Most research on mental health remains rooted in deficit models and focuses on mental ill health, e.g. anxiety, depression and stress rather than on well-being, e.g. emotional and psychological functioning and satisfaction with life. This is also true for research on adolescents’ mental health. Although time trends differ regarding an increase and stability across countries in Europe, Israel and North America , a clear, however minor increase in mental health complaints is evident for countries in Northern Europe since the 1980s . In Sweden, young people aged 16–29 have reported the highest levels of deteriorated mental health, i.e. symptoms of anxiety, depression, and stress, over the last decade compared to any other age group . The reasons for these heightened levels of self-reported symptoms are yet unclear, but two possible explanations stated in a literature review have gained the most recognition ; the first is a deterioration of the Swedish school system, which has undergone several changes in recent decades including a transformation from a centralised public system to a decentralised system and school choice. In parallel, a new school curriculum was introduced with an increased use of assessments, more strict achievement criteria and high-stakes national tests . These reforms might have led to a decline in school performance and increased degrees of internalised problems among children and adolescents. The second possible explanation concerns the more unstable labour market, which has led to higher demands on education and skills. It is probable that young people feel anxiety and stress about not performing well and about their future position in the labour market. However, alongside higher reported levels of mental health complaints, students’ positive mental health in terms of life satisfaction has not declined, but has instead remained relatively stable since 2001/2002 . This may seem contradictory, but it is possible given that positive and negative mental health may be seen as a dual continuum, whereby an individual can experience well-being despite a status of mental ill health [7, 8]. For example, it is not plausible that a mental health status of diagnosed depression could coexist with high life satisfaction, positive affect and happiness, or low negative affect. On the other hand, symptoms of depression may be transient and certainly can co-exist with stable subjective well-being (SWB).
The concept SWB, referred to in everyday speech as happiness, peace, fulfilment and life-satisfaction , is comprised of emotional well-being in terms of a high degree of positive affect and a low degree of negative affect, as well as high levels of satisfaction with one’s life . SWB has been researched for four decades and has been shown to build resilience as well as being beneficial for health, longevity, work performance and supportive social relationships . During adolescence, a period characterized by rapid developmental changes, a high level of SWB is an important tool to navigate life successfully . A representative study from the United States (US) showed that high levels of life satisfaction and positive affect in adolescence and young adulthood correlated significantly with higher levels of income at about age 29 . In college students, high levels of SWB, albeit not the very highest, were associated with study achievement measured by grade point average . In a cross-sectional study among 7th and 8th grade middle school students in the US, those with the highest levels of SWB were also high study achievers . Intervention studies indicate that SWB may be improved through school-based interventions .
In addition to measuring individual well-being, SWB is likewise an indicator of societal well-being and varies across nations and according to gross national income [17, 18]. The link between social capital (specifically the dimensions of trust, social interaction, and norms and sanctions) and the individual happiness aspect of SWB has been investigated in 29 European countries . The authors found that all three dimensions matter for happiness, especially social interaction and general social and institutional trust. However, significant differences were found in how social capital interacts with happiness, and in the Nordic countries only trust was associated with happiness. Diener and colleagues have stated that SWB seems to be a necessary, but not sufficient, characteristic of a good life and a good society, and it should be complemented by economic and social indicators .
Associations between personality traits and SWB
Whereas demographic factors such as age and sex are weakly related to SWB, personality traits, together with health and socioeconomic factors, have strong associations according to meta-analyses and reviews [20,21,22,23]. The traits extraversion, agreeableness, conscientiousness, neuroticism, and openness to experience are included in the Big Five Model [24,25,26]. Until the late 1990s, research on personality traits indicated that the mean levels of personality traits change during development in childhood and adolescence, but are principally fixed by age 30 . Later studies rebut these findings and show that changes are possible over the life course and that trait stability continues to increase until after 50 years of age , although most changes occur before the age of 40 [29, 30]. Studies on the association between SWB and personality (regardless of study population age) suggest that extraversion is moderately to strongly correlated with positive affect [20, 31,32,33] and high life satisfaction . Research furthermore shows that neuroticism is associated with negative affect [34,35,36] and low life satisfaction . In contrast to previous research a recent meta-analysis shows strong associations between conscientiousness and SWB beyond neuroticism and extraversion . Over a period of one year, a test–retest correlation of approximately 0.76 in the measurement of SWB has been observed . Furthermore, over a period of four weeks, a test–retest correlation of approximately 0.88 in the measurement of Big Five personality traits was found .
The association between personality and SWB has been mostly studied in adult populations or among young adults [18, 20, 39]. In adolescents, the associations have mainly been assessed in cross-sectional studies without predictive possibilities. In studies on adolescents in corresponding age ranges with our population and conducted in a Western context, neuroticism (low levels), extraversion, and conscientiousness seem to be the strongest factors associated with SWB. This is also apparent in studies on adolescents from Sweden , and from the US [41, 42], whereas other studies have only identified significant associations between neuroticism (negatively) and extraversion, and SWB [43, 44] in Swedish and American samples. Nonetheless, a longitudinal study in adolescents on vocational education and training shows another pattern . In this German cohort with a mean age of 18 years, agreeableness and conscientiousness exhibited the strongest associations with life satisfaction at a follow-up measurement of 15 months as well as after three years.
Until recently a common assumption was that personality is relatively stable and causes well-being, but not the reverse [20, 21, 27]. This postulation has been challenged in a large national representative Australian sample of persons aged 15–93 ; Soto found that higher levels of extraversion, agreeableness and conscientiousness and emotional stability predicted subsequent levels of SWB. Additionally, high levels of initial well-being predicted agreeableness, conscientiousness, emotional stability, i.e. the counterpart of neuroticism, and introversion, i.e. the counterpart of extraversion, at a later point in time. However, it should be noted that although many of the prospective associations in Soto’s study were significant, due to a large sample (N = 16,367) they were weak, with only one standardized association exceeding 0.1 (0.137). In the present study we intended to investigate if Soto’s findings are maintained when examined in a younger age group, namely adolescents between the ages of 16 and 18. In summary, several scholars before us have longitudinally examined the correlation between personality traits and their effects on SWB over time. Nonetheless, whether SWB may also predict personality has to our knowledge only been investigated and confirmed by Specht  and Soto  in their studies on predominantly adult populations.
Purpose of the study
The purpose of this longitudinal study was to analyse associations and prospective effects of SWB on personality traits, and vice versa, over 15–18 month follow up in a cohort sample of girls and boys in secondary schools in Sweden.