Table 3.

Within-pair fixed-effects negative binomial regression models for the depression score outcome (at the age of 17) by smoking behavior (at the age of 14)

Cigarette smokinga	All pairs						DZ						MZ
	Crude modelb			Adjusted model			Crude modelb			Adjusted model			Crude model			Adjusted model
	IRR	95% CI	p	IRR	95% CI	p	IRR	95% CI	p	IRR	95% CI	p	IRR	95% CI	p	IRR	95% CI	p
Lifetime cigarettes smoked	N = 1877 pairs			N = 1789 pairs			N = 1176 pairs			N = 1120 pairs			N = 614 pairs			N = 585 pairs
1–50	1.10	1.02–1.19	0.018	1.07	0.98–1.16	0.114	1.12	1.01–1.24	0.025	1.09	0.98–1.21	0.101	1.01	0.83–1.22	0.935	1.00	0.83–1.21	0.971
> 50	1.13	0.99–1.30	0.069	1.05	0.90–1.22	0.521	1.14	0.96–1.36	0.139	1.06	0.87–1.28	0.571	1.11	0.86–1.44	0.413	1.08	0.80–1.44	0.625
Current smoking status at the age of 14	N = 1910 pairs			N = 1819 pairs			N = 1199 pairs			N = 1140 pairs			N = 624 pairs			N = 595 pairs
Experimenters	1.09	1.00–1.18	0.041	1.06	0.98–1.15	0.155	1.10	1.00–1.21	0.056	1.08	0.97–1.20	0.143	1.03	0.86–1.24	0.719	1.04	0.87–1.24	0.655
Quitters or trying to quit	1.22	1.05–1.41	0.007	1.14	0.97–1.34	0.109	1.25	1.04–1.51	0.020	1.17	0.96–1.44	0.126	1.07	0.82–1.39	0.625	1.07	0.82–1.41	0.610
Regular smokers	1.23	1.08–1.40	0.002	1.10	0.95–1.27	0.205	1.26	1.07–1.48	0.006	1.12	0.93–1.35	0.219	1.09	0.85–1.40	0.481	1.08	0.82–1.42	0.581

^aReference category: never smokers

^bAdjusted for sex in all pairs and DZ pairs

IRR, incidence rate ratio; CI, confidence interval

Adjusted model: sex, school grades, drinking alcohol to intoxication, health status, and pre-existing depressiveness