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Kenneth S. Kendler, MD; Nancy Pedersen, PhD; Lars Johnson, MD; Michael C. Neale, PhD; Aleksander A. Mathé, MD, PhD

Arch Gen Psychiatry. 1993;50(9):699-706.

Abstract
Objective
We sought to compare the probandwise concordance rate (PRC) for affective illness (AI) in monozygotic (MZ) and dizygotic (DZ) twins in samples ascertained through psychiatric hospitalization vs samples from the general population.

Methods
Twins were ascertained through psychiatric hospitalization for AI from the Swedish Psychiatric Twin Registry or as a matched sample from the population-based Swedish Twin Registry. Lifetime diagnoses were based on a mailed questionnaire containing, in self-report format, DSM-III-R criteria for mania and major depression. Returned questionnaires were obtained from 1484 individuals and both members of 486 pairs, of whom 154 were classified as MZ, 326 as DZ, and six of unknown zygosity.

Results
No evidence was found for violations of the equal environment assumption. Using either a narrow or broad diagnostic approach, the risk for AI in cotwins of proband twins was independent of the gender, polarity (ie, unipolar vs bipolar) and mode of ascertainment of the affected proband (ie, via hospitalization vs from the general population). Combining both subsamples, PRC for total AI using narrow diagnostic criteria was 48.2% in MZ and 23.4% in DZ twins. Using broad diagnostic criteria, the parallel figures were 69.7% and 34.9%. The risk for bipolar illness was substantially increased in the cotwins of probands with bipolar AI.

Conclusions

Author Affiliations
From the Departments of Psychiatry (Drs Kendler and Neale) and Human Genetics (Dr Kendler), Medical College of Virginia/Virginia Commonwealth University, Richmond; the Department of Environmental Hygiene, The Karolinska Institute, Stockholm, Sweden (Dr Pedersen); and the Karolinska Institute Department of Psychiatry, St Göran's Hospital, Stockholm, Sweden (Drs Johnson and Mathé).

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