone addiction. The present study's research questions are as follows: (1) Does stress predict smartphone addiction among adolescents? (2) Does grit and components of grit predict smartphone addiction among adolescents? (3) Does grit and components of grit mediate the relationship between stress and smartphone addiction among adolescents?
We expected that stress would positively influence smartphone addiction among adolescents. In other words, students with higher stress would be more likely to be addicted to smartphones. Second, we hypothesized that grit would negatively influence smartphone addiction among adolescents. Less gritty students would be more likely to be addicted to smartphone addiction. Also, we hypothesized that grit would partially mediate the relationship between stress and smartphone addiction. In other words, when levels of stress increase, grit would decrease, which will increase the risk of smartphone addiction. The research model of the current study is presented in Fig. 1.

Participants
Participants were 605 adolescents (315 boys and 290 girls) from 12 to 16 years old and the mean age was 13.97 years. The participants consisted of 136 people aged 12, 101 people aged 13, 130 people aged 14, 121 people aged 15, 117 people aged 16. Most of the participants resided in Seoul and Gyeonggi province of the Republic of Korea.

Stress
Stress was measured using the Daily Hassles Scales for Children in Korea, developed by Han and Yoo (1995) to measure daily stress that is generally experienced by children in Korea. The scale consists of 6 factors: parent-related stress, home environment stress, friend-related stress, academic stress, teachers & school related stress, and surroundings related stress. Among these factors, items of parent-related stress (8 items), home environment stress (7 items), and academic stress (7 items) were selected for this study. A 6-point Likert scale is used for each item (ranging from 1 = strongly disagree; 6 = strongly agree). Higher scores represented higher levels of stress. The Cronbach's alpha of the scale was 0.85 in Han and Yoo's (1995) study. The Cronbach's alpha for the items used in the current study was 0.91.

Grit
Grit was measured by the Korean version of the Original Grit Scale (Duckworth et al., 2007;Park et al., 2020). The scale consists of 12 items assessing consistency of interest (6 items) and perseverance of effort (6 items). Answers were given on a 5-point Likert scale for each item (ranging from 1 = not at all like me; 5 = very much like me). Items for consistency of interest factor were reverse calculated. The higher the total score, the higher the level of grit. The reported Cronbach's alpha in Duckworth et al. (2007) was 0.85. The Cronbach's alpha in the current sample was 0.74.

Smartphone addiction
Smartphone addiction was assessed by Smartphone Addiction Proneness Scale for Youth developed by the National Information Society Agency (2011). The scale consists of 15 items and includes 4 factors: disturbance of adaptive functions, virtual life orientation, withdrawal, and tolerance. Participants answered items on a 4-point Likert scale ranging from 1 = not at all like me to 4 = very much like me. The higher the score, the more likely participants are to be addicted to smartphones. The Cronbach's alpha demonstrated by National Information Society Agency (2011) was 0.88. The Cronbach's alpha in the current study was 0.88.

Procedures
Participants arrived at the laboratory and entered a private room. A brief explanation of the purpose of the study and storage of the data were informed to the participants. Then they signed the consent form and agreed to participate in the study. They answered the questionnaires measuring stress, grit, and smartphone addiction. It took approximately 10 min for the participants to complete the questionnaires. Before the study was conducted, all procedures and measurement tools for this study were reviewed and approved by Seoul National University's Institutional Review Board (IRB).

Results
For statistical analysis, we used the Statistical Package for Social Science version 25 (SPSS 25) to analyze the relationship between stress, grit, and smartphone addiction. To investigate the mediating effect of grit on the relationship between stress and smartphone addiction, we used model 4 of PROCESS macro version 3.5 developed by Hayes (2017). Finally, bootstrapping was performed to test the statistical significance of the mediating effect. Bootstrapping was based on 5,000 samples with 95% confidence intervals.
Descriptive statistics of the variables, including stress, grit, and smartphone addiction were as follows (see Table 1 The correlations between stress, grit, and smartphone addiction are presented in Table 2. Correlations among variables were analyzed using Pearson's correlation analysis. As shown in Table 2, all of the variables significantly correlated with each other. Stress positively correlated with smartphone addiction (r = 0.37, p < 0.01) and negatively correlated with grit (r = -0.35, p < 0.01). Negative correlation was found between grit and smartphone addiction (r = -0.45, p < 0.01). Likewise, consistency of interest (r = -0.37, p < 0.01) and perseverance of effort (r = -0.35, p < 0.01) both negatively correlated with smartphone addiction. Other variables also showed significant correlations with each other, which are summarized in Table 2.
To examine the mediating effect of grit on the relationship between stress and smartphone addiction, model 4 of PROCESS macro version 3.5 developed by Hayes (2017) and bootstrapping was conducted. As shown in Table 3, adolescent stress significantly influenced smartphone addiction (β = 0.103, se = 0.017, z = 6.096, p < 0.001). Also, adolescent stress significantly influenced grit (β = -0.121, se = 0.014, z = -8.845, p < 0.001), and grit significantly influenced smartphone addiction (β = -0.449, se = 0.047, z = -9.488, p < 0.001). Finally, the path of "stress -grit -smartphone addiction" (β = 0.158, se = 0.017, z = 9.324, p < 0.001) was also significant, which means that grit mediated the relationship between stress and smartphone addiction. Therefore, grit appeared to partially mediate the relationship between stress and smartphone addiction. The results are summarized in Fig. 2. Also, the bootstrapped confidence intervals (CI = 0.0371-0.0755) of the mediation effect did not contain zero. Moreover, the mediating effects of two components of grit were separately analyzed. Similar to the previous result of the mediating effect of grit, consistency of interest and perseverance of effort both significantly mediated the relationship between stress and smartphone addiction (see Table 4). First, stress positively predicted smartphone addiction (β = 0.130, se = 0.017, z = 9.513, p < 0.001). Second, stress negatively predicted consistency of interest (β = -0.052, se = 0.009, z = -5.585, p < 0.001), and consistency of interest negatively predicted smartphone addiction (β = -0.598, se = 0.076, z = -7.833, p < 0.001). Also, the path of "stress -consistency of interest -smartphone addiction" (β = 0.161. se = 0.017, z = 9.513, p < 0.001) was significant, which indicates that consistency of interest mediated the relationship between stress and smartphone addiction. These results indicate that consistency of interest partially mediated the relationship between stress and smartphone addiction. The results are summarized in Fig. 3. In addition, the bootstrapped confidence interval (CI = 0.0186-0.0461) of the mediation effect excluded zero.

Discussion
The current study was conducted to shed light on the prevention and intervention of smartphone addiction among adolescents. In particular, we focused on grit, which is defined as "working strenuously toward challenges, maintaining effort and interest over years despite failure, adversity, and   (Duckworth et al., 2007(Duckworth et al., , p.1087(Duckworth et al., -1088 and investigated its association with stress and smartphone addiction. The main findings were as follows. First, results from the correlation analysis and mediation analysis indicated that adolescents' daily stress (parentrelated stress, home environment stress, and academic stress)    -.598 *** was significantly associated with smartphone addiction. This means that the higher the stress, the higher the possibility of smartphone addiction. This is consistent with previous studies, which reported that stress positively predicted smartphone addiction (Liu et al., 2018;Wang et al., 2015). When people experience high levels of stress, they were more likely to use the internet or smartphones to relieve stress and escape from internal discomfort (Wang et al., 2015). In line with these findings, our study revealed that high levels of stress can lead to increased possibility of smartphone addiction among adolescents.
Second, the adolescent's grit negatively correlated with smartphone addiction. The results from the correlation analysis and mediation analysis indicated that the lower the grit level, the higher the tendency for smartphone addiction. Also, both components of grit -consistency of interest and perseverance of effort -were negatively related to smartphone addiction. Previous studies reported that grit was negatively related to maladaptive behaviors such as delinquent behaviors among adolescents, substance use disorder, online game addiction, and more (Borzikowsky & Bernhardt, 2018;Griffin et al., 2016;Guerrero et al., 2016;Knauft et al., 2019). As grit refers to perseverance and sustained effort despite failure and adversity, it can contribute to focusing on long-term goals and making responsible life choices (Guerrero et al., 2016). Thus, grit may lead to less engaging in risky and maladaptive behaviors. The current result confirmed the previous findings in that lower grit was related to higher possibility of being addicted to smartphones, adding to a growing body of literature about the relationship between grit and addictive behaviors.
Finally, grit partially mediated the relationship between stress and smartphone addiction among adolescents. This finding showed that as levels of stress increased, grit decreased, which in turn increased the risk of smartphone addiction. In other words, higher stress led to smartphone addiction through lower grit. This is consistent with the previous finding (Yoo & Choi, 2019) that demonstrated the mediating effect of grit on the relationship between academic stress and smartphone addiction among college students. Although our study seems to replicate Yoo and Choi's (2019) study, there are some meaningful differences. First, while Yoo and Choi (2019) only considered academic stress as the independent variable, our study included not only academic stress but also parent-related stress and home environment stress. Moreover, unlike Yoo and Choi's (2019) study, which was conducted on a limited community sample of college students, our study targeted adolescents (12 to 16 years old) who are more vulnerable to smartphone addiction (Haug et al., 2015). According to the current study, adolescents' daily stress can not only directly lead to smartphone addiction but also lower the level of grit, which may lead to increased smartphone addiction. This result implies that high levels of grit can serve as a protective factor against smartphone addiction even under stressful situations. As gritty people minimize their attention to stress and focus on success, not failure, they are less influenced by stressful situations and tend to use more effective problem-solving strategies (Blalock et al., 2015;Marie et al., 2019). The salient presence of long-term goals may also motivate gritty people to engage in more effortful and less enjoyable activities toward long-term goals (Blalock et al., 2015;Duckworth et al., 2011). Thus, high grit may motivate adolescents not to indulge themselves in smartphones and to focus more on constructive behaviors for long-term goals.
In addition, unlike Yoo and Choi's (2019) study, we separately analyzed the two components of grit (consistency of interest and perseverance of effort), and found that they both partially mediated stress and smartphone addiction. As reviewed earlier, several studies argued that consistency of interest and perseverance of effort may bring different results (Datu et al., 2016;Disabato et al., 2019). However, our study found that consistency of interest and perseverance of effort were both negatively related to smartphone addiction and mediated the relationship between stress and smartphone addiction. This means that adolescents' daily stress can reduce consistent passion and continued effort for long-term goals, which might subsequently lead to engaging in excessive use of smartphones. Thus, these results indicate that possessing high levels of grit might block the path of stress leading to smartphone addiction.
This study has following limitations and suggestions for further research. First, our study was based on self-reported questionnaires, which assume that participants answered the questions truthfully. However, participants' emotional state, social desirability, and other factors may have influenced their response. Second, this study relied on non-experimental cross-sectional data, and findings were based on data from one period. Therefore, future longitudinal or experimental studies should be conducted to examine a more direct causal relationship between stress, grit, and smartphone addiction. Third, our study only focused on grit as a mediating factor of stress and smartphone addiction. However, several previous studies suggested the mediating effect of self-control (Liu et al., 2018) and resilience (Park & Kwon, 2019) on the relationship between stress and smartphone addiction. Thus, we recommend future research to comprehensively explore the mediating functions of personal factors, including not only grit but also self-control and resilience on the relationship between stress and smartphone addiction. Finally, as the importance of grit was found to extend beyond academic performance, it would be meaningful to investigate the predictive factors of grit. For instance, a recent longitudinal study by Park et al. (2020) demonstrated that the ability to delay immediate gratification at age four predicted greater grit at age 14. Unfortunately, most studies have rarely examined the predictive factors of grit in a longitudinal study. Thus, it is necessary for more studies to explore the predictors of grit in longitudinal settings .
However, despite these limitations, as the first empirical study to investigate the relationship between stress, grit, and smartphone addiction among adolescents, the current study has following implications. First, by exploring grit in relation to smartphone addiction, our study went one step further than relating grit with academic success and contributed to a growing body of literature that highlighted the protective function of grit against maladaptive behaviors (Borzikowsky & Bernhardt, 2018;Griffin et al., 2016;Guerrero et al., 2016;Knauft et al., 2019). As previously discussed, grit has significant implications for adolescents in that it leads to better academic performance (Duckworth et al., 2011) and mental health (Blalock et al., 2015;Marie et al., 2019). However, to the best of our knowledge, this is the first empirical study that examined the influence of grit on adolescent's smartphone addiction. Thus, the present study is meaningful in that we showed the negative relation between grit and smartphone addiction of adolescents and contributed to expanding the function of grit among adolescents. Moreover, by exploring the mediating effect of grit on stress and smartphone addiction, our study revealed the mechanism of how stress influences smartphone addiction and provided a personal factor that can prevent the path of stress leading to smartphone addiction. Therefore, educational programs fostering grit among adolescents would be helpful for the prevention and intervention of smartphone addiction due to daily stress.
Recently, several studies investigated how grit can be developed and improved. For instance, recent studies discovered that school environment (Park et al., 2018) and parent's mindset about failure  influenced the development of grit. As grit appeared to mediate the relationship between stress and smartphone addiction in the current study, further research could investigate how to encourage grit in adolescents to prevent smartphone addiction and alleviate the negative effects of stress.
Data availability The datasets generated during and/or analyzed during the current study are not publicly available due to privacy issues but are available from the corresponding author on reasonable request.

Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.