Twin Pair Resemblance for Psychiatric Hospitalization in the Swedish Twin Registry: A 32-year Follow-up Study of 29,602 Twin Pairs
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Allgulander et al. (Allgulander C, Nowak J, Rice JP (1991) Acta Psychiatr Scand 83, 12) published twin pair analyses of psychiatric hospitalization for like-sex pairs from the Swedish Twin Registry born 1926–1958. As noted in a subsequent letter (Allgulander C, Nowak J, Rice JP (1992) Acta Psychiatr Scand 86, 421), several features of the original study resulted in under-ascertainment of cases and underestimated heritability, particularly for alcoholism. The present report updates the prior results by using 17 additional years of follow-up, including members of opposite-sex twin pairs, and addressing biases arising from cohort effects and from excluding pairs with unknown zygosity.
Registry records for 29,602 twin pairs born 1926–1958 were matched against national databases of psychiatric and medical hospitalizations from 1972–2000 to obtain ICD diagnostic codes. Zygosity was known for 10,903 opposite-sex pairs and 15,401 like-sex pairs who participated previously in research. Twin-pair resemblance and genetic and environmental variance proportions were estimated for hospitalization for alcoholism, affective disorders, psychosis, and (in females) anxiety disorders.
Hospitalization rates during the ascertainment window were: alcoholism: males = 3.67%, females = 0.94%; affective disorders: males = 1.99%, females = 2.75%; anxiety disorders: males = 0.46%, females = 0.74%; and psychotic disorders: males = 1.70%, females = 1.96%. Twins from like-sex pairs with unknown zygosity had significantly higher prevalences than those with known zygosity. Tetrachoric correlations and heritability estimates were affected by the method used to model unknown zygosity and cohort effects.
Inclusion of additional follow-up information, opposite-sex twin pairs, age-adjustment, and use of current ICD definitions yielded higher heritability estimates for alcoholism, anxiety disorders, and psychosis than previously published for this nationally-representative sample of twins from Sweden. The results show that relatively small selection biases can alter twin study results and underscore the importance of addressing under-ascertainment of cases in genetic research based on volunteers.
KeywordsAlcoholism Depression Psychosis Anxiety Ascertainment
The Swedish Twin Registry is supported by grants from the Swedish Scientific Council, the Swedish Ministry of Higher Education, and Astra Zeneca. The authors are grateful to Lisa Halberstadt for helpful comments and J. J. McArdle for statistical advice. Preparation of this manuscript was supported by grants R01-AA-11408 and K01-AA-00236 from the U.S. National Institutes of Health and an Independent Investigator award from the National Alliance for Research on Schizophrenia and Affective Disorders (to CAP). Rebecca Ortiz assisted with manuscript preparation.
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