The associations between genetic factors
The associations between genetic factors and socio-emotional functioning are complex and almost certainly not direct. They are mediated through a variety of developmental processes including individual differences in personality traits (Davis and Loxton, 2013, Saphire-Bernstein et al., 2011). For the COMT gene, the genotype effects on personality traits may be more prominent than its influence on socio-emotional functioning (Calati et al., 2011); thus personality traits may serve as intermediate phenotypes that bridge the gap between the COMT gene and socio-emotional functioning. As demonstrated in the current study, the effects of the COMT gene on well-being and depression are substantially mediated by dispositional gratitude and forgiveness, two positive personality traits that serve as keys to pursuing well-being and combating depression (Bono et al., 2008, Reed and Enright, 2006; Wood et al., 2008a; Wood et al., 2008b). In recent years, positive psychology has paid extensive attention to the measurement, application, and improvement of the 24 positive personality traits that contribute to human well-being (Park et al., 2004, Peterson et al., 2005, Peterson and Seligman, 2004, Seligman et al., 2005). They found that individuals vary considerably in positive personality traits (Peterson et al., 2005) and that a large portion of well-being could be accounted for by the 24 positive personality traits (Park et al., 2004). However, little attention has been paid to the genetic basis underlying the heterogeneity of positive personality traits as well as the potential mediating roles of these traits in the impacts of genetic and environmental factors on well-being and depression. The current findings suggest that a large portion of genetic contributions to well-being shown in the twin studies (Lykken and Tellegen, 1996, Røysamb et al., 2002, Stubbe et al., 2005) results from the effects of the GSK1904529A on positive personality traits. One might wonder how the gene polymorphism affects positive personality traits. Given the increased negativity bias in the Met allele carriers (Gao et al., 2016, Kia-Keating et al., 2007, Ohara et al., 1998, Smolka et al., 2005, Williams et al., 2010) and the detrimental effects of the negativity bias on the development of positive personality traits (Hanson, 2013), our findings suggest that the COMT Met allele predisposes individuals to be hyposensitive to positive life events but hypersensitive to negative life events. These individuals may gradually form, over the developmental course of life, a habit of neglecting the positive aspects of life events and complaining about misfortunes, resulting in decreased positive personality traits, such as gratitude and forgiveness. As for how gratitude and forgiveness promote well-being, Bono and McCullough have given extensive review on the issue (Bono and McCullough, 2006). Previous studies have shown that non-genetic factors, such as socioeconomic status, religious beliefs, relationship status, and parenting environment, are crucial for the cognitive, affective, and psychological aspects of well-being (Dush, 2005, Ellison, 1991, Huppert et al., 2010, Pinquart and Sörensen, 2000). Nevertheless, our results showed that the effect of the COMT gene on well-being and the mediated paths from the gene through gratitude and forgiveness to well-being continued to hold after controlling for these environmental factors. This finding suggests that COMT may play an important role in fostering positive experiences and attitudes toward life independently of environmental contributions. Several limitations of this study should be noted. First, like all candidate gene association studies (Ebstein et al., 2012), the sample size of this study was relatively small, particularly for the male group. As women in general report lower well-being than men (Pinquart and Sörensen, 2001), we examined whether the observed genotype effects depended on gender but found a non-significant gene×gender interaction. However, because of the relatively small sample size for the male group, this study could be underpowered to detect a small interaction effect in the two-way between-subjects ANOVA. As such, further studies with larger samples are needed to investigate the potential gene×gender interaction. Second, COMT Val158Met polymorphism accounted for 1.2% variance of well-being while the heritability of well-being was estimated at 38–54% (Lykken and Tellegen, 1996, Røysamb et al., 2002, Stubbe et al., 2005), suggesting that well-being are likely to be influenced by multiple polymorphisms (Okbay et al., 2016). It would be important for future studies to consider simultaneously a variety of genetic factors underlying individual differences in well-being. Finally, all the participants in the current study were Chinese. Therefore, our findings should be interpreted with caution in generalizing to other populations. As some studies have showed that the associations between genes and positive personality traits can be modulated by culture (Kim et al., 2011, Kitayama et al., 2014), it would be interesting to investigate the potential cultural differences in the relationship between COMT polymorphisms and well-being.