Introduction Subjective well-being is an important psychological construct linked to various relevant life outcomes. Diener (1984) proposed the original conceptualization of the term subjective well-being by defining it as a multidimensional construct consisting of three components – life satisfaction, positive affect, and negative affect. Those three components of subjective wellbeing are considered theoretically diverse and independent constructs that separate in factor analyses and have different patterns of relations with other variables. So, for example, the cognitive component is less determined by the personality traits than the affective component. The positive and negative affect are also theoretically separated constructs, i.e., not just the opposite poles of one affective dimension. Therefore, in order to adequately assess subjective well-being, it is essential to measure all three components. The subjective well-being can be seen as a trait. This is supported by the findings that subjective well-being has a long-term stability, as well as the fact that it is a heritable construct significantly determined by personality traits. The term heritability denotes the proportion of genetic variance attributable to phenotypic variance. It is examined within behavioral-genetic research, which aims to decompose the total variance of a phenotype into different types of genetic and environmental influences. Meta-analyses show that various measures of subjective well-being are substantially heritable; that is, about 30-40% of individual differences between people in the assessment of subjective well-being can be attributed to differences in their genotype (Bartels, 2015; Nes & Røysamb, 2015; Vukasović et al., 2012). The rest of the variance is attributed to non-shared environmental factors. Subjective well-being is significantly related to personality traits, which are one of its strongest predictors. More specifically, studies show that neuroticism and extraversion are most closely related to measures of subjective well-being. In addition to those two key traits, metaanalyses show that conscientiousness also significantly predicts some measures of subjective well-being, and less strong effects are sometimes found for agreeableness as well (Anglim et al., 2020; Busseri & Erd, 2023; Steel et al., 2008). The relationship between subjective wellbeing and personality can be explained by their shared etiology, that is, by sharing common genetic and environmental contributions to individual differences. A smaller number of studies examining this etiological connection have shown that subjective well-being and personality share the same genetic contributions to a significant extent. However, the previous studies disagree on whether a portion of the genetic variance in subjective well-being is unrelated to personality or whether their genetic variances are completely overlapping. Studies examining the relations between personality and subjective well-being have mainly operationalized personality within the framework of Big-Five/Five-factor model. The goal of this research is to operationalize personality within the alternative personality model - HEXACO - which defines the existence of six basic personality factors. Although it has some similarities with the five-factor models, the HEXACO also has significant differences compared to the five-factor models, and it can be expected that it would show different patterns of association with measures of subjective well-being. So far, no research has been published in the literature that examined the etiological connection between subjective well-being and HEXACO personality traits. The aim of this study is to examine individual differences in subjective well-being from two perspectives - by examining the etiology of these differences and by examining their relationship to personality traits operationalized through the HEXACO personality model on phenotypic but also on etiological level. We, therefore, aimed to examine: (i) the heritability of the subjective well-being composite score and its three components (life satisfaction, positive affect, negative affect); (ii) the phenotypic relationships of the subjective well-being composite score and its components with HEXACO personality domains and facets; (iii) the etiological relationships of the subjective well-being composite score and its components with HEXACO personality domains and facets; (iv) the etiological relationships of the three components of subjective well-being. Methodology The research was conducted using the classic twin study method. Data were collected on a total of 830 individuals, or 412 pairs of twins. Of these, 144 pairs are monozygotic (MZ) twins, and 268 are dizygotic (DZ) twins. The average age of the participants is 22.15 years, with a range of 19 to 28 years. Also, 63% of the total number of participants were female. The cognitive component of subjective well-being is operationalized by the Satisfaction With Life Scale (SWLS; Diener et al., 1985), consisting of five items, which refer to a global assessment of the extent to which we are satisfied with our own life as a whole. The affective component was measured with a shortened version of the Positive and Negative Affect Schedule (PANAS; Watson et al., 1988), consisting of a total of 16 items. The respondents were instructed to rate how they felt in life in general. The Croatian version of the HEXACO-PI-R questionnaire was used to assess personality traits (Babarović & Šverko, 2013; Lee & Ashton, 2018). It is a questionnaire designed to assess six domains and 25 facets of the HEXACO personality model with a total of 100 items. Results In order to answer the first problem of this research, four univariate behavioural-genetic models were conducted to examine the sources of individual differences in the variance of life satisfaction, positive affect, negative affect, and the composite score of subjective well-being. The results showed that the data for all four measures are best described by the AE model, which defines the existence of additive genetic variance (A) and non-shared environmental influences (E). It was determined that there is a significant degree of heritability of all measures of subjective well-being, as well as the composite result. More precisely, the heritability of life satisfaction is 54%, positive affect 49%, negative affect 47%, while the heritability of the composite score is 58%. Non-shared environmental effects account for the remainder of the variance. In order to answer the second problem of this research, a series of correlation and regression analyses were conducted. Results showed that six HEXACO personality domains explain between 31 and 52% of the variance of measures of subjective well-being, and at the facet level between 40 and 63%, the most for the composite score and the least for life satisfaction. All four measures of subjective well-being have the highest correlation with the extraversion domain, ranging between -.45 for negative affect and .65 for the composite score. Some differential relations of individual components with certain domains and facets were also determined. A higher level of life satisfaction is predicted by higher levels of extraversion, conscientiousness, or, more precisely, narrower traits of social self-esteem, liveliness, and diligence. A higher level of negative affect is predicted by lower extraversion, higher emotionality, and lower agreeableness. More specifically, a greater tendency towards unpleasant emotional tendencies is predicted by higher anxiety, lower social self-esteem, lower liveliness, and lower patience. When it comes to positive affect, it is associated with a higher level of extraversion and conscientiousness, that is, with higher levels of liveliness, social selfesteem, sociability, social boldness, and diligence. The same pattern is found for a high level of subjective well-being in general, as a composite result, with the addition of it being predicted by a low level of anxiety. The etiological association between personality and subjective well-being was examined by conducting bivariate and multivariate modelling. Results showed that genetic effects explain between 54% and 76% of the phenotypic covariance between all measures of subjective well-being and personality, and strong genetic correlations were also established between all examined variables. Multivariate Cholesky models showed that all measures of subjective well-being significantly shared genetic variance with personality, somewhat more with narrower facets than with broader personality domains. However, genetic variance unrelated to these dispositional traits was also established. On average, about 30% of the total genetic variance is specific to measures of subjective well-being. Finally, results showed that life satisfaction has strong genetic correlations with both affective components, while positive and negative affect are moderately correlated at the genetic level. Multivariate analysis showed that the common factor model best explains the etiological connection between the three components. Thus, between 60 and 70% of the heritability of the three components is determined by the genetic variance on a common latent factor. In comparison, around 30-40% of the genetic variance is specific to each individual component. Conclusion In conclusion, this research showed that genetic effects significantly contribute to individual differences in subjective well-being, which are shared to a significant extent with personality traits, primarily with extraversion and its narrower facets that are measured within the HEXACO model. Therefore, the same genetic effects that determine our level of extraversion, conscientiousness, or narrower facets of social self-esteem and liveliness contribute to a large extent to our life satisfaction and positive and negative affect. Nevertheless, although biological factors shared with personality play an essential role in defining the variance of all components of subjective well-being, it was also shown that there is a significant part of the genetic variance of these constructs that is not attributable to personality. Therefore, this research indicates that happiness is not only a matter of personality.