Table 1. Descriptive characteristics for 44 pairs of monozygotic twins discordant for chronic fatiguing illness.

Variable	Affected twins	Unaffected co-twins	Statistical comparison
Met criteria for CFS	32/44, 73%	0/44, 0%	By design
Met criteria for ICF	12/44, 27%	0/44, 0%	By design
Female sex	39/44, 89%	39/44, 89%	Identical by design
Median age at evaluation, IQR	51, 39–59 years	51, 39–59 years	Identical by design
Median body mass index, IQR	24.6, 22–30 kg/m2	24.4, 22–31 kg/m2	Paired t43 = 0.2, p = 0.87
Median SF-36 physical function, IQR	41.2, 28–48	48.2, 40–52	Paired t43 = 3.0, p = 0.004
Median SF-36 mental function, IQR	38.9, 29–47	51.6, 40–56	Paired t43 = 5.0, p = 1×10−5
Median current fatigue by VAS, IQR	69, 49–77	19, 10–51	Paired t43 = −7.2, p = 6×10−9
Median duration of fatigue, IQR	7.6, 4–16 years	NA	–
Sudden onset of fatigue	2/44, 4.5%	NA	–

Abbreviations. CFS = chronic fatigue syndrome, ICF = idiopathic chronic fatigue, IQR = inter-quartile range, VAS = visual analog scale (0–100, higher means greater fatigue at clinical examination).