Table 1.

Proportions (%) of use of illicit drugs by potential categorical predictors

			All		Boys		Girls
		na	%	P- valueb	%	P- valueb	%	P- valueb
Ever smoking c	No	2371	5.0	<0.001	4.5	<0.001	5.5	<0.001
	Yes	1735	25.0		22.6		27.3
Age of smoking initiationd	No initiation	1205	0.8	<0.001	0.5	<0.001	1.2	<0.001
	Late (> 12)	1914	14.0		12.5		15.2
	Early (≤ 12)	960	28.8		26.1		32.1
Drinking to intoxicationc	No	3156	7.5	<0.001	7.1	<0.001	7.8	<0.001
	Yes	930	34.0		33.9		34.0
Number of smoking peersc	None	1611	5.8	<0.001	5.8	<0.001	5.8	<0.001
	One	463	9.3		7.9		10.6
	2–5	1106	13.8		13.9		13.8
	More	888	29.6		25.7		32.6
Number of acquaintances with drug experienced	None	3234	9.0	<0.001	9.1	<0.001	8.9	<0.001
	One	452	22.3		17.8		25.7
	2–5	361	34.3		36.9		33.0
	More	65	56.9		60.0		55.6
Mother’s drinking to intoxicatione	Never	1675	9.7	<0.001	10.5	0.20	9.1	<0.001
	Less often than monthly	2046	14.7		12.4		16.9
	Monthly or more often	321	22.7		16.1		28.1
Father’s drinking to intoxication e	Never	1326	9.1	<0.001	8.2	<0.01	10.0	<0.001
	Less often than weekly	2052	13.3		11.4		15.1
	Weekly or more often	305	19.3		17.6		21.0
Mother’se smoking	No	3309	12.5	<0.01	11.5	0.36	13.5	<0.01
	Yes	717	16.7		13.5		20.5
Father’se smoking	No	2939	12.0	0.39	10.4	0.38	13.6	0.66
	Yes	746	13.4		12.2		14.6
Pre-natal exposure to nicotinee	No	2821	11.2	<0.001	10.3	<0.01	12.1	<0.001
	Yes	1118	18.3		15.6		21.2
Mother’s SESe	Employer	438	11.9	0.68	11.2	0.39	12.6	0.85
	Self-employed	721	13.9		12.1		15.5
	Upper-level employee	2096	13.8		13.3		14.3
	Lower-level employee	683	11.7		8.5		14.5
	Manual worker	30	6.7		5.9		7.7
	Pensioner	0	-		-		-
	Other	59	15.2		8.0		20.6
Father’s SESe	Employer	765	11.2	0.42	9.5	0.34	12.9	0.80
	Self-employed	922	13.7		12.2		15.1
	Upper-level employee	603	13.9		12.5		15.3
	Lower-level employee	1389	11.2		9.5		12.8
	Manual worker	6	33.3		33.3		-
	Pensioner	2	0		-		0
	Other	0	-		-		-

^aTotal numbers of observations in each predictor category

^bFisher’s exact test for dichotomies and Chi Square test for other variables, adjusted for correlated twinship

^cReported at age 14

^dReported at age 17

^eReported by the parent at age 12 of the twin