Overall, more than half of the included studies showed a negative association between religiosity and mortality. The other studies reported mixed results; that is, they found a significant negative association only for specific subgroups of the population (e.g., women) or components of the spiritual activities.

Several mechanisms have been hypothesized to explain the link between religious attendance and longevity. Among these mechanisms, the potential mediating factors of social support, health behavior, and mental health were the most studied (Hill et al. 2005). For instance, religious attendance may reduce the risk of mortality, partly through the promotion of social contacts and social resources (Ellison and Levin 1998; George et al. 2002). In fact, people who regularly attend a religious community may benefit from greater social support, community involvement, and access to material and psychological help (Fraser et al. 2020), which can reduce stress and provide options for assistance that may also affect mortality (Ellwardt et al. 2015; Olaya et al. 2017).

Moreover, people who are more religious are likely to engage in healthier lifestyles (Ellison and Levin 1998; George et al. 2002). For example, religious involvement may deter drinking and smoking by increasing exposure to anti-abuse norms and peers and by reducing contact with deviant networks (Gorsuch 1995; Ellison and Levin 1998; Hill et al. 2005). Previous literature reviews also suggested that attendance at religious services is associated with better mental health and psychological well-being (Hackney and Sanders 2003), as religiously motivated expressions such as hope, forgiveness, altruism, and love have been proposed as psychological factors that may strengthen host resistance (Levin 1996) and satisfy the need for social contact and meaning in life (Oman and Reed 1998).

Although social support, mental health, and health behavior differences have often been suggested as the primary mechanisms of health benefit among religious people, several religious and mortality studies found that adjustments for those mediating factors did not fully account for the survival benefit that the religious participants received (Oman and Reed 1998; Koenig et al. 1999; Helm et al. 2000). This suggests that other important mechanisms that may connect religious involvement to reduced mortality are not yet understood.

Concerning the included studies that showed mixed results, two revealed gender-related differences, with religiosity being associated with better survival among women but not among men (Teinonen et al. 2005; Zhang 2008). The two studies were conducted in two countries, China and Finland, where the social component of religion is weaker than in other states. In particular, the fairly regularized religious lives typical of Western religions, characterized by attending weekly religious services and other activities, do not exist in China (Zeng et al. 2011), and Finland presents a large discrepancy between engagement in private prayer and public worship, with almost half of all Finns usually praying at least once a month, but only 14% of them attending religious services as often (Teinonen et al. 2005). Moreover, a previous meta-analysis revealed that the impact of religiosity on longevity was weaker in studies that used measures of private religious involvement (McCullough et al. 2000) and that the association between religious attendance and mortality was weaker among Finnish women than among the older population in the United States (Koenig et al. 1999), where the social component of religious activity is stronger. In addition, praying has been described as more helpful to females than males (Pargament 1997).

An interesting finding on the impacts of social religious activities was reported by McDougle et al. (2016) in their investigation of the effects of various coping strategies, including social (e.g., church attendance) and individual religious activities (e.g., prayer), on mortality risk. The participants were asked how often they typically sought comfort through praying or church attendance when they had problems or difficulties in their family, work, or personal life. As a result, a reduction in mortality risk was observed among people who attended church more frequently to cope with stress, whereas an increase in mortality risk was found among those who used prayer more frequently (McDougle et al. 2016). In other words, social approaches to religious coping appear to be more protective than individual approaches. This finding seems to support the protective role of social approaches to religion that may be a way for individuals to not only relieve their anxiety but also allow them an opportunity to obtain the relevant affirmations needed that will enable them to build their own coping abilities (McDougle et al. 2016). In general, it confirms the impacts that social integration and support can buffer against negative health outcomes (Thoits and Hewitt 2001).

On the other hand, Parks and colleagues reported that spiritual peace and not religious attendance was associated with lower mortality in a sample of patients with congestive heart failure (Park et al. 2016). This result is in line with a previous study that showed that the association between religiousness, particularly service attendance, and reduced risk of mortality was usually found in healthy subjects but not in populations already diagnosed with a serious disease (Chida et al. 2009). Similar results were obtained by Helm et al., who specifically investigated private religious activities and concluded that they had a protective effect only among participants with good functional ability and not among those with impaired performance in activities of daily living (Helm et al. 2000). One explanation proposed for this phenomenon is that religion may be more important in resisting disease than in helping people already diagnosed with a disease and undergoing treatment (Powell et al. 2003; Chida et al. 2009). However, spirituality was confirmed to be related to a lower mortality risk, even after considering many other variables (Park et al. 2016). The authors defined spirituality in terms of a sense of inner peace and harmony and focused specifically on the spiritual component of deep peacefulness (Park et al. 2016), which has been shown to be critically important to individuals with serious and life-limiting illnesses (Ironson et al. 2002; Steinhauser et al. 2006; Canada et al. 2008; Whitford and Olver 2012). This finding is consistent with those of other studies that linked spirituality and mortality in patients with serious illness, in which this inner experience of a sense of peace may matter most in terms of survival and exert the strongest protective effects on mortality risk (Ironson et al. 2002; Whitford and Olver 2012).

This systematic review has several limitations. First, the number of studies that assessed the association between spirituality and mortality among older adults was small compared with the number of those that focused on religious attendance. Further studies are needed to better understand the possible link between spirituality and health status. Second, most of the included studies were conducted in the United States, where the Christian religion is predominantly practiced. Studies from other parts of the world and on other religious beliefs would broaden the generalizability of the results of the present study.