Table 3.

Path estimates for the best-fit model of substance consumption^a

Factor (age)	Factor loadings			Genetic factors				Shared environmental factors				Unique environmental factors
	SM	ETOH	DU	Total				Total				Total
				a2, %	A1	A2	A3	c2, %	C1	C2	C3	e2, %	E1	E2	E3
1 (13–14)	0.94	0.82	0.62	52	0.72			35	0.59			13	0.36
2 (16–17)	0.87	0.76	0.85	54	0.70	0.21		34	0.43	0.40		12	−0.03	0.35
3 (19–20)	0.72	0.53	0.82	53	0.63	0.32	0.36	19	0.30	0.31	−0.07	18	0.05	0.13	0.40

Factor loading, depicted in Fig. 1, connects the common factor to self-reported substance use; a², heritability or proportion of variance in substance use resulting from genetic factors; c², proportion of variance in substance use resulting from shared environmental factors; e², proportion of variance in substance use resulting from unique environmental factors; A1, A2 and A3, additive genetic path estimates at ages 13–14, 16–17 and 19–20 respectively; C1, C2 and C3, shared environment path estimates at ages 13–14, 16–17 and 19–20 respectively; E1, E2 and E3, unique environment path estimates at ages 13–14, 16–17 and 19–20 respectively; SM, regular smoking; ETOH, alcohol intoxication; DU, illicit drug use.

^aResults for ‘ upper portion ’ of the model only (Fig. 1).