title: Genetics of the association between intelligence and nicotine dependence: a study of male Swedish twins.
abstract: OBJECTIVE
  Previous studies have found inverse associations between intelligence quotient (IQ) and cigarette smoking, but the causal pathways linking IQ with smoking status and nicotine dependence (ND) are not well understood. The aim of this study was to explore the associations between IQ and ND using a genetically informative twin design to detect whether any association is because of shared genetic or environmental factors.
  
  
  METHODS
  A population-based twin cohort with IQ measured in adolescence and ND later in life, analysed by classical twin modeling based on linear structural equations.
  
  
  METHODS
  Swedish national registry data.
  
  
  METHODS
  A total of 5040 male twins born 1951-84.
  
  
  METHODS
  IQ was measured at military conscription at a mean age of 18 years. ND was self-reported at the ages of 22-57 years using the Fagerström Test for ND scale (FTND). Both cigarette smoking and Swedish snus use were analysed.
  
  
  RESULTS
  Both IQ and ND showed moderate heritability (0.58 and 0.39, respectively). The heritability of ND was similar for cigarette smoking and snus use. The phenotypic correlation between IQ and ND was weak: -0.11 (-0.16, -0.06) for total ND. Bivariate analysis revealed that this correlation was mainly because of genetic factors, but still the genetic correlation between IQ and ND from cigarette smoking was only -0.24.
  
  
  CONCLUSIONS
  Nicotine dependence, as measured by the Fagerström Test for Nicotine Dependence, shows moderate heritability in both smokers and snus users but is only weakly associated with intelligence quotient; common genetic factors underlying nicotine dependence and intelligence quotient probably account for little of the observed association between smoking and intelligence quotient.
authors.affiliation: Department of Public Health Sciences, Karolinska Institute, Stockholm, Sweden. karin.modig@ki.se
authors.forename: Karin
authors.initials: K
authors.lastname: Modig
authors.forename: Karri
authors.initials: K
authors.lastname: Silventoinen
authors.forename: Per
authors.initials: P
authors.lastname: Tynelius
authors.forename: Jaakko
authors.initials: J
authors.lastname: Kaprio
authors.forename: Finn
authors.initials: F
authors.lastname: Rasmussen
mesh_terms: D000293:Adolescent; D000328:Adult; D015984:Causality; D002648:Child; D015331:Cohort Studies; D003627:Data Interpretation, Statistical; D004200:Diseases in Twins / Q000235:genetics* / Q000523:psychology; D015438:Health Behavior; D006801:Humans; D007360:Intelligence / Q000235:genetics*; D007361:Intelligence Tests; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D012042:Registries; D012907:Smoking / Q000009:adverse effects / Q000453:epidemiology / Q000235:genetics*; D012931:Social Environment; D012959:Socioeconomic Factors; D013548:Sweden / Q000453:epidemiology; D014029:Tobacco Use Disorder / Q000453:epidemiology / Q000235:genetics*; D014030:Tobacco, Smokeless / Q000009:adverse effects; D014427:Twins / Q000235:genetics; D055815:Young Adult