It is also important to identify which aspects of the depression phenotype are most strongly associated with smoking. Prior twin studies of depression�Csmoking associations have primarily selleck chemicals llc modeled depressive symptoms as a unitary phenotype. However, depressive symptoms may be more aptly characterized as a collection of multiple intermediate phenotypes that can each be isolated by parsing depression into its constituent symptom dimensions (Hasler, Drevets, Manji, & Charney, 2004). For example, investigators have used the four-factor model based on the Center for Epidemiological Studies Depression Scale (CESD; Radloff, 1977; Shafer, 2006), which separates depression into discrete dimensions of negative affect (NA; sadness, distress, and worthlessness), somatic features (SF; poor appetite, low energy, and sleep disturbance), low positive affect (PA; diminished positive emotions and pleasure), and interpersonal problems (IP; poor social adjustment).

Separate dimensions of depressive symptomatology exhibit disparate patterns of association with some smoking characteristics in adults (Korhonen et al., 2011; Leventhal, Ramsey, Brown, LaChance, & Kahler, 2008; Mickens et al., 2011; Pomerleau, Zucker, & Stewart, 2003; but also see Prochaska et al., 2004). Thus, it is of interest to examine the relation between smoking initiation and depressive symptom dimensions in adolescents. The present study explored genetic and environmental sources of covariation between depressive symptom dimensions and smoking initiation in Chinese twins aged 9�C16 years old.

Examining smoking initiation in this age group is important, given evidence that those who have their first cigarette before (vs. after) age 17 are more likely to be persistent smokers in adulthood (Breslau & Peterson, 1996). Of particular interest was whether the magnitude of correlation and influences on covariation between depressive symptoms and smoking initiation differed across separate symptom dimensions. Methods Participants and Procedure Participants in this cross-sectional study were members of the Qingdao Twin Registry (QTR; Pang et al., 2006), which is a subset of the national Chinese Twin Registry. Twins were recruited to join the QTR via the general newborn registry for the city of Qingdao, connections with school nurses, and outreach using media campaigns (Pang et al., 2003).

As of 2005, the QTR had 10,655 twin pairs and was estimated to include 74% of all twins living in Qingdao (Pang et al., 2006). The current sample comprises a youth cohort of 321 monozygotic (MZ) pairs and 281 dizygotic (DZ) twin pairs who were contacted from the GSK-3 QTR in 2005 and 2006 and asked to take part in a multiwave study of health and behavior (Unger et al., 2011); the current study focuses on only the first wave to maximize data and offset attrition at later waves. Of the 281 DZ pairs, 109 were opposite sex pairs, 81 were same-sex male pairs, and 81 were same-sex female pairs.