The goal of this study was to provide indirect evidence regarding causality in the association between declining religious attendance and health. If active religious attendance did improve health and promote better health behaviors, we would expect people’s health to get worse after they quit active religious attendance. Data from four prospective cohort studies provided little if any evidence to support this hypothesis.

Heavy alcohol consumption, smoking, physical inactivity, cannabis use, and body mass index increased, and self-rated health decreased, already in the 10-year follow-up period before the change from active to inactive religious attendance, and these trajectories did not become steeper in the following years of inactive religious attendance. These results lend support for a non-causal interpretation in which religious attendance is a correlate—not a cause—of poor health-behavior trajectories. It seems plausible that people who are on a life-course trajectory characterized by poorer health behaviors are also more likely to quit active religious attendance. Crucially, this non-causal interpretation appears more plausible than a bidirectional causal association between religious attendance and health, which has been suggested by some previous studies [11]. If religious attendance had been a causal protective factor, poorer health and health behaviors should have worsened even more after the participants gave up religious attendance, but this was not the case. This speaks against the hypothesis of bidirectional causality.

Smoking increased in the 10 years before the change from active to inactive religious attendance but remained flat afterwards. Similarly, the increase in BMI became less steep, and heavy alcohol consumption showed slightly (but not significantly) more gradual increase after versus before the change. These patterns were unexpected, and this study design was not equipped with measures to examine these trends with sufficient depth. It is possible that religious engagement induces psychological distress (and associated poor health behaviors) in some individuals [18], and that this distress is relieved after giving up religious engagement. More specific measures of religiosity would be required to test this hypothesis. For the other direction of change in religious attendance—from inactive to active—there was evidence for decreasing smoking and slightly improving self-rated health after starting active religious attendance.

None of the above is to say that people who attend religious services don’t have better health and health behaviors than those not attending. Previous studies have also reported poorer health among those who have left religion [19–21], but these retrospective studies have not been able to determine temporal precedence between health and leaving religion. The current longitudinal findings show that the unhealthy developmental trajectories were observed already several years before becoming religiously inactive, that is, during the time the participants still reported at least monthly religious attendance. It appears more likely that the poorer health among those who have left religion [19–21] represents adverse health trajectories preceding religious disaffiliation—and not its consequences.

It can be argued that most causal questions are meaningful only when considered against the backdrop of an intervention—even an imagined intervention that would be impossible to carry out in the real world [13]. This helps to clarify the specific causal claim being evaluated. The present study was motivated by the question of whether declining religious attendance will lead to deteriorating population health, and so the analysis focused on changes in health and health behaviors among individuals who initially engaged in religious activities but then became disengaged. It is safe to assume that most people who quit religious attendance in the United States do so voluntarily, without external coercion, so the study design should be suitable for answering the question concerning religious decline due to people leaving their religion. But the study design did not address all aspects of religious decline, as religious engagement may also decrease due to birth-cohort effects by which more recently born individuals are less religious to begin with [22]. It is unclear whether the present conclusions extend to birth-cohort effects, as different social and causal mechanisms might be involved. The focus on changing religious attendance also means that the participants might not have been representative of all religiously active and inactive individuals; study designs attempting to address specific causal claims often need to rely on non-representative sampling [23]. Other study designs would be needed to assess causality of religiousness among those who remain either religiously active or inactive throughout their life.

In 2018–2019, 65% of Americans identified themselves as Christians and 26% as religiously unaffiliated, which leaves only 9% for all the other major religions, such as Hinduism, Islam, and Buddhism [1]. The present study did not have large enough samples to examine differences between religions, so it is uncertain whether the present results generalize to other religions besides Christianity. Moreover, some Christian denominations (e.g., Mormons and Jehovah’s Witnesses) within the United States often have stricter social norms that might influence health behaviors more strongly than what was observed in the present study [20], but these denominations cover only a small proportion of the population.

It should also be emphasized that the current study focused on religious attendance and did not address religious disaffiliation, that is, the increasing prevalence of the religiously unaffiliated, or any other measures of religiosity/spirituality. By itself, religious affiliation is unlikely to have causal effects on health, whereas religious attendance has been shown to be the indicator of religiosity that has the strongest correlations with health, or at least with depressive symptoms [3]. Religious attendance has also been shown to account for the health correlates of religious affiliation [21], which further justifies the use of religious attendance as the measure of religiousness when examining health outcomes. Health changes associated with changes in other indicators of religiosity/spirituality should nevertheless be examined with similar study designs. People may exaggerate their frequency of religious attendance due to social desirability bias, which may have introduced measurement inaccuracy in our analysis [24, 25].

In sum, the present study suggests that the declining rates of religious attendance are unlikely to have a major influence on population health: most of the adverse health correlates of religious non-attendance were observed already many years before the participants gave up active religious attendance, and the change from active to inactive religious attendance did not change the trajectories of health or health behaviors for the worse. It is more likely that religious disengagement is an expression of broader life-course trajectory accompanied by poorer health and health behaviors.