Most tobacco use begins in childhood or adolescence when an individual is most vulnerable to social influences that promote tobacco use, such as having friends who smoke, engaging in problem behaviors, having a person at home who smokes, policies on sales to minors, smoke-free policies, and tobacco company marketing [1]. Furthermore, initiation of tobacco use at an early age extends the potential duration of tobacco use, leading to stronger nicotine addiction from increased tobacco consumption [2]. Therefore, understanding the course of tobacco use in childhood and adolescence is an integral component of tobacco control and the prevention of lifelong adverse effects on health.

Longitudinal analysis of trajectory data allowed for identification of several different trajectories or subgroups, which were then classified by individual response patterns and finer modeling of individual variations. Analysis results were applied over time to understand factors associated with the different subgroups and to develop prevention strategies that were more tailored to each subgroup [3].

Trajectory analysis from previous studies divided the course of childhood and adolescent tobacco use into several categories such as never-smokers, triers, experimenters, and regular smokers. These studies established common predictors of becoming a sustained smoker through several distinct stages [4-10]. These studies have indicated that childhood and adolescent tobacco use patterns follow different trajectories, involving a complex transition from experimentation to addiction. In addition, variability and inter-individual changes over time in tobacco use behaviors among children and adolescents could be examined. Lastly, factors such as gender, ethnicity, socioeconomic status, academic performance, parent and/or peer tobacco use, tobacco bans at home and/or in public spaces, and perceptions of the tobacco industry have been identified, among others, as predictors of tobacco use trajectories [6,8,9].

However, studies investigating the tobacco use trajectories of children and adolescents have provided insufficient information on (1) the risk factors associated with tobacco use initiation and/or continuation among people of different ethnicities or cultures, (2) the affordability of tobacco use, (3) the prevalence of tobacco use, and (4) tobacco control measures. Furthermore, although lifelong adverse health effects can be expected with a high prevalence of smoking among adolescents, a limited number of studies have explored predictable connections between adolescent health and juvenile delinquency in Asian countries.

The prevalence of smoking among Korean adolescents in 2006 and 2017 was 16.0% and 9.5%, respectively, for boys and 9.2% and 3.1%, respectively, for girls, even though the age for initiating daily cigarette smoking had decreased since 2006 (14.4 and 13.8 years for boys and 14.2 and 13.7 years for girls in 2006 and 2017, respectively) [11]. The decrease in age for initiating daily cigarette consumption emphasizes a need for action to reduce youth access to tobacco products. Therefore, it is important to develop more intensive and effective cessation interventions tailored to the specific needs of smoker subgroups.

Accordingly, the aim of this study was to identify the smoking trajectories and potential predictors of smoking during adolescence in Korea in order to develop appropriate strategies for the primary prevention of tobacco use.

Respondents who reported never having smoked were specified as an a priori group in the GBTM. A cubic polynomial growth mixture model was finalized with 2 or 3 groups based on having BIC values closest to 0. The 2-group model performed the best statistically (BIC2=-507.72; BIC3=-520.92; BIC4=-533.95) and was selected as the final trajectory model. Figure 2 presents the a priori group and the 2 cigarette smoking trajectories.

The 3 trajectories were named based on their shapes: non-users (class 1: n=775, 82.3%), temporary users (class 2: n=32, 3.4%), and regular users (class 3: n=135, 14.3%). Temporary and regular users were also combined into a single trajectory designated as ever-users (classes 2 and 3 17.7%). The smoking experience rate of experimenters rose sharply between grades 6 and 7, peaked, and then decreased gradually until falling to zero at grade 9. For regular users, the rate of tobacco use began mounting in grade 6 and increased until grade 10 (Figure 2).

Respondent characteristics were compared by grade: ever-smoking experience was 1.7% in grade 6 and regularly increased, reaching 9.1% in grade 10; the percentage of respondents dissatisfied with their grades increased with age; and older respondents spent less time sleeping and reading. However, the percentage of respondents with a girlfriend and at least 1 delinquent friend increased with age. Interestingly, grade 7 had the highest percentage of respondents who had engaged in at least 1 type of delinquent behavior, and grade 6 had the highest percentage of respondents who had been victimized at least once (Table 1).

Four observations were noted when controlling for demographic and adolescent behavioral factors. First, when compared to non-smokers, ever users (temporary and regular users) had a significantly higher likelihood of having a girlfriend, committing at least 1 type of delinquent behavior, and never reading at most grade levels. Dissatisfaction with grades and lower annual household income were associated with being ever-users in grade 6, while having at least 1 delinquent friend in grades 8 and beyond was associated with being an ever-user (Table 2). Second, compared to non-smokers, temporary users had a higher likelihood of living with a single parent in grades 6 and beyond. In grade 7, when smoking experimentation was at its peak, dissatisfaction with grades, having a girlfriend, being victimized at least once, and having at least 1 delinquent friend were identified as factors associated with temporary users. Among temporary users, committing at least 1 type of delinquent behavior in grade 8 and having a girlfriend in grade 9 were also relatively higher (Table 3). Third, when compared to non-smokers, regular users had a higher likelihood of having a girlfriend, committing at least 1 type of delinquent behavior, and never reading in all grades, although never reading showed only a marginal significance in grades 8 and 9. In grade 6, reading less than 1 hour per day and belonging to the lowest household income group (< 30,000,000 Korean won) were significant factors associated with regular use. In grade 8, respondents who self-reported that they were regular users were more likely to have at least 1 delinquent friend (Table 4). Fourth, when compared to temporary users, regular users had a higher likelihood of committing at least 1 type of delinquent behavior and having at least 1 delinquent friend in grades 9 and beyond (Table 5).

This study was the first to identify multiple trajectories of smoking behavior as well as the smoking-related factors associated with each trajectory using a representative sample of Korean adolescents. Therefore, the goals of this analysis were to identify the distinct developmental trajectories of cigarette use spanning adolescence and examine the key risk factors associated with these particular smoking trajectories.

As shown in Table 1 for grade 7 (12-13 years), the number of respondents who reported dissatisfaction with their grades, having a girlfriend, having committed at least 1 type of delinquent behavior, having at least 1 delinquent friend, and making time for reading increased. For this same age group, respondents who were victimized at least once and sleeping time decreased significantly. The number of ever-users also dramatically increased from ages 11-12 (1.7%) to 15-16 (9.1%). These years constitute a critical time during which young people are especially vulnerable due to significant life changes such as starting middle school, increasing academic workloads, and various societal influences (e.g., peer relationships and growing social awareness), all of which can affect their interest in trying, experimenting, and continuing smoking. These results corresponded with the findings of previous studies which investigated the association between the characteristics of puberty and smoking experimentation [2,15]. Therefore, understanding the specific characteristics of individual behaviors and changes in social environments during puberty should be considered when developing an appropriate program to prevent children and adolescents from smoking.

Considering previous studies [1,5,10,15,16], the longitudinal patterns of smoking that were observed in this analysis were best described by 3 smoking trajectory groups: non-smokers (class 1), temporary users (class 2), and regular users (class 3). Compared to non-smokers, the other 2 trajectories were characterized by smoking continuity. Respondents assigned to class 2 (3.4%) started smoking when they turned 12 (the first year of junior high school), their tobacco use peaked at age 13, and then they quit gradually. Individuals assigned to class 3 (14.3%) continued smoking. Distinguishing class 2 from class 3 was of considerable importance for adolescent smoking prevention because it was key to understanding why some individuals discontinue tobacco use while others continue.

Analysis enabled us to examine how smoking risk factors were organized in meaningful patterns that distinguished subgroups of smokers. By incorporating risk factors into the trajectory model, hypotheses that related these risk factors with the probability of inclusion in a trajectory group were tested. Living with a single parent was the significant factor associated with the temporary-user group for all ages. Peak increases in smoking experimentation at age 13 were associated with grade dissatisfaction, having a girlfriend, being victimized at least once, and having at least 1 delinquent friend. At grade 6, belonging to the lowest family income demographic and spending less time reading (never or < 1 hr/d) were identified as significant factors for smoking initiation in the regular-user group, even if the factors did not continue in later ages. Having a girlfriend and having committed at least 1 type of delinquent behavior were significant factors associated with regular use in all grades. These findings suggested that respondents who neither commit delinquent behaviors nor regularly have a girlfriend have a greater possibility of smoking discontinuation, even if smoking experimentation was initiated because of a lack of daily care or parental concern or the societal influence of their peers (e.g., having delinquent friends and being victimized by peers). Furthermore, concern about grades and engaging in positive leisure activities such as reading could be considered factors that help prevent an interest in smoking.

These results were supported by the findings of previous studies published in other countries, including recent studies in the United States and Europe which revealed distinct smoking trajectory patterns and smoking-related factors during adolescence [6,9,14,16-19]. As in this current study, these previous studies suggested that a single-parent family, dating, less sleeping time, delinquent behavior, and poor academic performance were factors associated with smoking initiation and continuation for the period spanning early adolescence to young adulthood. Some of the factors showed a significant association with an increase in adolescent smoking or distinguished smoking trajectories from non-user trajectories. In addition, the current study included new factors such as experiences of victimization, having delinquent friends, absence of a guardian after school, reading time, and PC/video game-playing time. Although previous studies did not include these factors, it was nevertheless important to investigate them in terms of their potential effect on interest in smoking at an early age in adolescence or childhood. The current study also revealed that victimization and reading time were significant factors for distinguishing both temporary users (class 2) and regular users (class 3) from non-smokers (class 1), respectively. The findings are unique and contribute meaningful insights into early adolescent smoking behaviors and explain why different trajectories occur, especially in Asian countries where such information is not available. Furthermore, taking into account that the age of puberty has been decreasing worldwide, including in Europe, early onset of puberty could also be considered a factor associated with early smoking initiation, based on the early maturation hypothesis [20]. Accordingly, the factors included in the current study and their effect on grouping individuals into different smoking trajectories contributed to understanding the influences of the gap between physical maturity and social maturity on health and social behaviors, as well as how these influences relate to smoking initiation and continuation in early adolescence [20,21].

This study differed from previous cross-sectional studies that investigated the factors associated with smoking status and/or behaviors observed in adolescent subjects at a specific time. The current study investigated these factors by analyzing trajectory groups with longitudinal follow-up. Even though results from cross-sectional studies, using data from the Korea Youth Risk Behavior Web-based Survey, suggested that student performance in school, alcohol consumption, and the influence of peer smoking were the factors associated with adolescent tobacco use, the results did not identify the factors that affect which smoking trajectory is followed or which step of the natural history of tobacco use is followed, including never-use, trying, experimentation, and continuation or discontinuation [22]. Whereas, in the current trajectory analysis, factors associated with initiation, continuation, and discontinuation of tobacco use could be identified and fluctuations in their influence tracked over the span of adolescence. For example, living with single parents was a significant factor associated with temporary users through all grades but dissatisfaction with grades, having a girlfriend, being victimized at least once, and having at least 1 delinquent friend were identified as factors associated with temporary users in grade 7 when smoking experimentation was at its peak. Having a girlfriend, committing at least 1 type of delinquent behavior, and never reading were identified as the factors associated with regular users in all grades, but were different for temporary users. These subgroup differences provide significant scientific evidence for developing tailored prevention strategies to address tobacco use in adolescence.

The current study of Korean adolescents offered unique data on smoking trajectories and the factors used to include individuals in different trajectory groups using a representative sample and longitudinal follow-up design. The results were meaningful in terms of understanding the multiple smoking trajectories that exist in early adolescence in relation to certain characteristics of puberty. However, there were a few limitations.

First, although adolescent smoking patterns follow different trajectories and some previous studies have suggested heterogeneity of trajectories in smoking onset and progression, the current study identified 3 kinds of smoking trajectories: non-smokers, temporary users, and regular users [5,15]. In the current study, the 3 kinds of smoking trajectories were retained because heterogeneity did not identify a significantly different trajectory in smoking onset and progression; a statistically best fit for differentiation of the smoking pattern was also shown. In addition, comprehensive, statistically significant comparison of the classes was not possible due to the very small number of subjects in class 2. Second, the current study omitted certain variables that have demonstrated diverse trajectory patterns in previous studies, such as personality, depression, family problems (including parental conflict or violence), school attendance, conduct problems, sibling smoking, parental education level, parental smoking, smoking intensity, and other adolescent behaviors [9,15,16,23-25]. The current study analyzed secondary data from KCYPS; therefore, it was not possible to gather additional information on other variables used in previous studies. In particular, smoking intensity and other more specific smoking behaviors that could further differentiate tobacco users were not considered. Non-smoking policies that affect smoking behaviors were also not included in this study. Variables frequently used in previous studies such as family problems and parental education level might be interchangeable with variables in this study, such as absence of a guardian after school and level of household income. Future studies could also compensate for this limitation by including other variables that have not been considered in previous studies, potentially yielding additional implications. Third, the current analysis was limited to boys. Thus, results for girls were not available. However, as mentioned in the Methods section, this could not be avoided due to the very small number of girls who had ever smoked, thus preventing an appropriate statistical analysis.

In conclusion, this study suggested that adolescent tobacco use in Korea follows 3 distinct smoking trajectories: non-smokers, temporary users, and regular users. Temporary users were associated with living with a single parent. The peak increase in smoking experimentation among grade 7 temporary users was influenced by having a girlfriend, being victimized at least once, and having at least 1 delinquent friend. Having a girlfriend and having committed at least 1 type of delinquent behavior were significant factors for regular users while belonging to the lower family income demographic and spending less time reading at age 12 were significant factors for smoking initiation among regular users. Identification of smoking trajectory patterns and factors associated with inclusion in specific trajectory groups can contribute to the development of prevention and early cessation programs for adolescents tailored to the specific characteristics of subgroups.