Table 2.

Overview of twin studies into problematic cannabis use. Studies in bold are used in the present meta-analysis.

Cohort	Reference	Country	Age	Measure	Prevalence (%)	Sex	MZ (pairs)	DZ (pairs)	A (in %)	C (in %)	E (in %)
1	Agrawal et al., 2007 (47)	Australia	24–36	Abuse/Dependence problems	M: 28 F: 15	M F	487 696	387 506	68 (65–69) 55 (30–63)	14 (12–14) 16 (11–19)	18 (16–19) 29 (20–31)
1	Lynskey et al., 2002 (33)	Australia	24–36	Lifetime dependence; 2 or more symptoms	M: 15.1 F: 7.8	M F	487 699	387 507 655	56 (23–77) 21 (0–67)	13 (0–40) 39 (0–65)	31 (21–45) 40 (27–54)
2	Agrawal et al., 2005 (49)	US, Virginia cohort 1	M 20–58 F 21–62	Abuse/Dependence	8–19	M F	702 556	489 378	31 36	0 0	69 64
2	Agrawal et al., 2004c (57)	US, Virginia cohort 1	M 20–58 F 21–62	Abuse/dependence	M: ~18 F: ~8	M F	2632 MM individuals 1943 FF individuals		76 69	- -	24 31
2	Agrawal et al., 2004d (53)	US, Virginia cohort 1	M 20–58 F 21–62	Abuse/dependence	M: 18.0 F: 7.5	M F	702 556	489 378	76 48 (47–56)	0 28 (0–31)	24 24 (18–26)
2	Gillespie et al., 2009 (50)	US, Virginia cohort 1	24–62 (at wave 3)	Symptoms of abuse	Not reported	M	1772 twins		35	34	31
2	Kendler et al., 2007 (83)	US, Virginia cohort 1	F 36.3 (8.2) M 37.0 (9.1)	Number of abuse/dependence symptoms	M: 20.5 F: 9.4 (at least one symptom)	M F	1666 i 1151 individuals	1269 779 individuals	71	-	29
2	Kendler et al., 2003 (54)	US, Virginia cohort 1	20–58	Lifetime abuse/dependence	18.3	M	704	492	73	1	26
2	Kendler et al., 2000 (75)	US, Virginia cohort 1	20–58	Abuse	MZ: 16.6 DZ: 20.1	M	708	490	76 (42–82)	1 (0–31)	23 (17–33)
2	Kendler et al., 1999 (76)	US, Virginia cohort 1	37.7 ± 7.5	Lifetime use	7.9	F	1934 individuals		47	19	34
2	Kendler et al., 1998 (77)	US, Virginia cohort 1	22–62	Lifetime Abuse	MZ: 7.8 DZ: 7.6	F	485	335	72 (56–84)	-	28 (16–44)
2	Neale et al., 2006 (78)	US, Virginia cohort 1	Not reported	Abuse	Not reported	F	499	327	48	30	33
3	van den Bree et al., 1998 (67)	US, Minnesota cohort 1*	15–63	Abuse and or dependence	Treatment sample	M F	56 38	66 28	68 53	24 0	8 47
4	McGue et al., 2000 (27)	US, Minnesota cohort 2	17 years old	Lifetime Abuse/dependence	M: 7.1 F: 6.7	M F	188 223	101 114	54 6	27 68	19 26
5	Rhee et al., 2003 (34)	US, Colorado	12–19	One or more abuse/dependence symptoms	M: 13.7 F: 12.7	M F	159 186	113 101 123 DOS 306 siblings 74 adoptive	34 (0–67)	36 (10–60)	30 (16–48)
5	Young et al., 2006 (31)	US, Colorado	12–18	One or more abuse/dependence symptoms	M: 0.8–28.8 F: 0.0–22.9 (age 12 to 18)	M/F	645 MZ, 702 DZ 1945 males, 1799 females including siblings		55 (29–80)	24 (2–44)	21 (13–33)
6	Fu et al., 2002 (84)	US, VETR	33–52	Lifetime dependence	6.6	M	1868	1492	50	13	37
6	Grant et al., 2006 (68)	US, VETR	36–55	Lifetime abuse/dependence	~8	M	1583	1255	39	20	40
6	True et al., 1999 (85)	US, VETR	33–52	3 or more lifetime symptoms of abuse/dependence	6.7	M	1856	1479	43.9	21.3	35.8
6	Tsuang et al., 1999 (82)	US, VETR	36–55	Transition to abuse and dependence	7.2 (abuse/dependence)	M	1874	1498	22 (0–49)	9 (0–40)	69 (51–88)
6	Tsuang et al., 1998 (48)	US, VETR	36–55	Lifetime abuse or dependence	7.2	M	1874	1498	33	29	38
6	Tsuang, et al., 1996 (86)	US, VETR	36–55	Lifetime abuse or dependence	7.2	M	1874	1498	33	29	38
6	Xian et al., 2008 (55)	US, VETR	33–55	Lifetime dependence diagnosis	6.6	M	1857	1482	38	26	35
7	Kendler et al., 2006b (30)	Norway	Mean = 28.2 (3.9)	Symptoms of abuse or dependence	1.6	M F	220 448	117 263 338	75 (34–89)	0 (0–34)	23 (11–47)

A = additive genetic variance, C = shared environmental variance, E = unshared environmental variance