Background. Stressful life events associated with forced migration and war increase vulnerability for adverse health consequences, including health-related behaviors. Due to WWII Finland lost 10% its land area to the Soviet Union in 1944, and the entire population (420,000 persons) of Ceded Karelia was settled in the rest of Finland after the war. We studied how war experiences, especially forced migration, are associated with subsequent cigarette smoking within the nationwide Finnish Adult Twin Cohort.
Methods. Altogether 12933 twin individuals born before 1945 replied to a questionnaire in 1975 of the Finnish Twin Cohort (89% response rate) and were included in the present analysis. The exposure (forced migration due to war, categorized as ‘no’, ‘once’,’ 2+’) used following measures: the municipality of birth and whether the respondent had moved municipality ever and if so, for what reason (10 options, including war). For a subset, we had a PRS for smoking initiation from GSCAN (gscan.sph.umich.edu) omitting Finnish data.
As a measure of the validity of the item, we found that of the respondents born in Ceded Karelia 83% replied that they had had to move due to war, with the corresponding percentages varying from 5% to 20% in other provinces (due to fighting in border areas and bombings of urban areas). Detailed smoking questions were used to create the smoking status variable: ‘ever smoker’ versus ‘never smoker’ with an ever smoker defined as a person who has smoked more than 100 cigarettes lifetime. Logistic regression with correction for sampling of twins as twin pairs was used for analyses of individuals, while twin tetrachoric correlations were computed to assess similarity within pairs.
Results
Those who had experienced forced migration due to World War II showed higher likelihood of being ever smoker (age, sex, education, and birth area adjusted Odds Ratio =1.70, 95% Confidence Interval=1.43-2.02; p<0.001) than those without forced migration experience. Adjustment for smoking initiation PRS did not attenuate this. For pairwise analyses, we focus on male twin pairs as smoking rates were low in women. Among male pairs with no forced migration experience (71.6% ever smokers), the correlation for being an ever smoker was in MZ pairs 0.83 (standard error (se) 0.021, N=1162 pairs) and in DZ pairs 0.55 (se 0.026, 2640 pairs); a finding consistent with genetic influences on smoking. Among pairs in which both twins experienced forced migration (78.7% ever smokers), the MZ correlation (r=0.71, se 0.09, 138 pairs) and the DZ correlation (r=0.79, se 0.05, 306 pairs) were very similar and indicate absence of genetic effects. The average age in 1975 was 44.6 years in the non-exposed pairs and 46.7 in the exposed group, some 30 years after the end of the war.
Conclusions
Forced migration experience as a stressful life event seems to increase vulnerability for use of addictive substances, such as cigarette smoking, and suppresses familial/ genetic influences on the liability to initiate smoking. Forced migration due to war represents a true environmental influence that the twins were not able to control.
