The strongest links between positive emotions and health were found in studies that examined "trait" emotions, which reflect a person's typical emotional experience, rather than "state" emotions, which reflect momentary responses to events. People who typically report more positive emotions experience lower rates of chronic illness, symptoms and pain. Moreover, among the elderly who live on their own or with family rather than in retirement homes, positive emotional dispositions are linked to living longer. In contrast, positive emotions are not associated with increased longevity in studies of other populations, and though possibly beneficial for recovery from less serious diseases, extremely positive emotions are in some cases associated with poorer outcomes among those with serious illness.
"Overall, the literature suggests an association between positive emotions and some measures of good health, but there are many subtle weaknesses in these studies and it would be inappropriate to make any strong conclusions," Cohen said.
One problem in interpreting the literature is that in many cases, it is difficult to distinguish between the effects of positive and negative emotions. For example, do elderly living on their own or with family live longer because they are happy or because they are not sad? Interestingly, people's experiences of positive and negative emotions are partly independent in some circumstances. For instance, in looking back over the last month or year, one can reasonably report having been both happy and sad. A definitive answer to whether positive or negative emotions are contributing to a health outcome can only come from studies that measure both types of emotions and examine their independent effects. Consequently, it is difficult to conclude from the existing literature whether happiness leads to a healthier and longer life or unhappiness results in a less healthy, shorter one.
The authors also were concerned with the possibility that some measures of positive emotions may themselves be direct indicators of physical health. For example, adjectives such as "energetic," "full-of-pep," and "vigorous" may reflect a positive mood, but may also reflect how healthy one feels. Self-rated health has been found to predict illness and longevity above and beyond objective health measures such as physician ratings. Consequently, it is important for future research to include standard measures of self-rated health to help exclude the possibility that researchers are merely predicting good objective health from good perceived health masquerading as positive emotions.
Cohen and Pressman suggest that future research focus on determining how emotions "get under the skin" to influence health. In other words, what behavioral or physiological changes do positive emotions trigger to ward off illness? The authors propose that emotions can have a direct impact on health; for example, they may influence lifestyle choices, or the function of the immune and autonomic nervous systems. Alternatively, they suggest that positive emotions may also influence health by mitigating the harmful effects of stress.
"Overall, we consider this literature provocative but not definitive. It does not unequivocally indicate that positive emotions are beneficial for health, but instead suggests a more divergent view of when positive emotions may have positive, negative or no effects," Cohen said.