This study is focused on the biological correlates of individual differences in affective well-being, and we aggregated momentary samples to generate a single measure of happiness. The complete range of possible happiness levels was covered from individuals who never rated themselves happy to those with positive happiness ratings on all samples. As might be expected, happiness levels were higher on the leisure than working day, and individual differences were consistent across days. People who were happy most of the workday were also happier during the leisure day. Aggregation of the momentary assessments appears therefore to have generated relatively robust estimates of positive affective state. A complementary approach is to study within-subject covariation of positive affect with biology, but this involves different analytic methods (33). As anticipated, happiness was inversely associated with psychological distress measured by using the GHQ. Substituting subjective stress ratings for the GHQ as an indicator of distress did not alter the associations between positive affect and biology.
The relationship between reduced cortisol and positive affect is potentially relevant to health. Cortisol is a key stress hormone related to a range of pathologies including abdominal obesity, Type 2 diabetes, hypertension, and autoimmune conditions (34, 35). The average difference in cortisol of 32.1% between the lowest and highest happiness quintiles is substantial and might contribute to health risk if it persists over months or years. Because high cortisol is characteristic of some depressed individuals (36), it is important to note that the link with happiness was independent of negative affect in this study.
Elevated heart rate has been shown to predict mortality and cardiovascular disease risk in prospective epidemiological studies (37, 38). The associations we found with happiness in men were independent both of standard covariates and of ratings of physical activity that have previously been shown to correlate with objective energy expenditure (27). The explanation of the gender difference in heart rate is not clear, but observational epidemiological studies have shown consistent associations between mortality and heart rate more in men than women (38).
The laboratory phase of the study involved assessment of subjective, cardiovascular, and inflammatory responses to standardized behavioral tasks. The advantages of this method of assessing psychobiological responses have been well documented (39). Laboratory stress testing permits the measurement of sophisticated markers of biological activation under controlled conditions, eliminating variability due to factors such as ongoing activities, physical environment, consumption of caffeine and tobacco, that complicate naturalistic ambulatory monitoring. Assessment of fibrinogen responses in everyday life would be very difficult because of the need to take repeated blood samples. By imposing standardized behavioral challenges, individual differences in stress responsiveness can be revealed. Interestingly, happiness was not related to subjective or cardiovascular stress responses, nor did appraisals of task difficulty and controllability vary with happiness. We had expected that happier individuals might feel less stressed by the behavioral tasks, but this was not the case. This finding reinforces the observation that happiness reports were partly independent of negative affect by showing that acute episodes of stress were not related to happiness. In analyses that have not been presented in detail, we failed to observe any association between happiness and poststress cardiovascular recovery, as reported by Tugade and Fredrickson (19).
Plasma fibrinogen is an inflammatory marker and predictor of future coronary heart disease, heightening risk through increasing blood viscosity, infiltration of the arterial wall, stimulation of atherogenic cell proliferation, and platelet aggregation (40, 41). Although the fibrinogen stress responses were small in absolute terms, the relative increase was >12 times greater in the lowest compared with highest happiness group. If differences of this magnitude are elicited in everyday life when people are exposed to daily hassles and challenges, the result could be a marked difference in cardiovascular disease risk.
Research in positive psychology has begun to document the significance of positive well-being to creativity, leadership, and the realization of human potential (5, 42). Our findings indicate that positive affective states are related to favorable profiles of functioning in several biological systems and may thereby be relevant to risk of development of physical illness. The participants in this study were in relatively good health, so no objective health outcomes were analyzed.
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