Skip to main content
. Author manuscript; available in PMC: 2019 Jan 1.
Published in final edited form as: Am J Psychiatry. 2017 Aug 25;175(1):63–70. doi: 10.1176/appi.ajp.2017.17010009

Table 2.

Results of Individual-Level and Twin Difference Analysesa of Baseline ADHD Symptoms on Smoking Initiation, Progression to Daily Smoking, Cigarettes Per Day, and DSM-IV Nicotine Dependence Symptoms by Age 17

ADHD Inattentive Symptoms (raw- or z-score)
Gender Moderation?b Individual-Level Analysisc Within-Pair Twin Difference Analysis (in complete pairs only)
Smoking Involvement by Age 17 All Twins
N=3629		 Dizygotic Pairs
N=652		 Monozygotic Pairs
N=1154
Initiation of Use – age in years No Hazard Ratiod 95% CI Hazard Ratio 95% CI p Hazard Ratio 95% CI p
1.16 1.13, 1.20 1.06 1.01, 1.11 0.023 1.08 1.02, 1.14 <0.010
Progression to Daily Smoking Yes; Female>Male All Twins
N=3478		 Dizygotic Pairs
N=617 (321 female;296 male)		 Monozygotic Pairs
N=1092 (570 female;522 male)
Odds Ratio 95% CI Odds Ratio 95% CI p Odds Ratio 95% CI p
0 = none Female Female Female Female Female Female Female Female
1 = initiated, never daily 1.27 1.17, 1.37 1.12 0.99, 1.26 0.063 1.19 1.01, 1.40 0.041
2 = smoking daily starting since age 16 Male Male Male Male Male Male Male Male
3 = smoking daily before age 16 1.19 1.11, 1.27 1.03 0.92, 1.14 0.609 1.06 0.92, 1.22 0.392
Maximum Cigarettes Per Day(z)e Yes; Female>Male All Twins
N=3457		 Dizygotic Pairs
N=609 (318 female;291 male)		 Monozygotic Pairs
N=1085 (568 female;517 male)
β 95% CI β 95% CI p β 95% CI p
Female Female Female Female Female Female Female Female
.17 .13, .21 .10 .01, .18 0.030 .17 .10, .24 <0.0001
Male Male Male Male Male Male Male Male
.07 .03, .11 −.04 −.12, .05 0.421 .01 −.06, .08 0.749
Symptoms of Nicotine Dependence(z)e Yes; Female>Male All Twins
N=3478		 Dizygotic Pairs
N=617 (321 female;296 male)		 Monozygotic Pairs
N=1092 (570 female;522 male)
β 95% CI β 95% CI p β 95% CI p
Female Female Female Female Female Female Female Female
.17 .12, 21 .10 .01, .18 0.047 .17 .09, .25 <0.0001
Male Male Male Male Male Male Male Male
.09 .05, .14 −.01 −.09, .07 0.799 .04 −.03, .12 0.258
ADHD Hyperactive-Impulsive Symptoms (raw- or z-score)
Gender Moderation?b Individual-Level Analysisc Within-Pair Twin Difference Analysis (in complete pairs only)
Smoking Involvement by Age 17 All Twins
N=3629		 Dizygotic Pairs
N=652 pairs		 Monozygotic Pairs
N=1154 pairs
Initiation of Use – age in years No Hazard Ratiod 95% CI Hazard Ratio 95% CI p Hazard Ratio 95% CI p
1.24 1.20, 1.29 1.07 1.01, 1.14 0.029 1.06 0.98, 1.14 0.150
Progression to Daily Smokinge Yes; Female>Male All Twins
N=3478		 Dizygotic Pairs
N=617 (321 female; 296 male)		 Monozygotic Pairs
N=1092 (570 female; 522 male)
Odds Ratio 95% CI Odds Ratio 95% CI p Odds Ratio 95% CI p
0 = none Female Female Female Female Female Female Female Female
1 = initiated, never daily 1.45 1.32, 1.59 1.31 1.14, 1.52 <0.001 1.20 0.98, 1.45 0.073
2 = smoking daily starting since age 16 Male Male Male Male Male Male Male Male
3 = smoking daily before age 16 1.24 1.15, 1.34 1.03 0.84, 1.26 0.773 0.98 0.88, 1.08 0.643
Maximum Cigarettes Per Day (z)e Yes; Female>Male All Twins
N=3457		 Dizygotic Pairs
N=609 (318 female; 291 male)		 Monozygotic Pairs
N=1085 (568 female; 517 male)
β 95% CI β 95% CI p β 95% CI p
Female Female Female Female Female Female Female Female
.21 .16, .25 .20 .11, .29 <0.0001 .13 .04, .21 0.003
Male Male Male Male Male Male Male Male
.10 .06, .14 −.03 −.12, .06 0.547 −.03 −.11, .05 0.432
Symptoms of Nicotine Dependence(z)e Yes; Female>Male All Twins
N=3478		 Dizygotic Pairs
N=617 (321 female;296 male)		 Monozygotic Pairs
N=1092 (570 female;522 male)
β 95% CI β 95% CI p β 95% CI p
Female Female Female Female Female Female Female Female
.20 .15, .24 .22 .12, .31 <0.0001
DZ>MZf 		.06 −.03, .16 0.175
Male Male Male Male Male Male Male Male
.11 .06, .15 −.06 −.14, .04 0.205 .02 −.08, .11 0.738
a

Individual-level analyses include all adolescents; twin difference analyses include pairs in which both twins have data. Significant effects are in bold. Because all individual-level effects of ADHD were significant (all ps <.0001), specific significance levels are given only for within-pair effects.

b

Gender moderation test statistics and significance levels are given in the text.

c

Individual-level estimates were covariate-adjusted for age at assessment (except for initiation), cohort, parental SES, and gender (except when presented separately by gender). Within-pair estimates are not affected by adjusting for shared covariates.

d

Effects for initiation are given as hazard ratios; effects for progression to daily smoking are given as odds ratios. Those significantly greater than 1.0 correspond to the increased likelihood of initiating at each age (ages 8–18) or progressing a level toward daily smoking (e.g., from 1 to 2), respectively, associated with a 1 symptom increase in ADHD. Within-pair estimates reflect differential likelihood of initiating or progressing in frequency associated with a twin having 1 more ADHD symptom than his or her cotwin.

e

Effects for CPD and nicotine dependence are given as standardized beta coefficients (β), because ADHD symptoms, CPD, and log-transformed nicotine dependence symptoms were all converted to standardized (z) scores, with a mean of 0 and SD of 1. Individual-level estimates thereby reflect the increase in CPD or symptoms (in SD units) associated with a 1 SD increase in ADHD; within-pair estimates reflect the difference (in SD units) associated with a twin being 1 SD higher in ADHD than his or her cotwin.

f

DZ>MZ indicates that the within-pair effect was significantly greater for DZs than MZs (p<.05).