Happiness has been viewed as a temporary emotional state (e.g., pleasure) and a 
relatively stable state of being happy (subjective happiness level). As previous 
studies demonstrated that individuals with high subjective happiness level rated 
their current affective states more positively when they experience positive 
events, these two aspects of happiness are interrelated. According to a recent 
neuroimaging study, the cytosine to thymine single-nucleotide polymorphism of 
the human cannabinoid receptor 1 gene is associated with sensitivity to positive 
emotional stimuli. Thus, we hypothesized that our genetic traits, such as the 
human cannabinoid receptor 1 genotypes, are closely related to the two aspects 
of happiness. In Experiment 1, 198 healthy volunteers were used to compare the 
subjective happiness level between cytosine allele carriers and thymine-thymine 
carriers of the human cannabinoid receptor 1 gene. In Experiment 2, we used 
positron emission tomography with 20 healthy participants to compare the brain 
responses to positive emotional stimuli of cytosine allele carriers to that of 
thymine-thymine carriers. Compared to thymine-thymine carriers, cytosine allele 
carriers have a higher subjective happiness level. Regression analysis indicated 
that the cytosine allele is significantly associated with subjective happiness 
level. The positive mood after watching a positive film was significantly higher 
for the cytosine allele carriers compared to the thymine-thymine carriers. 
Positive emotion-related brain region such as the medial prefrontal cortex was 
significantly activated when the cytosine allele carriers watched the positive 
film compared to the thymine-thymine carriers. Thus, the human cannabinoid 
receptor 1 genotypes are closely related to two aspects of happiness. Compared 
to thymine-thymine carriers, the cytosine allele carriers of the human 
cannabinoid receptor 1 gene, who are sensitive to positive emotional stimuli, 
exhibited greater magnitude positive emotions when they experienced positive 
events and had a higher subjective happiness level.