Table 1.

Danish twin studies of asthma

Population (birth year)	Year studied	Age (years)	Participants (%)	Males (%)	Females (%)	Intact pairs	References
1931–1952
Questionnaire study	2002	50–71	13,649 (75)	6,415 (47)	7,234 (53)	4,240	[I, II, VI, IX]
1953–1982
Questionnaire study	1994	12–41	29,183 (86)	14,074 (48)	15,106 (52)	11,231	[IV, V]
Questionnaire study	2002	20–49	21,133 (75)	9,431 (47)	11,702 (53)	7,201	[I, II, III, VI, IX]
Clinical study	2004	21–51	575 (67)	240 (42)	335 (58)	256	[III]
1983–2000
Questionnaire study	2003	3–20	19,748 (68)	9,896 (50)	9,850 (50)	9,694	[I, V, VI, VII, VIII]

Three individuals from 1994 and two individuals from 2003 were of unknown sex.

Intact pairs are twin pairs with complete data on asthma.