The research literature relating positive well-being with physical health is growing
(Pressman and Cohen, 2005; Veenhoven, 2008). Theoretically, the strongest research design for establishing causality is the experimental study, in which participants are assigned at random to different levels of positive well-being and tracked for health outcomes. Although several controlled positive psychological intervention studies have now been conducted, the emphasis to date has been on subjective well-being and the alleviation of depression rather than on physical health and biological outcomes (Sin and Lyubomirsky, 2009).
One of the stronger population-based research designs for studying predictors of the development of physical illness is the prospective epi-demiological cohort study. This involves recruiting a sample of initially healthy individuals, assessing positive well-being (along with traditional risk factors for disease), then tracking the cohort over several years. The relationship between positive psychological factors and future illness can then be investigated. It is essential in such a study to have as complete a follow-up of participants as possible, since loss to follow-up may distort the results. It is also important to assess potential confounders. This has been a limitation in some studies of positive well-being. For example, Danner and coworkers (2001) published a well-known study of elderly Catholic nuns, showing that those whose writings in early life contained high levels of positive emotional content had a reduced risk of mortality when the participants were aged over 75 years.
Interesting though this association is, it supplies weak evidence for a protective role for positive emotions; no health assessments were performed at baseline, so it is possible that the nuns whose writings had less positive content had clinical or subclinical health problems before the writing task was carried out; only women who survived to age 75 were included, so it is conceivable that a different association between well-being and survival was present earlier in life; and apart from age and education, no other factors that might potentially influence either positive emotion or future mortality were controlled.
Fortunately, other studies provide stronger evidence. A good example is Kubzansky and Thurston's (2007) study of emotional vitality and coronary heart disease (CHD). A cohort of 6025 men and women aged 25-75 years who were initially free of CHD were followed for an average of 15 years, during which time 1141 developed CHD. Emotional vitality, a combination of vitality (sense of energy and pep), positive well-being (happiness and life satisfaction), and emotional self-control (feeling emotionally stable and secure), was assessed at baseline. Participants with greater emotional vitality were at markedly reduced risk for CHD, and this effect remained significant after accounting statistically for age, gender, ethnicity, marital status, educational attainment, blood pressure, cholesterol, body mass index (BMI), smoking, alcohol use, physical activity level, diabetes, hypertension, and psychological illness. All three components of emotional vitality appeared to contribute to the health outcomes in this study. In the model adjusting for risk factors, the relative risk of CHD in the highest compared with lowest tertile of emotional vitality was 0.74 (95% confidence intervals 0.64-0.85), indicating a 26% reduction in relative risk.
These effects are not only observed in Western populations. An analysis was conducted of 88,175 Japanese men and women aged 4069 years at baseline who were followed up for an average of 12 years (Shirai et al, 2009). A simple rating of enjoyment of life that correlates well with more elaborate measures of happiness was administered. Over the follow-up period, 3523 had newly diagnosed cardiovascular disease, and there were 1860 fatalities. Low enjoyment of life was associated with increased risk of cardiovascular disease incidence (hazard ratio 1.23, C.I. 1.05-1.44) and mortality (1.61, C.I. 1.32-1.96) in men, after adjustment for age, occupation, BMI, smoking, physical activity, alcohol consumption, diabetes, hypertension, and participation in health screening. Interestingly, effects were maintained when the deaths within the first 6 years were excluded, arguing against the possibility that some participants were already sick with the early stages of cardiovascular disease (and therefore unhappy) at the start of the study. It is not clear why there were no significant associations among women, but their low disease rates may have been responsible.
We published a meta-analysis of prospective studies relating psychological well-being with mortality in 2008 (Chida and Steptoe, 2008). Twenty-six articles involving initially healthy populations and 28 articles studying people with an established illness such as head and neck cancer or HIV/AIDS were identified. The follow-up periods ranged between 2 and 44 years in the healthy population studies and 1-20 years in studies of illness groups. We found that positive affect and positive traits such as optimism and hopefulness were associated with reduced mortality, with stronger effects in healthy populations (adjusted hazard ratio 0.82, 95% C.I. 0.76-0.89, p < 0.001) than in those with existing illnesses (hazard ratio 0.98, C.I. 0.95-1.00, p = 0.03). These associations persisted when negative affect was controlled, and were also strong in studies that were judged to be of high quality on the basis of criteria such as the measurement of covariates and the rigor of outcome ascertainment. However, there were indications of publication bias, implying that studies finding a positive association were more likely to be published than those which did not. It is also possible that, despite controlling for standard risk factors such as age, socioeconomic position, gender, and marital status, other unmeasured factors were responsible for the apparent protective effects.
Positive affect is related to other severe health outcomes as well. For example, Ostir and colleagues (2001) have reported that positive affect was associated with a reduced risk of stroke in a population of older men and women after controlling for relevant covariates, and in another analysis with lower blood pressure and less hypertension (Ostir et al, 2006). A study of older patients with coronary artery disease demonstrated that functional decline was reduced in those reporting high levels of positive emotion (Brummett et al, 2009b). Cohen and colleagues (2003, 2006) have used experimental exposure to infectious respiratory pathogens such as influenza virus, in order to analyze the role of emotional states under relatively controlled conditions. In these studies, volunteers were administered standard doses of virus and were then quarantined for a number of days to assess the development of objective illness. It was found that participants with a more positive emotional style (those with high levels of positive affect over several days) had reduced risk of developing upper respiratory illness. Interestingly, these effects were independent of optimism, extraversion, self-esteem, purpose in life, and other covariates, suggesting rather specific associations between positive affect and health outcome.
Not all studies of positive affect or happiness have shown health protective effects (Eaker et al, 2007; Lampert et al, 2002). Nevertheless, from this brief review it can be seen that much of the evidence to date does suggest that there are important associations between positive well-being and health outcomes, and that these are not merely the mirror image of the effects of depression and distress.
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