Table 2

Adjusted relative risk of schizophrenia in a population-based cohort of 2.66 million Danish people according to family history of mental illness and place and season of birth

Variable Relative risk (95% Cl)1
Mother born in Denmark later than 1 April 1935 Mother either born in Denmark before 1 April 1935 or born outside Denmark No maternal restrictions
Study Population A Study Population B Study Population C Study Population D Total study population
Follow-up 1970 to 1993 Follow-up 1994 to 1998 Follow-up 1970 to 1993 Follow-up 1994 to 1998 Follow-up 1970 to 1998
1.The relative risk was adjusted for age and its interaction with gender, calendar year of diagnosis, ages of the mother and father at the time of child's birth, and all variables in the table.
2.A sine function with an amplitude of one has no time of peak.
Maternal history
Schizophrenia 9.37 (7.67-11.4) 7.31 (5.76-9.27) 6.12 (4.88-7.67) 4.19 (2.65-6.62) 7.10 (6.28-8.01)
Schizophrenia-like psychoses 4.46 (3.61-5.50) 4.20 (3.33-5.31) 3.07 (2.52-3.76) 3.07 (2.15-4.37) 3.68 (3.28-4.13)
Other mental disorders 2.09 (1.89-2.32) 2.34 (2.10-2.60) 1.73 (1.58-1.90) 1.65 (1.39-1.95) 1.95 (1.85-2.06)
Mother not affected (reference) 1.00 1.00 1.00 1.00 1.00
Paternal history
Schizophrenia 6.54 (4.84-8.84) 4.74 (3.40-6.61) 4.95 (3.19-7.69) 4.05 (1.92-8.54) 5.39 (4.45-6.53)
Schizophrenia-like psychoses 3.36 (2.43-4.66) 3.43 (2.47-4.78) 2.69 (1.91-3.80) 2.78 (1.57-4.92) 3.11 (2.60-3.74)
Other mental disorders 1.77 (1.57-2.00) 1.77 (1.56-2.00) 1.64 (1.47-1.83) 1.72 (1.42-2.09) 1.73 (1.62-1.84)
Father not affected (reference) 1.00 1.00 1.00 1.00 1.00
History in siblings
Schizophrenia 6.12 (4.73-7.90) 5.11 (4.08-6.40) 6.74 (5.60-8.10) 4.45 (3.38-5.85) 5.68 (5.07-6.37)
Schizophrenia-like psychoses 3.82 (2.69-5.42) 2.82 (2.03-3.91) 3.28 (2.33-4.60) 3.77 (2.63-5.39) 3.36 (2.83-3.99)
Other mental disorders 2.16 (1.83-2.54) 1.74 (1.49-2.02) 2.13 (1.84-2.47) 1.75 (1.42-2.16) 1.96 (1.80-2.13)
No affected siblings (reference) 1.00 1.00 1.00 1.00 1.00
Place of birth
Capital 2.16 (1.92-2.43) 2.02 (1.77-2.30) 2.10 (1.92-2.30) 2.32 (1.98-2.73) 2.13 (2.01-2.25)
Capital suburb 1.56 (1.33-1.83) 1.54 (1.31-1.81) 1.64 (1.44-1.86) 1.71 (1.37-2.13) 1.60 (1.48-1.73)
Provincial cities 1.54 (1.34-1.77) 1.57 (1.36-1.82) 1.37 (1.22-1.54) 1.30 (1.05-1.61) 1.46 (1.36-1.56)
Provincial towns 1.26 (1.11-1.42) 1.19 (1.05-1.35) 1.25 (1.14-1.37) 1.07 (0.90-1.27) 1.21 (1.14-1.28)
Rural area (reference) 1.00 1.00 1.00 1.00 1.00
Season of birth
Amplitude 1.11 (1.05-1.17) 1.01 (0.95-1.07) 1.00 (0.96-1.05) 1.07 (0.99-1.16) 1.02 (1.00-1.05)
Time of peak (day/month) 6/3 (7/2-5/4) 2 2 24/5 (16/3-1/8) 25/3 (14/1-5/6)