title: Risk factors influencing smoking behavior: a Turkish twin study.
abstract: OBJECTIVE
  In this study, we introduce the first twin study in Turkey, focusing on smoking behavior, and laying the foundation to register all twins born in Turkey for research purposes. Using Turkish twins will contribute to our understanding of health problems in the context of cultural differences.
  
  
  METHODS
  We assessed 309 twin pairs (339 males and 279 females) aged between 15 and 45 years living in the Kırıkkale and Ankara regions of Turkey, and administered a health and lifestyle interview that included questions about smoking status and smoking history. We analyzed the data using descriptive statistics, t-tests, chi-square tests, and bivariate and multivariate clustered logistic regression. In addition, we fit bivariate Structural Equation Models (SEM) to determine contributions of latent genetic and environmental factors to smoking outcomes in this sample.
  
  
  RESULTS
  One hundred seventy-eight participants (28.8%) were identified as smokers, smoking every day for a month or longer, of whom 79.2% were males and 20.8% were females. Mean values for number of cigarettes per day and the Fagerstrom Test of Nicotine Dependence (FTND; Fagerstrom, 1978) score were higher in males than in females, and age of onset was earlier in males. There was a significant positive correlation between the FTND score and number of cigarettes smoked per day, and a significant negative correlation between both variables and age at onset of smoking. Our study showed that gender, presence of a smoking twin in the family, age, alcohol use, marital status, daily sports activities, and feeling moody all played a significant role in smoking behavior among twins. The twin analysis suggested that 79.5% of the liability to FTND was influenced by genetic factors and 20.5% by unique environment, while familial resemblance for smoking initiation was best explained by common environmental factors.
  
  
  CONCLUSIONS
  Marked differences in the prevalence of smoking behavior in men versus women were observed for the Turkish population. Genetic analyses showed that common environmental factors primarily contributed to smoking initiation, while genetic factors explained a greater proportion of variance in liability to nicotine dependence. Our study shows higher heritability estimate of the FTND scores and higher shared environmental influence on smoking initiation for both males and females than reported in previous studies.
authors.affiliation: Department of Statistics,Faculty of Arts and Sciences,Kırıkkale University,Kırıkkale,Turkey.
authors.forename: Sevgi Yurt
authors.initials: SY
authors.lastname: Oncel
authors.affiliation: Departments of Psychiatry,Psychology,and Human and Molecular Genetics,Virginia Institute for Psychiatric and Behavioral Genetics,Virginia Commonwealth University,Richmond,Virginia,USA.
authors.forename: Danielle M
authors.initials: DM
authors.lastname: Dick
authors.affiliation: Departments of Psychiatry,Psychology,and Human and Molecular Genetics,Virginia Institute for Psychiatric and Behavioral Genetics,Virginia Commonwealth University,Richmond,Virginia,USA.
authors.forename: Hermine H
authors.initials: HH
authors.lastname: Maes
authors.affiliation: Departments of Psychiatry,Psychology,and Human and Molecular Genetics,Virginia Institute for Psychiatric and Behavioral Genetics,Virginia Commonwealth University,Richmond,Virginia,USA.
authors.forename: Fazil
authors.initials: F
authors.lastname: Alıev
mesh_terms: D000293:Adolescent; D000328:Adult; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D012307:Risk Factors; D012907:Smoking / Q000453:epidemiology* / Q000235:genetics*; D014421:Turkey / Q000453:epidemiology; D014427:Twins / Q000235:genetics*; D055815:Young Adult