Table 3.

Model fitting of bivariate risk indicator and causal models on the association between age of alcohol initiation and alcohol use disorders in 1336 young adult twins sampled from a population-based registry.

	Parameters estimated
Model	ai	ci	ei	c	cc	ec	ad	cd	ed	b	Δχ2	Δdf	p	AIC
1. Risk indicator modela	x	x	x	x	x	x	x	x	x
2. Risk indicator model	x	x	x	x	x	x	x		x		0.6	1	0.56	−1.4
3. Risk indicator model	x	x	x		x	x	x		x		5.4	2	0.07	1.4
4. Risk indicator model b	x	x	x	x		x	x		x		1.1	2	0.58	−2.9
5. Risk indicator model	x	x	x	x	x		x		x		6.2	2	0.05	2.2
6. Causal model	x	x	x				x		x	x	5.0	3	0.17	−1.0

^aRisk indicator model # 1 had a −2loglikelihood of 22867.7 and 25 free parameters. Model #1 serves as a reference for fit of the other models.

^bBest fitting model. A description of the parameters estimated is given in the legend for Figure 2. The parameters estimated correspond to the ones given in Figure 1a and Figure 1b. The a, c, and e denote additive genetic, shared environmental, and individual-specific environmental risk factors, respectively. Subscript i, c, and d denote indicator-specific effects, effects common for indicator and disorder, and effects specific for disorder, respectively. The b denotes a phenotypic association between the indicator and the disorder. Δχ² is the difference in −2loglikelihood as compared to the reference model. Δdf is the difference in degrees of freedom as compared with the reference model. Akaike’s information criterion (AIC) is calculated as Δχ²-2Δdf.