Our analysis of psychological ratings revealed that the mean subjective happiness score was similar to that reported in a previous study19. The correlation analysis revealed that subjective happiness was negatively associated with negative emotional traits, as previously reported18, but not with the negative emotional states. The regression analysis revealed that the subjective happiness score could be explained by the combined intensity of positive and negative emotions and purpose in life scores. Our results support previous theoretical and empirical data1 and indicate that subjective happiness is a stable construct consisting of emotional and cognitive components.

More important, our analysis of structural MRI data revealed that the subjective happiness score was associated with increased gray matter volume in the precuneus. This finding is largely in line with those of previous functional neuroimaging studies showing that the induction of happy emotional states was associated with activation of the medial parietal cortex4,5,7,8,9. However, these previous studies investigated a transiently induced happy mood state, whereas subjective happiness, which is a more complex and stable subjective experience, has not been evaluated. Involvement of the precuneus in subjective happiness is consistent with empirical and theoretical findings regarding this region. Previous functional neuroimaging studies have shown that the precuneus region has the highest level of cortical glucose metabolism in the brain, highlighting the importance of this region for subjective consciousness in humans20. Several neuroimaging studies revealed that the precuneus is involved in changes in subjective experience (e.g., the gradual transition of consciousness level following the induction of anesthesia21,22), suggesting that this region may be involved in the production and/or alteration of subjective experiences20,23. Although this psychological function fits well with the concept of subjective happiness, no previous study investigated precuneus involvement in subjective happiness. To our knowledge, our study is the first to show that the precuneus is associated with subjective happiness.

Furthermore, our results revealed an association between the precuneus and the combined positive and negative emotional intensity and purpose in life scores. These results indicate that the widely accepted psychological model postulating emotional and cognitive components of subjective happiness24 may be applicable at the level of neural structure. A previous neuroanatomical study in monkeys has found that the precuneus receives projections from widespread cortical and subcortical regions25. Several neuroimaging studies in humans also revealed that the precuneus is involved in the default mode network and communicates with widespread brain regions22,26. Moreover, the precuneus is involved in self-referential processing, which integrates the information of one’s current internal experience, past memory and future plans20,27,28. Thus, the region is thought to be involved in the integration of internal and external information22. Together with these findings, our results suggest that the precuneus may play an important role in integrating different types of information and converting it into subjective happiness.

Our finding that subjective happiness can be predicted using structural measurements has practical implications. Interest in subjective happiness has increased as its importance as the “the ultimate currency” and association with life success and good physical health has been highlighted by recent psychological studies29. In terms of public policy, subjective happiness is thought to be a better indicator of happiness than economic success30. However, the subjective measures of happiness have inherent limitations, such as the imprecise nature of comparing data across different cultures and the difficulties associated with the applications of these measures to specific populations, including the intellectually disabled31. Our results show that structural neuroimaging may serve as a complementary objective measure of subjective happiness.

Although our results provide evidence for the existence of a neural substrate underlying subjective happiness, they do not show that the construct is unchangeable. On the contrary, previous structural neuroimaging studies have shown that training in psychological activities, such as meditation, changed the structure of the precuneus gray matter32. These findings are consistent with those of previous studies showing that meditation training increased subjective happiness33. Together with these findings, our results suggest that psychological training that effectively increases gray matter volume in the precuneus may enhance subjective happiness.

Our study has several limitations, some of which may account for the discrepancies between our results and those of previous studies. First, although the precuneus was the only brain region significantly associated with subjective happiness in our study, several previous functional imaging studies found that other brain regions, such as the anterior cingulate gyrus and amygdala, were active during the induction of happy emotions4,5,6,7,8,9. This disparity may be explained by the methodological differences among the studies when measuring hemodynamic responses versus gray matter volumes. Alternatively, it may be that our small sample size lacked the power to detect an association of subjective happiness with other brain regions. Consistent with this notion, we found non-significant negative associations between subjective happiness and various brain regions, such as the insular cortex, previously reported to be involved in negative emotional states10. Future studies with a larger sample size may reveal the involvement of other brain regions and neural networks in the experience of subjective happiness.

Second, although we found that the cognitive and emotional components of happiness were associated with precuneus volume, recent structural MRI studies using a different methodology to investigate the structural neural substrates of the cognitive component of happiness reported different findings34,35. Specifically, these studies found that eudaimonic well-being36 was positively associated with right insular cortex volume35 and that life satisfaction37 was positively associated with right parahippocampal gyrus volume and negatively associated with left ventromedial prefrontal cortex and left precuneus volumes34. We speculate that the cognitive component of happiness may contain multiple subcomponents and our use of a single purpose-of-life measure may not tap into all of these subcomponents. Further studies are needed to clarify the relationships between cognitive subcomponents of happiness and their associations with structural neural substrates.

Finally, the present study assessed the psychological ratings of the participants after the MRI scans and this procedure may not have been ideal for the measurement of subjective happiness as a stable construct. It is possible that the MRI acquisition procedure altered the transient mood of some participants which, in turn, had a confounding effect on the subjective happiness ratings. Future research that measures subjective happiness under calm conditions independently from MRI scanning would be preferable for rigorously examining the structural neural substrates of subjective happiness.

In conclusion, our investigation using structural MRI and questionnaires revealed a positive relationship between the subjective happiness score and gray matter volume in the right precuneus. Furthermore, we found a correlation between the combined purpose in life and positive and negative emotional intensity scores and gray matter volume in the same region. Our findings suggest that the precuneus mediates subjective happiness by integrating the emotional and cognitive components of happiness.