South Korea has one of the most advanced IT systems in the world, with a fast internet speed and overall nationwide easy internet accessibility. Therefore, the adaptation of internet use in their lives has become a common phenomenon. A survey on the use internet revealed that 99.9% of adolescents.1 Internet addiction has been recognized as an international problem. Studies in other countries have also been conducted. The prevalence rate of internet addiction in the USA is 9.8-15.2% among people in their teens and twenties.2 In Greece, prevalence rate of potential problematic internet use (PIU) is 19.4% and the PIU rate is 1.5%. In this study, potential PIU is defined as internet use which fulfils some, but not all, of the propose criteria of PIU. They used Young Internet Addiction Test for grouping internet use characteristics. PIU means an individual's inability to control one's use of the internet, so have marked distress and/or functional impairment. In Taiwan, the prevalence rate of the internet addiction is 15.3% among university students.3 A number of studies have revealed the incidence rates of Internet addiction among Korean adolescents to be between 2.6 and 14.9%.1,4,5 A number of factors such as location, screening tool and target age may have contributed to the differences of incidence rates in these studies.

Internet addiction disorder (IAD) is defined as one's inability to control his use of the internet, which could leads up to physical, psychological, social difficulties.6 In 1998, Goldberg suggested IAD to be a psychiatric disorder based on pathological gambling as described in the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV). Along with pathological gambling, IAD show features similar to those of substance dependency like salience, mood modification, tolerance, withdrawal symptoms, conflict and relapse.6 Clinical concern requiring intensive evaluation and treatment on Internet addiction has magnified over the past few years. But it was debated whether it should be added as a disorder in the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5). It is not clear whether internet addiction has manifested from a preexisting disorder, or is truly a discrete disease entity. As of 2013, 'Internet use gaming disorder' has been incorporated into DSM-V Section 3, a category of disorders needing further research.7

Internet addiction has demonstrated a correlation with depression, attention-deficit hyperactivity disorder (ADHD) and impulse control disorder.8,9,10,11 Of the 1618 students aged 13 to 18 years, 6.4% were at a moderate to high risk of pathological internet use. Those with pathological use were 2.5 times more likely to experience depression at the 9-month follow-up compared with the control group. This result suggests that young people who are initially free of mental health problems but pathologically use the internet are at risk of developing depression.11 On the other hand, depression was one of the preceding mental health issues of Internet addiction as reported by Young.6

The development of internet addiction was shown to be higher in the population with ADHD. In a report by Yoo et al.,12 it was stated that elementary students with internet addiction had a higher rate of ADHD symptoms. The adolescent population has also shown an associations between ADHD and internet addiction.13 It is suggested that to satisfy their need for immediate gratification, child and adolescents with ADHD seek comfort in the internet probably leading to internet addiction. Because they have a harder time to retain their interest and have an aversion for delayed reward, this usually results in poor academic function and difficulties in peer relationships. This is probably one of the main causes of their internet addiction because internet activities usually provide multimodal stimuli, immediate response and reward.

In addition, exposure to the internet at an early age and poor family cohesion, adaptability and communication were environmental factors of internet addiction.13 Ni et al.14 stated that the age of first exposure to internet use was significantly associated with internet addiction. Surveys on other addictions such as pathologic gambling and alcohol drinking suggest that early age exposure is closely related to its severity or dependency.15,16,17 If we take into account the results of these researches, exposure to the internet at a young age may be an associated factor to Internet addiction.

In this study, our aim was to examine 1) the prevalence of problematic internet use and the extent of internet addiction, 2) the factors associated with internet addiction.

Among the 1217 middle school students enrolled in this study, it was proven that 29 subjects (2.38%) were internet addicted users, 449 subjects (36.89%) were overusers and 739 (60.72%) were normal internet users (Table 1). Sex, age, smoking experience and age of initial internet use differed among the subgroups but alcohol drinking and economic status sh-owed little difference (Table 1).

The average scores were 77.41±7.80 of internet addicted users, 49.42±7.65 of overusers and 30.20±5.13 of normal internet users (Table 2). In the higher internet addicted group, the score of sub-item of K-YSR was high and the differences were statistically significant (p<0.01) except withdrawn item. There was no difference between overuser and internet addicted user but normal internet user showed a difference from the other two in the withdrawn item. In K-CDI, the higher internet addicted group showed a correlation with higher K-CDI scores and the difference among the three groups was significant (p<0.01) (Table 2).

Internet addiction was related to K-YSR total and sub-items and also to K-CDI (Table 3, p<0.01). Factors that could explain the severity of internet addiction were attention problems (β=0.578, t=3.36), delinquent problems (β=0.900, t=4.02), thought problems (β=0.727, t=3.80) and aggressive behaviour (β=0.264, t=3.25) in K-YSR and sex (β=5.498, t=8.65), age (β=1.591, t=4.29), K-CDI scores (β=0.382, t=6.50) (Table 4). Age of first internet use showed counter correlation with internet addiction (β=-0.090, t=-3.71). It meant that when we start using internet younger age, we are easily addicted to internet (Table 4). Above eight items consisted of 31.5% of factors explaining internet addiction [R2=0.315, F(8)=68.41, p<0.01] (Table 4).

This study was about the internet addiction rate in middle school students and related socioeconomic characteristics, emotional and behavioural factors.

Males were more closely related to internet addiction than females which had consistent results with other studies.1,3,8,9,24 When multiple regression was conducted, the male gender was a strong predictor of internet addiction (Table 4).

This study proved there was a relationship between internet addiction and the older the subject were. Studies in other countries have also concluded that internet addiction is highly prevalent in adolescents.25,26,27 But there has been no in-depth study into the reason why internet addiction affects adolescents between the ages of thirteen and fifteen. Generally, high school students are more likely to be addicted to the internet than middle and elementary school students. Therefore we speculate that as middle school students near high school age, their internet addiction becomes more prominent.5,28

The younger the age of first internet use showed a higher tendency of a more severe internet addiction. A research in China on the starting age of internet use (age 8-12) in university freshman confirmed an internet addiction.14 There is no definite reason but this result may mean that exposing children to the internet later in age could be a protective factor of internet addiction. To protect children from excessive internet exposure, the family environment is important. Parents have to enforce measurements on the internet use of their children. Younger children are influenced easily by their parents' internet use.

Other family environmental factors also influence internet addiction. Kim et al.29 reported that communication problems within the family and a weak family cohesion were related to severe internet addiction. A survey showed that the atmosphere surroundings within the family would probably be an important factor in reducing internet addiction.30

Internet addiction was correlated with a high K-CDI score and depression/anxiety and social problems: sub-items of K-YSR which was consistent with the results of other studies.26,27,31,32,33,34 Based on Khantzian's hypothesis, we suggest that the cyber-world as a method of self medication can easily regulate the affection, self-esteem, relationship or self-care of the user even though they suffer from depression in the real world.36 Lee et al.34 stated that those with severe internet addiction sh-owed problems with adjusting to school life and low self-efficacy. Therefore, adolescents with depression or social problems are more likely to use the internet as a means of escaping problems in the real world.

Items associated with a social problem of K-YSR include not getting along, being teased and not liked, feeling persecuted, and being the target of fights and attacks.

The result that attention problem was a predicting factor with internet addiction was a replication of results of other stu-dies.26,37,38 Ko et al.32 also reported that ADHD was the strongest predictor of internet addiction in a prospective study over two years. ADHD patients cannot endure one thing for an extended period of time and they have difficulties waiting for delayed reward and only respond to immediate reward. So they become easily addicted to games that are immediately rewarding.39 Being able to multitask while on the internet is another attractive trait to ADHD patients.

In this study, delinquent problems, externalizing problems and aggressive behavior were associated with internet addiction (Table 2), moreover delinquent problems and aggressive behavior independently were predictors of internet addiction (Table 4). There are several researches which reported that impulsivity and delinquency are associated with internet addiction regardless of having an attention problem.9,24,40,41 Adolescents with aggressive or delinquent behavior have difficulties forming a relationship in the real world but find it easier to form and break in the cyber-world. But it is not clear whether aggression or delinquent behavior is a direct cause of internet addiction, more focus on this topic is needed.

Adolescent internet use could not be seen as a wasteful activity but as an indirect expression of difficulties associated with school adjustment and peer relationship. Internet addiction can be a sign of depression, anxiety or ADHD so the evaluation of comorbidity is needed. We have to also evaluate the severity of the internet addiction and its implication and give the internet addicts help.

This research has some limitations and the first limitation is geographical limitation because subjects were in a city in Korea, making it hard to generalize the results. The second limitation is that we couldn't explore more about correlation of depression, K-YSR sub-items or sociodemographic data with contents of internet use due to no data about the contents. It is possible that correlations between them are associated with internet contents. The third limitation is about to thought problems of YSR. There has been no definite explanation about the relationship between thought problems and internet addiction yet. This study couldn't explain the association either. It could be a method like exploring about the contents of internet user to explain it. The third limitation is that problematic behaviors were not psychiatric disorders. So we cannot think that a person with high score in attention problems is an ADHD patient or a person with high delinquent problems is a patient with conduct disorder. The forth is that this study is cross-sectional study so we cannot explain exact causal relationship.

This result showed similar to other researches about sociodemographic, emotional or behavioral factors related to internet addiction. Male, older age, attention problems, delinquent problems, aggressive behavior, K-CDI scores and age of first internet use were related to internet addiction. Generally, subjects with more emotional or behavioral problems showed more severe internet addiction. It means that they already have had various difficulties when we found internet addiction of adolescents. Therefore it is necessary to evaluate whether the subjects have any emotional or behavioral troubles and to intervene to prevent internet addiction.