Abstract: Epidemiological and developmental studies showed that one of the obvious phenomena during adolescence is a marked increase in depressive symptoms. Behavioral genetic studies found that shared environmental influences account for the majority of the variance for children’s symptoms while genetic factors are substantial for adolescent depressive symptoms. However, whether the results of behavioral genetic studies from Western countries can be generalized to adolescents in China with a collectivism culture remains unexplored. First, prevalence of depression varies dramatically across cultures with a lower rate in China than in other countries. Second, factors that are associated with depressive symptoms and the extent to which these factors are consequential for adolescents’ mood are also different across cultures. Existing evidences showed that the quality of family relationships and grades in school had significantly stronger associations with depressive symptoms among Chinese youths than among U.S. youths. Third, genes that contribute to depressive symptoms have different frequencies in different races. Moreover, culture may moderate the expression of genes. Therefore, the first objective of the study was to examine the heritability of adolescent depressive symptoms for boys and girls in China, respectively. Results from the family study, the adoption study and the twin study suggest that adolescent depressive symptoms are heritable (range 15-80%). Moreover, empirical work showed that the relative contributions of genetic and environmental influences to depressive symptoms vary as a function of age. The second objective of the study was to extend previous ones by examining whether the heritability of depressive symptoms in early-adolescence was different from that in mid-adolescence. We hypothesized that the heritability of depressive symptoms in early adolescence was higher than that in middle adolescence according to the theory and empirical work. Depressive symptoms tend to persist over time and the stability is evident in general population samples. The third objective of the study was to examine the contributions of genetic and environmental factors to the stability of adolescent depressive symptoms in early and middle adolescence, respectively. We hypothesized that genetic factors accounted for the stability of depressive symptoms in early adolescence whereas shared environmental influence contributed to the stability of depressive symptoms in middle adolescence. Data for the current study were from the Beijing Twin Study (BeTwiSt). Given obtaining written informed consents from twins and their parents, arrangements were made for the twins to stay in their classrooms after school time. Research staffs were there to answer any questions that students might ask about the questionnaires. After the twins completed the questionnaires, they were asked to provide their saliva samples using the Oragene® DNA self-collection kit (Genotek Inc.). Zygosity was determined by DNA analyses (89.5%) and questionnaire (10.5%). In DNA analyses, nine short tandem repeat (STR) loci which are highly heterogeneous in Chinese population were used. Same-sex twins with at least one different genetic marker were classified as dizygotic twins, otherwise classified as monozygotic twins. 508 MZ, 176 DZ twins participated in the longitudinal study with the interval of 1.37 years (SD=0.44), and the percentage of male was 46.2%. CDI (Children’s Depression Inventory) was used to measure adolescent depressive symptoms and Mx software was used to conduct the unique genetic analyses and bivariate genetic analyses. No gender difference was found in the present study. Genetic factors accounted for more variance of adolescent depressive symptoms in early adolescence than in middle adolescence, and the magnitude of nonshared environmental influence increased with time. Genetic factors contributed to the stability of depressive symptoms in early adolescence whereas environmental influence contributed to the stability of depressive symptoms in middle adolescence.

Key words: twins, genetic, shared environment, nonshared environment, depressive symptoms