Table 2.

Twin studies of bipolar disorder

Study	Country	Age (years)	Basis for Diagnosis	Diagnostic Criteria	Number of Pairs a	Heritability
Kendler et al., 1993 and 1995	Sweden	37-57	Questionnaire	DSM-III-R	35	79%
McGuffin et al., 2003	U.K.	≥ 15	Medical record review,+/− research interview, outside informants	DSM-IV	67	85%
Kiesseppa et al., 2004	Finland	53 (±13) b	Medical record review, diagnostic interview, +/− outside informants	DSM-IV	25	93%
Summary						85%

^aat least one twin affected

^bmean ± standard deviation