South Korea is one of the most rapidly aging countries in the world. According to the United Nations report, it is expected to have the highest predicted growth rate (23.0%) of population aged 65 years and above (hereinafter referred to as older adults) in East Asia [1]. The number of elderly Koreans increased from 6.757 million in 2016 to 8.125 million in 2020, and the share of older adult is expected to account for 42.5% of the Korean population by 2065 [2].

The population of older adults suffering from depressive symptoms is also increasing significantly due to the increase in their population. In fact, according to the Ministry of Health and Welfare [3], the proportion of people experiencing depression in 2017 was the highest (17.3%) among older adults aged 70 years and above. As the number of elderly people receiving medical treatment for depressive episodes increases, social and economic costs have also increased sharply from approximately 89 billion Won in 2016 to about 1.1 trillion Won in 2018, and depression is predicted to be among the top three diseases that would cause a global burden by 2030 [4,5].

Depressive symptoms negatively affect individuals’ various functional areas, including physical health [6]. In particular, depression in old age can cause chronic diseases to deteriorate, increase the likelihood of other psychiatric diseases such as dementia, and ultimately shorten the life expectancy, as well as reduce the quality of life [7,8]. Furthermore, mental illness such as depression can increase the risk of death [9,10].

Meta-analyses and longitudinal studies that analyzed the relationship between depression and risk of death in patients suffering from type 2 diabetes [11], heart diseases (including failure, acute myocardial infarction, cardiovascular diseases, etc.) [12,13], end-stage renal disease [14], stroke [15], and cancer [16] found that these patients’ risk of death increased significantly as they became more depressed.

As such, it has been reported relatively consistently that depressive symptoms are significantly related to the risk of mortality [11,17,18]. However, most previous studies had limitations in estimating the causal relationship between the two variables having used a cross-sectional design. In addition, the effect of depressive symptoms on mortality may vary depending on the age group, but a detailed study is lacking. Therefore, this study aims to verify the longitudinal effect of depressive symptoms on all-cause mortality after controlling age, education, sex, residential region, marital status, employment status, health insurance, and a number of comorbidities, as covariates. In addition, the longitudinal effects of depression between middle-aged adults (aged 64 and below) and a group of older adults were compared.

This study’s primary purpose was to examine the association between depressive symptoms and all-cause mortality in middle-aged and older adults using the longitudinal models for nationally representative data from 2006 to 2016 in South Korea. The following are the main results of this study.

First, this study used the 10-item version of the CES-D scale to assess feelings and behaviors during the past week, and categorized these items as two factors (negative and positive affects) using exploratory factor analysis. These results support the various studies that had analyzed CES-D [20,21], and found that it was composed of two factors.

Second, Q1 (high) of the negative affect factor was significantly associated with the increased risk of all-cause mortality, whereas Q1 (high) of the positive affect factor was not significantly associated with the risk of all-cause mortality. In particular, it was found that the risk of mortality increased significantly in both the middle-aged and older adults, when they experienced more negative affect factors. These results suggest there is a possibility that people who experiencing negative affects more than recognizing lack of positive things in daily life could be more vulnerable to the depression. Depressive symptoms were significantly associated with the risk of mortality, and these mechanism could be explained by response expectancy theory.

According to the response expectancy theory, negative mood regulation (NMR) expectancies could be defined as “the expectancy that some behavior or cognition will alleviate a negative mood state [22].” NMR expectancies are a major variable influencing not only how to control emotions when facing stress or negative affects in daily life, but also on how to interpret these emotions and choose the appropriate coping strategies, and are closely related to the severity and longevity of depressive symptoms [23,24]. When people with lower NMR expectancies experience negative feelings and behaviors in daily life, it is difficult to alleviate their negative moods because they tend to use avoidant coping strategies, and this maladaptive coping which worsens their depressive symptoms is likely to lead to a decrease in NMR expectancies again [22]. Additionally, depressed people who have a low-level NMR expectancies tend to see the future pessimistically and expect a shorter lifespan than others [8]. Since subjective expectations for the future play an important role in decision making [25,26], negative subjective expectations for the future can lead to hopelessness, low well-being, or negative lifestyles. People who suffered from psychological distress were more likely than nondepressed people to choose and maintain maladaptive behaviors, including a tendency to be less physically active, adopt more avoidance coping, become underweight/overweight, or indulge in substance abuse [6,12,25,27]. Also, since maladaptive behavior increases the likelihood of re-increasing depression, this negative cycle could be a cause of the increased risk of mortality.

Third, regarding age-specific associations with depressive symptoms and all-cause mortality, Q1 (high) of negative affect factors were likely to increase the risk of all-cause mortality regardless of age. Middle-aged adults who experienced negative feelings and behaviors in their life had a higher risk of all-cause mortality than older adults. This result could be interpreted by Korean social perspectives. According to the Korean Statistical Information Services [28], the participation rate in economic activities was 31.5% for those aged over 65 years, whereas it was 81.1% and 77.0% for those in their 40s and 50s, respectively. Older adults were often unemployed due to retirement and were more likely than other age groups to receive pensions, aid through government policies, or financial support from their children, which is why they experienced relatively less negative stress when they were socially and economically active [25]. However, middle-aged adults actively engage in economic activities and incur a large amount of expenditure due to child rearing expenses, including private education expenses [29], so they cannot discontinue their economic activities, even if they experience negativism in social activities. Thus, there is a possibility that middle-aged adults are more sensitive to negative experiences because they are more likely to be exposed to negative emotions, behaviors, and stress from relationships with others or from their surroundings, whereas older adults are more likely to develop depression due to loneliness caused by social isolation [30,31].

The limitation of this study is the result that Q1 (high) of positive affect factors were not significantly associated with the risk of all-cause mortality even though all the individual items were significantly associated with it, which may have been due to the relatively small number and vague content of items about positive feelings and behaviors. According to the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition [32], loss of interest or pleasure is essential symptom to diagnose major depressive disorder. However, factor 2 items that translated in Korean and used in KLoSA to check positive affect were not only small in number as two questions but also consisted of ambiguous content to measure loss of interest or pleasure (e.g., do you think you’ve been relatively doing well for the past week?) [33]. Therefore, future studies need to consider increasing the number and clarifying the content of items on positive affects and assessing other factors that may account for the association between depressive symptoms and mortality, such as family medical history or kinds of occupation.

Despite these limitations, this study has strengths and contributions. In particular, we used a nationwide representative sample and the 10-year follow-up database. These findings suggest that confirming the longitudinal effects of the negative affect factor on the risk of mortality provide clear information on depressive symptoms influencing the increase in allcause mortality even when covariates such as demographic variables and health status are in control, as against the limitation of previous studies that focused only on the relationship between depression and suicide. In addition, through exploratory factor analysis, it was confirmed that the negative affect had a more significant effect on the worsening of depressive symptoms and the increase in mortality than the positive affect on the relief from the depressive symptoms and mortality. Also, middle-aged adults could be more sensitive to negative feelings and behaviors than older adults. Therefore, the results of this study suggest that specialized psychological services or programs for every age group are needed to help in adaptively dealing with negative feelings and behaviors in daily.