Table 1

Lifetime rates of affective disorder in co-twins of bipolar twin probands

SampleLifetime rates of affective illness in co-twin of bipolar twin probands (probandwise concordance rate)Comment
41 Norway twin and psychosis register. 6 MZ pairs with BP probandBP-BP:MZ: 67%Small sample and no operationalised diagnostic criteria
42 USA Veteran twin register. 5 MZ, 15 DZ pairs (out of 15‚ÄČ909 twin pairs on register)BP-BP:MZ: 20%Inferior methodology. Note the very low rate of BP disorder detected in the twin sample (0.07%)
DZ: 0%
43 Denmark twin and psychiatry registers. 34 MZ, 37 DZ pairsBP-BP:MZ: 62%Despite lack of operationalised diagnostic criteria this is a detailed study and is the best available
DZ: 8%
BP-BP/UP:MZ: 79%
DZ: 19%
44 Norway twin register. 4 MZ, 6 DZ pairsBP-BP:MZ: 75%Small sample which may overlap partly with that of ref 41
DZ: 0%
BP-BP/UP:MZ: 100%
DZ: 0%
45 Sweden twin and psychiatric registers. 13 MZ, 22 DZ pairsBP-BP:MZ: 39%This has the strength of being a large epidemiological twin sample but the weakness of using questionnaire assessments. Likely to underestimate concordance
DZ: 5%
BP-BP/UP:MZ: 62%
DZ: 14%
46 UK psychiatric hospital twin register. 22 MZ, 27 DZ pairsBP-BP:MZ: 36%Diagnoses based on hospital notes. Likely to underestimate concordance
DZ: 7%
  • MZ: monozygotic; DZ: dizygotic. Sample size refers to number of twin pairs in which at least one twin suffered with bipolar disorder. BP-BP refers to twin pairs in which both have narrowly defined bipolar disorder. BP-BP/UP refers to twin pairs in which one has bipolar disorder and the other has broadly defined bipolar phenotype (including unipolar depression).