1 Introduction

Social capital (SC), “resources that are accessed by individuals as a result of their membership of a network or a group,” [1] has been considered a determinant of health [2]. People obtain various resources from social relationships, and these may affect their lives. Such phenomena are sometimes serious, and the consequences may reach beyond health. In January 2018, Tracey Crouch was appointed as the first Minister for Loneliness in the UK in move that is indicative of the worldwide concern about loneliness—the lack of social relationships—not only in terms of its effects on health, but also for the effects on the lives of individuals and on society. This chapter introduces SC with a focus on its effects on health.

2 The Roseto Effect

In the 1950s, a strange phenomenon was discovered in the small town of Roseto, Pennsylvania, in the USA. Many of the residents of Roseto were Italian emigrants, who lived in a close-knit community. Although their lifestyles were not superior to their neighbors, it was observed that the residents of Roseto showed lower mortality from myocardial infarction than neighboring areas [3].

SC was considered to be the cause of the lower mortality in Roseto [4]. SC is a concept that has been used in various academic fields. Putnam [5] defined SC as “features of social organization, such as trust, norms and networks that can improve the efficiency of society by facilitating coordinated actions.” Until now, epidemiology researches have reported that people who live in communities with deep trust, helping each other and enjoying social participation, have better health conditions.

3 Social Capital

There are various definitions and measurements of SC, because it has been discussed and considered in the fields of sociology, economics, and political science [6, 7]. Recently, the definition mentioned above, “resources that are accessed by individuals as a result of their membership of a network or a group,” [1] seemed suitable for social epidemiology.

In addition to this definition, there are many classifications and subordinate concepts in SC [8, 9], including classification by cognitive SC and structural SC. Cognitive SC includes cognitive components of SC such as social trust and social support, while structural SC includes concepts related to network structure such as social networks and social participation. Classification as horizontal SC and vertical SC focuses on the structure of networks: SC obtained from horizontal and vertical networks can be distinguished. Classification as bonding SC, bridging SC, and linking SC focuses on the characteristics of networks: bonding SC is obtained from close and intense social ties; bridging SC is obtained from weak and diverse social ties; and linking SC is obtained from different power levels and positions. Following the background and hypothesis of a social epidemiological study as needed, these classifications and subordinate concepts should be selected as appropriate.

Figure 17.1 shows the theoretical explanation of the pathways between community-level SC and health. Living in a community with rich SC produces various resources. Social influence and informal social control affect health behaviors. Collective efficacy and social security contribute to establishment of health care policies. SC also buffers psychological stress. These mechanisms from community-level SC are considered to promote the health of residents [1].

Fig. 17.1
figure 1

Theoretical explanation of the pathways between community-level social capital and health

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4 Individual and Community Social Capital and Multilevel Analysis

In social epidemiology, the distinction of individual-level SC and community (neighborhood)-level SC is an important issue. Studies often use components of SC such as social participation, social trust, and social networks as the variables of individual-level SC. Aggregated values of individual-level SC or community variables such as voting rate are often used as the variables of community-level SC.

Social epidemiology studies using multilevel analysis have revealed that SC in the community affects the health of residents regardless of individual-level SC. Such types of multilevel studies are interesting because they indicate that the health of an individual is not only determined by the individual’s characteristics but also by the social environments in which they live. Such community-level SC also contributes to health inequalities between areas or groups. This community-level effect is called the “contextual effect”. By contrast, the “compositional effect” causes some health inequality owing to differences among individuals in areas or groups. The concept of the contextual effect was new when SC studies began in the health field. Therefore, studies of SC have emphasized community-level SC.

However, studies of individual-level SC also seem to be important, especially when determining the mechanism of SC in health. Recent studies of social epidemiology seem to focus more on individual-level variables related to SC.

5 Social Capital and Health: Benefits and Downsides

Since the late 1990s, associations between SC and various health outcomes have been examined. Many studies of SC have considered effects on mental health, but other studies have also focused on other outcomes. SC is considered to affect mental well-being and to reduce the risk of mental health problems [10,11,12], mortality [13], and chronic noncommunicable diseases such as diabetes [14, 15], cardiovascular disease [13, 15], cancer [13], and oral diseases [16]. Recently, intervention studies relating to SC have suggested beneficial effects of SC on the health of older people [17, 18]. Although the associations of SC and health are not always robust, and high-quality research is required, SC generally seems to be beneficial for health.

Studies have also suggested a buffering effect of SC on socioeconomic inequalities in health [19, 20], and SC is considered a beneficial resource for health in developing countries [21]. In spite of these positive effects, we should also consider the potential downsides of SC in terms of health [22]. A systematic review reported behavioral contagion cross-level interactions as the primary negative effect of SC on health. As a behavioral contagion, SC may contribute to the diffusion of harmful health behaviors and negative health. A study that examined cross-level interactions reported that people with low trust experienced more harmful health effects in high-trust communities. Such downsides should be considered when SC is used for health promotion.

6 Social Capital Studies in Japan

There are relatively large reports of SC and health in Japan, even though SC studies have mainly been conducted in Western countries [23]. Although fewer longitudinal studies of community-level SC were reported, several longitudinal studies have been conducted in Japan; the outcomes considered were functional disability [24], cognitive decline [25], and oral health [26]. Many studies focused on the components of individual-level SC such as social support, social participation, social networks, and social trust. There were also studies of the development of measurements of community-level SC [27]. As a result of these studies, the Japanese government includes the concept of SC in the health field, especially in the prevention of functional disabilities among the older population. Therefore, levels of SC in communities are sometimes measured for health policy planning. In this section, two recent important topics are introduced: SC intervention and SC in the disaster context.

6.1 Intervention Study of Social Capital and Health

Possibilities for interventions that improve SC and promote health have been reported [17, 18]. In Japan, local governments and researchers in the Japan Gerontological Evaluation Study (JAGES) conduct a community-based intervention program that aims to improve SC and health. The Taketoyo town government has established community salons, where older people participate in several activities; the salon is managed by older volunteers. This community-based intervention program provides the opportunity for improved social participation and interaction among older people and reduces the problem of being homebound, which increases the risk of frailty. Its effects on health were evaluated by researchers, showing reduction in the risks of poor self-rated health [28], onset of functional disability [29], and cognitive decline [30]. This kind of community intervention program has been introduced to other municipalities in Japan, and further reports on its effects in other municipalities are anticipated.

6.2 Social Capital and Health in a Post-disaster Context

The health effects of natural disasters are enormous around the world, making it an important public health issue [31]. Disasters change various social determinants of health including SC. SC is considered to play an important role in mitigating the health effects of disasters [32,33,34]; for example, as in the Great East Japan Earthquake and Tsunami in 2011 during which 15,894 people lost their lives, and 2546 people remain missing. The Tohoku area was the main affected area, and one of the areas, Iwanuma City, was the site of a JAGES cohort health survey before and after the disaster. Studies in Iwanuma contribute important insights to SC and health in a post-disaster context.

SC prior to the disaster contributed to preventing incidents of post-traumatic stress disorder following the disaster [35]. Because the tsunami destroyed housing, residents were obliged to move to other residences. The detrition of SC owing to the disaster increased the risk of cognitive decline [25], and obligatory relocation changed the SC of disaster survivors [36]. As an important implication of this study, group relocation to temporary housing with neighbors by the government aimed at maintaining social networks of neighbors, and was reported to protect SC. Other studies showed that physical exercise and participation in sports activity groups reduced the risk of depression after a disaster [37]. Therefore, intervention to provide opportunities for sports activity group participation may reduce risk of depression among disaster survivors.

7 Summary

SC is considered a social determinant of health. SC in a community affects the health of residents regardless of individual characteristics. Recent studies have suggested the possibility that intervention to improve SC reduces health risks. SC is also important in the post-disaster health of survivors. Studies are required to determine the effect of SC on health as a health promotion resource of communities.