Table 2.

Standardized variance components and 95% confidence intervals between brackets from fitting two multivariate models to data on the initiation and progression of smoking and cannabis and other illicit drug use

	Smoking		Cannabis use
Variable	Initiation	Progression	Initiation	Progression
Causal model (−2 log likelihood = 9,963.550, df = 10,601, AIC = −11,238.450)
Additive genetic	.58	.43	.32	–
	[.44, .72]	[.32, .55]	[.12, .53]
Common environment	.34	–	.20	–
	[.21, .47]		[.02, .37]
Unique environment	.08	.10	.08	.17
	[.05, .12]	[.05, .16]	[.03, .14]	[.07, .55]
Initiation of smoking	na	.41	.40
		[.24, .57]	[.32, .48]
Initiation of cannabis	na	.06	na	.83a
		[.02, .13]		[.44, .93]
Cholesky factor model (−2 log likelihood = 9,978.852, df= 10,599, AIC = −11,219.148)
Additive genetic	.61	.13	.59	–
	[.53, .69]	[.02, .24]	[.51, .70]
Common environment	.31	.13	.30	–
	[.24, .37]	[.02, .24]	[.19, .40]
Unique environment	.08	.10	.11	.20
	[.06, .10]	[.07, .14]	[.08, .15]	[.09, .33]
Initiation of smoking	na	.64	na	na
		[.56, .67]
Initiation of cannabis	na	na	na	.80b
				[.67, .89]
Correlations
Additive genetic	.57	–
	[.44, .65]
Common environment	.82	–
	[.72, .95]
Unique environment	.99	–
	[.89, 1.00]

Notes: AIC = Akaike's Information Criteria; na = not applicable.

^aAlso includes path from initiation of smoking through initiation of cannabis/drug use (i.e., part of liability of cannabis/drug progression is attributable to the effect of liability to smoking initiation on cannabis/drug initiation);

^balso includes liabilities common to smoking initiation and initiation of cannabis/drug use.