The few representative studies distinguishing bipolar I disorder from unipolar depressive disorder have all had some methodological limitations (2– 4). Disparities in diagnostic criteria, ascertainment method, or sample compilation among studies can explain this large variability. Probandwise concordance rates have varied from 0.33 to 0.75 for monozygotic twins and from zero to 0.13 for dizygotic twins. Twin studies of bipolar disorder have frequently shown a higher concordance for the disease in monozygotic than in dizygotic twins, indicating the importance of a genetic contribution to the liability to this disorder (1). CONCLUSIONS: The high heritability of bipolar disorder was demonstrated in a nationwide population-based twin sample assessed with structured personal interviews. The best-fitting model was the one in which genetic and specific environmental factors explained the variance in liability, with a heritability estimate of 0.93 (95% CI=0.69 to 1.00). The model with no familial transmission was rejected. The correlations in liability were 0.85 and 0.41, respectively. RESULTS: The probandwise concordance rates were 0.43 (95% CI=0.10 to 0.82) for monozygotic twins and 0.06 (95% CI=0.00 to 0.27) for dizygotic twins. The authors calculated probandwise and pairwise concordances and correlations in liability and applied biometrical model fitting. Lifetime diagnoses were made by using the Structured Clinical Interview for DSM-IV. Thirty-eight pairs were thereby identified and invited to participate in the study the participation rate was 68%. METHOD: All Finnish same-sex twins (N=19,124) born from 1940 to 1957 were screened for a diagnosis of bipolar I disorder as recorded in the National Hospital Discharge Register between 19 or self-reported in surveys of the Finnish Twin Cohort in 1975, 1981, and 1990. The authors report what appears to be the first twin study of bipolar I disorder involving a population-based twin sample, in which the diagnoses were made by using structured, personal interviews. OBJECTIVE: The few studies of bipolar I disorder in twins have consistently emphasized the genetic contribution to disease liability.
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