Introduction

Achieving a tobacco-free world depends largely on the prevention of tobacco use among young generations. Although adolescents’ tobacco use has been decreasing globally since the early 2000s,1 it is still a major public health concern, as it is a strong predictor of continued smoking in adulthood and future detrimental health outcomes.2 Among the wide range of interventions that help reduce adolescent tobacco use, a key measure is reinforcing the public perception that tobacco use is not a mainstream activity in society, that is, denormalisation.3

Efforts to denormalise tobacco use have resulted in substantial reductions of smoking at a population-level, including youths.4–7 Key approaches to tobacco denormalisation include limiting the locations available to smoke in, increasing the prices of tobacco products and controlling how tobacco products are sold and advertised.3 Although changing social norms is generally the main goal of tobacco control policies,8 it is difficult to directly obtain an objective measure of social norms in a population. The ‘cohort effect’, that is, the changes across different generations, can be used to detect shifts in social norms which are predominantly formed among those within the same generational group.9

Each new generation or cohort have unique experiences that provide an opportunity for societal transformation,9 such as a world free of tobacco use. This is because cohort membership formed during adolescence endures throughout the rest of life.10 In addition, assessing cohort effects can help elucidate the impact of macroenvironmental events through studying the responses to shared exposure among a group of individuals, during a certain period. Furthermore, the presence of cohort effects in smoking trends raises an important methodological issue. Cross-sectional studies, such as the Global Youth Tobacco Survey, often examine smoking behaviours using age-specific or year-specific statistics,11 but these indicators can be confounded by changes in age composition or the timing of policies.12 Thus, trends should be studied within the context of birth cohorts, and the investigation of cohort effects with regard to tobacco control is particularly important.

In South Korea, comprehensive tobacco control measures began with the enactment of the Health Promotion Act in 1995. This act prohibits tobacco sales to minors; restricts tobacco sales, smoking in some public places through the designation of smoke-free areas and tobacco advertising, promotion and sponsorships; and requires text-only warning labels on cigarette packs. Since the ratification of the Framework Convention on Tobacco Control (FCTC) in 2005, additional tobacco control policies have been introduced and strengthened (figure 1).

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Figure 1

Landmarks of tobacco control policies in South Korea. FCTC, Framework Convention on Tobacco Control.

During the post-FCTC period, smoking prevalence among South Korean adolescents declined markedly, from 12.8% in 2006 to 6.4% in 2017. This population-level decrease indicates presence of a significant transition in social norms among generations of youths. Two important components of the tobacco control legislation in South Korea that contributed to tobacco denormalisation6 were the banning of smoking in public places in December 2011, and the largest ever increase in cigarette prices (2500 WON/pack→4500 WON/pack), in January 2015.

Prior studies have yielded favourable impacts of smoke-free legislation and price increase on adolescent smoking prevalence and frequency.13–15 For example, Hawkins et al 14 emphasised that youngest adolescents were most sensitive to prices, while smoke-free legislation was effective in all age groups. One limitation of these studies is that age-related changes represent mixed effects of birth cohort and age on adolescent smoking. Therefore, we investigated whether these policies contributed to the observed decline in adolescent smoking prevalence, particularly through lasting effects on different cohorts that goes beyond declining trends in all age groups. The presence of cohort effects as a result of tobacco control policy would indicate the likelihood of social norms trending towards a generation where tobacco use is not the norm.

Using segmented regression and age-period-cohort (APC) analysis, this study seeks to (1) determine whether the implementation of smoke-free legislation in public places and the increase in cigarette prices were associated with reductions in cigarette use in adolescents of all ages and (2) whether these policies resulted in cohort effects that lowered cigarette smoking prevalence beyond that expected from the decreasing trends alone.

Methods

Results

Prevalence of cigarette smoking

We used data from 853 441 adolescents (439 783 boys (52%) and 413 658 girls (48%)) over the study period of 2006 to 2017 in our analyses. Trends in the prevalence of adolescent cigarette smoking are displayed in figure 2. Smoking prevalence (unadjusted) among South Korean adolescents remained steady between 12% and 13.2% from 2006 to 2011. Then, from 2011 to 2017, a decrease in smoking prevalence from 12% to 6.2% was observed. For boys, smoking prevalence fluctuated between 15.8% and 17.2% during 2006–2011. Smoking prevalence in boys then decreased from 17.0% in 2011 to 9.2% in 2017. Among girls, the prevalence of smoking decreased gradually from 8.9% in 2006 to less than 3% in 2016. However, the most recent statistics in 2017 indicate that smoking among girls is on the rise.

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Figure 2

Prevalence of cigarette smoking among South Korean adolescents, 2006–2017.

APC model

The full APC (IE) model provided the best fit for both sexes (online supplementary table S2 in supplementary file 1). Figure 3 presents the model-derived effect of all three parameters, age, period and cohort, on the prevalence of adolescent cigarette smoking. A progressive increase in smoking prevalence with increasing age was found in both sexes. Although the patterns were similar between the sexes, the magnitude of the age effect was observed to be larger in boys than in girls.

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Figure 3

Estimated age-period-cohort effects for cigarette smoking prevalence.

The period effect of adolescent cigarette use was also significant. Among boys, an estimated decrease in risk of approximately 40% was observed followed by a period of fluctuation between 2006 and 2008. The period effect was larger for girls, with the estimated PRR decreasing by more than 50%.

Cohort effect in the mid-panel of figure 3 and table 2 was most striking among the three effects of the APC model. An increase in the risk of smoking was observed from 1994 to 1998, followed by a decrease from 1998 to 2005 for boys, indicating a cohort effect. From 1998 onwards, the risk of cigarette use declined across every consecutive birth year for boys. Similarly for girls, the PRRs increased from 1994 to 1997. The risk of cigarette use also declined yearly for girls born between 1997 and 2005. The peak PRR from 1994 onwards was higher, and the association between birth year and smoking prevalence stronger, for boys than for girls.

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Table 2

Estimated cohort effects for cigarette smoking prevalence by sex

Discussion

This study illustrated the impact of two tobacco control policies, smoke-free legislation in public places, including schools and increasing cigarette prices, by applying segmented regression and APC models to a nationally representative sample of adolescents. Our results suggest that the decrease in prevalence of adolescent cigarette smoking seen for both sexes was associated with smoke-free legislation. The prevalence of cigarette smoking decreased immediately after the increase in prices, although statistically significant only in girls. The decline in cigarette smoking prevalence was influenced by birth cohort, as the risk of cigarette smoking decreased every consecutive year for those born after 1998.

In the segmented regression analysis, smoke-free legislation in public places, including schools, in December 2011 was associated with a significant decreasing trend in smoking prevalence which is in line with previous studies on adolescent smoking.14 15 23 24 Reductions in smoking prevalence can be achieved through the designation of smoke-free areas by limiting opportunities to engage in smoking and building social pressure not to smoke.25 26

Our data fell short of showing a sustainable favourable association between increasing the price of cigarettes and a reduced prevalence of cigarette smoking. This finding may be explained by a few factors. First, the level of price increase was insufficient to result in a noticeable difference given that other policies were already in effect to reduce the prevalence of smoking. A study of South Korean adolescents in 2014 reported that the median cigarette price per pack at which adolescents intended to stop smoking was 5000 KRW27 which is higher than the cigarette price of 4500 KRW after the increase. Additionally, the acquisition of tobacco via social sources such as borrowing, sharing or stealing would have dampened the responsiveness to a price increase,28 as only about half of South Korean adolescent smokers actually buy cigarettes directly with their own money.29 Furthermore, an inadequate amount of time may have passed after the price increase to observe a statistically significant effect on smoking behaviour. Although insignificant in boys, we note that there was an immediate decrease in smoking prevalence after cigarette prices were increased.

Results from the APC analysis also illustrate the possible effects of macroenvironmental circumstances on cigarette smoking prevalence in adolescents. The positive association between age and smoking found in earlier studies was confirmed in this study.30–32 The APC model suggests secular dynamics between tobacco control policies and prevalence by period effects and importantly, explains the impact of policies on different generations of adolescents via the effects of birth cohorts.

Smoke-free legislation was associated with a long-term decrease in the prevalence of adolescent cigarette smoking, implying the presence of cohort effects.10 Different risks of adolescent smoking were found in different birth cohorts. According to the cohort analysis, likelihood of smoking declined after peaking in 1998 for boys and 1997 for girls. This decreasing trend in birth cohorts born after 1997 and 1998 indicates that, along with early initial exposure to smoke-free legislation, continued exposure to the policy helped deter adolescents from smoking.

Overall, our study shows that implementation of smoke-free legislation in public places, including schools, was the major factor reducing the prevalence of cigarette smoking in adolescents. The continued decrease in smoking prevalence can be explained by cohort effects. More than other tobacco control measures, smoke-free legislation is renowned for its ability to denormalise tobacco use.23 Since social norms are predominantly generated within a birth cohort,9 effects of smoke-free legislation in establishing the norm of a society where smoking is not conceived as the prevailing norm can be inferenced by cohort effects. Smoke-free legislation could have created a virtuous circle as the presence of fewer sensory cues of others smoking would send a powerful message of what behaviours are appropriate and expected among adolescents.33 Constructing a clear identity of a generation that has a norm of a tobacco-free environment results in a prolonged effect among the cohorts sharing this experience.

Lowered risks of smoking among cohorts born after 1998 may have arisen from exposure to smoke-free policies in public places before the age of 14 years. It appears that intervening before adolescents enter secondary education is critical in the formation of an appropriate norm. This is due to the persistency of social norms. Once norms are solidly formed, they are unlikely to change for long periods of time.34 The average age of smoking initiation in Korean adolescents is 13 years,35 and the rate of smoking initiation increases more than threefold when adolescents advance to middle school.36 An increased presence of smoking peers may obstruct the formation of a generation with a smoke-free identity despite policy changes. Those born in 1997 and 1998 had reached the age of 15 and 14 years in 2012, and, as ‘first impressions matter,’ those who had been exposed for as long as a decade to schools where teachers and visitors smoked freely may have already established a norm about smoking before the time when complete bans were implemented. The gap between the younger, non-smoking generation and the relatively older, smoking generation can trigger a ‘norm cascade’37 or the view of smoking as an outdated concept.38

The marked reduction in smoking initiation rate, and the small increase in smoking cessation after the implementation of smoke-free legislation, demonstrate that cohort effects can influence adolescents to avoid ever trying a cigarette. Thus, positive results in terms of the reduction of smoking prevalence can be expected following the proposal for a tobacco-free generation38 which restricts tobacco sales and supply to adolescents by birth year to prevent smoking initiation.

It is intriguing that higher risks of smoking were found among adolescents who were only exposed to smoke-free legislation in their latter school years. Although additional work is required before an understanding of this phenomenon, speculations can be made based on previous studies. First, policies designed to restrict certain behaviours, such as the ban on smoking in public places, can backfire in adolescents due to arousal of a motivation to regain behavioural freedom,39 particularly when considering the inherently rebellious nature of this group. Second, the onset of early puberty, which is associated with higher risks of smoking,40 is more common among recent cohorts. For example, early menarche in girls increased from 17.2% for those born in 1988 to 24.6% for those born in 1998.41 Third, the behaviour of these cohorts may have been influenced by macroenvironmental events. The severe economic crisis in 1997, and its aftermath, could have reduced parenting quality, increasing the risk of smoking among children.42 Furthermore, tobacco sales were at a record high of 5.3 billion packs in 1997, where sale statistics started being documented in 1985.43

From our results, we recommend the use of birth year-specific statistics to examine cohort effects in smoking behaviour, in addition to the age-specific or year-specific statistics used conventionally in such studies. Furthermore, future research should focus on defining the characteristics and determinants of a generation where tobacco is greyed out, together with investigating the process of accepting this generational identity and the potential resistance to the initial implementation of tobacco control policies.

This study is subject to limitations. First, only speculations can be made about the dynamics of the observed changes due to the ecological design and descriptive nature inherent in the APC model. Still, APC models can provide valuable insight about macroenvironmental factors inducing temporal changes. Second, the study may not represent all adolescents in South Korea as participants of the survey were limited to those attending school. Nevertheless, less than 2% of people aged 12–17 years were reported not to be enrolled in school.16 In addition, the study relied on self-reported measures of tobacco use rather than biochemically verified measures. Self-reporting of tobacco use by adolescents can result in under-reporting; however, the estimates of self-reported smoking in the KYRBS exhibited good validity when compared with urine cotinine estimates.44

In conclusion, our study demonstrates that smoke-free legislation is associated with substantial declines in smoking prevalence among adolescents. More importantly, the changes in smoking prevalence from generation to generation indicate that tobacco control policies can create social norms in which smoking is viewed as an outdated behaviour.

Discussions on attaining a tobacco-free generation have mostly centred on legislative measures, such as restricting tobacco sales and supply by birth year.38 However, generational norm against tobacco can be constructed through establishing a ‘collective consciousness’10 of tobacco use as an unacceptable behaviour, in addition to legislative measures, as exemplified by the presence of cohort effects reducing smoking among South Korean adolescents during the past decade. More research on norm-changing tobacco control measures is still required.