Somatization is known to be more prevalent in Asian than in Western populations. Using a South Korean adolescent and young adult twin sample (N = 1754; 367 monozygotic male, 173 dizygotic male, 681 monozygotic female, 274 dizygotic female and 259 opposite-sex dizygotic twins), the present study aimed to estimate heritability of somatization and to determine common genetic and environmental influences on somatization and hwabyung (HB: anger syndrome). Twins completed self-report questionnaires of the HB symptoms scale and the somatization scale via a telephone interview. The results of the general sex-limitation model showed that 43% (95% CI [36, 50]) of the total variance of somatization was attributable to additive genetic factors, with the remaining variance, 57% (95% CI [50, 64]), being due to individual-specific environmental influences, including measurement error. These estimates were not significantly different between the two sexes. The phenotypic correlation between HB and somatization was .53 (p < .001). The bivariate model-fitting analyses revealed that the genetic correlation between the two symptoms was .68 (95% CI [.59, .77]), while the individual-specific environmental correlation, including correlated measurement error, was .41 (95% CI [.34, .48]). Of the additive genetic factors of 43% that influence somatization, approximately half (20%) were associated with those related to HB, with the remainder being due to genes unique to somatization. A substantial part (48%) of individual environmental variance in somatization was unrelated to HB; only 9% of the environmental variance was shared with HB. Our findings suggest that HB and somatization have shared genetic etiology, but environmental factors that precipitate the development of HB and somatization may be largely independent from each other.
Keywords: Comorbidity; environment; genes; hwabyung (anger syndrome); somatization; twin.