Table 4. Multivariate ORs jointly estimated for individual current smoking and excess concordance of smoking within twin pairs.

	Model 1			Model 2
Factor	OR	95% CI	P-value	OR	95% CI	P-value
Individual current smoking
Monozygosity	0.77	0.63–0.94	0.01	0.77	0.62–0.94	0.01
Parents smoke	1.86	1.63–2.11	<0.001	1.75	1.45–2.09	<0.001
Wave 2	2.72	2.29–3.22	<0.001	1.20	0.99–1.46	0.06
Wave 3	1.70	1.42–2.04	<0.001	1.61	1.29–2.02	<0.001
Wave 4	1.93	1.64–2.27	<0.001	1.08	0.87–1.34	0.5
Friends' smoking	NA			10.90	7.90–15.10	<0.001
Excess concordance of current smoking
Monozygosity (wave 1)	1.99	1.26–3.15	0.003	2.60	1.47–4.60	0.001
Monozygosity (wave 2)	1.83	1.22–2.76	0.004	1.12	0.78–1.60	0.5
Monozygosity (wave 3)	1.68	1.07–2.66	0.03	1.13	0.79–1.62	0.5
Monozygosity (wave 4)	1.30	0.86–1.98	0.2	1.18	0.66–1.68	0.5
Concordance of friends' smoking	NA			1.55	1.30–1.85	<0.001

CI=confidence interval; OR=odds ratio; NA=not applicable.

Excess concordance also adjusted for main effect of wave (no significant association).

Estimates shown adjusted for all variables in model.