a1 Department of Public Health, University of Helsinki, Helsinki, Finland; Department of Mental Health and Alcohol Research, National Public Health Institute, Helsinki, Finland. email@example.com
Tishler and Carey (this issue, 2007) have analyzed the difference in ever smoking prevalence between monozygotic (MZ) and dizygotic (DZ) twins in 8 published twin studies. They conclude that the lower prevalence in MZ twins might invalidate conclusions from classic twin models. I critique the conducted meta-analysis and suggest additional analyses to be conducted. A reanalysis of the Finnish Twin Cohort 1975 questionnaire data (included in Tisher and Carey) indicates that when intrapair correlations are correctly taken into account, the MZ-DZ difference in that study is no longer statistically significant. Even if the Tisher/Carey finding holds up in further analyses, it pertains only to the observed trait, the effect is small in size, and the mechanism underlying it needs to be explored.
(Received July 30 2007)
(Accepted August 02 2007)
c1 Address for correspondence: Jaakko Kaprio, Department of Public Health, PO Box 41, University of Helsinki, FIN-00014 , Helsinki, Finland.