Table 1.

Descriptive statistics and phenotypic correlations

	Age 4 years	Age 7 years	Age 9 yearsa	Age 16 years
Sample size, n
MZ	5072	5202	2421	3497
DZ	9671	9216	4103	6150
Total	14 743	14 418	6524	9647
Descriptive statistics
Mean age, years (s.d.)	4.04 (0.13)	7.07 (0.25)	9.02 (0.29)	16.32 (0.68)
Cronbach's α for OCB scale	0.59	0.59	0.62	0.61
Mean OCB score (s.d.)	2.56 (1.75)	1.89 (1.56)	0.94 (1.13)	0.82 (1.26)
Phenotypic correlations
Age 4 years
r	1
n	14 743
Age 7 years
r (95% CI)	0.45 (0.43–0.46)	1
n	11 276	14 418
Age 9 years
r (95% CI)	0.35 (0.32–0.37)	0.44 (0.42–0.46)	1
n	5409	5771	6524
Age 16 years
r (95% CI)	0.23 (0.21–0.26)	0.32 (0.30–0.34)	0.37 (0.34–0.40)	1
n	7891	8053	4054	9647

MZ, Monozygotic; DZ, dizygotic; s.d., standard deviation; OCB, obsessive–compulsive behaviour; CI, confidence interval.

^aAt age 9 years, the study was restricted to just two out of three cohorts, resulting in a reduced sample size.