THE NEW ZEALAND

MEDICAL JOURNAL
Journal of the New Zealand Medical Association

Association of parent and best friend smoking with stage of

adolescent tobacco smoking
Robert Scragg, Marewa Glover, Janine Paynter, Grace Wong, Judith McCool
Abstract
Objective compare the effect of parental and best friend smoking across the stages of
adolescent smoking, from being a never smoker susceptible to smoking, to being a
daily smoker
Method national cross-sectional annual survey (2002-2006 combined) of 157,637
Year 10 students aged 14 and 15 years who answered an anonymous self-
administered questionnaire.
Results the effects of smoking by parents and best friend varied with stage of
adolescent tobacco smoking. Attributable risk calculations showed that parental and
best friend smoking explained only 6.3% of susceptibility to smoking among never
smokers, and 21.7% of non-smoking students who had ever experimented with
cigarettes. The attributable risk for parental and best friend smoking progressively
increased with smoking frequency, up to 78.7% for daily smoking. The effect of best
friend smoking was stronger than parental smoking, although there was a synergistic
effect of both variables on the risk of daily smoking.
Conclusion smoking by best friend and parents are strongly associated with current
smoking by adolescents, but unrelated to susceptibility to smoke among those who are
non-smokers.

The majority of adult tobacco smokers take up smoking during adolescence. A

number of stages in the pathway to becoming a regular smoker during adolescence
have been described including ‘triers’, ‘experimenters’, ‘regular users’ and
‘dependent users’.1 Cohort studies of adolescents have identified various trajectories
through these stages to adult smoking, which combine measures of stability in
smoking status over time along with early/rapid or late/slow adopters of smoking.2-6
However, once adolescents start trying or experimenting with smoking, the transition
is mostly a one way process towards regular smoking,7 although some smokers start
quitting in their early 20s.2 5
An early review of the predictors of youth smoking concluded that there was a high
level of support for social learning variables, such as peer and family smoking, being
involved in the initiation of tobacco smoking.8 In particular, the review found stronger
evidence for an effect from peer smoking than for parental smoking. However,
subsequent reports have provided conflicting evidence with regard to this conclusion.
A Californian cohort study observed that friends’ smoking had a stronger effect on
adolescent smoking behaviour, particularly initiation, than parental smoking;9
although a subsequent report from this study found that, while smoking by friends
was important in the transition from trial to experimental smoking, parental smoking
predicted the transition from experimental to regular smoking.10

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The latter finding is supported by a US Mid-West cohort study which found that
parental smoking was associated with regular smoking in adolescence and adulthood,
but not with adolescent smoking experimentation; although the parental effects were
not as strong as peer effects.2
In contrast, a New Zealand cohort study observed that parental smoking predicted
smoking experimentation by age 13 years, while smoking at age 16 years was most
strongly predicted by affiliation with smoking peers at 15 years.11 Further, the US
National Longitudinal Study of Adolescent Health concluded that adolescent smoking
is more influenced by friend smoking than parent smoking, after comparing the
relative sizes of the risk ratios for these two variables.12
A recent review has concluded, based on the strengths of relative risks, that peer or
friend smoking is more strongly related to adolescent smoking than parental
smoking.13 However, this conclusion has recently been challenged by the argument
that the preferred measure of effect for ranking public health risk factors is the
population attributable risk, which integrates into a single measure both the strength
of a risk factor (i.e. the relative risk) and its frequency (prevalence).14 The population
attributable risk (or fraction) can be interpreted as the proportion of outcome events
(e.g. adolescent smoking) that can be attributed to (or explained by) an exposure
variable (assuming the latter is causative).15
Applying this calculation to a national sample of New Zealand Year 10 students
produced attributable risk values of 67% for best friend smoking and 64% for parental
smoking combined with exposures under parental control such as allowing smoking
in the home or amount of pocket money.14 Further, the influence of parents precedes
that of peers, and previous studies which have controlled for the effect of friend and
older sibling smoking in multivariate analyses will have underestimated the effect of
parental smoking.16 17
In this current paper we extend earlier results from the national Year 10 (aged 14 -15
years) surveys by comparing the relative importance of the influence of parental
smoking and best friend smoking on the various stages of adolescent smoking, along
the continuum from being a never smoker susceptible to smoking, to becoming a daily
smoker.
Method
Annual national surveys of tobacco smoking by Year 10 (4th form) students (ages 14-15 years) have
been carried out yearly since 1999.18 Each year, all New Zealand schools with Year 10 students were
invited to participate in the survey by administering a short questionnaire to their Year 10 students in
November. The current paper reports data from the 2002-2006 surveys which collected information on
smoking by parents and best friend of students. The annual school response rate was 67% in 2002 (n =
309), 66% in 2003 (n = 312), 65% in 2004 (n = 319), 58% (n = 278) in 2005, and 78% (n = 291) in
2006. The Ethics Committee of the Ministry of Health in Auckland granted a waiver of the formal
review and consenting processes.
School principals gave permission for teachers to supervise students while they completed the
anonymous self-administered questionnaires in class. To maintain confidentiality, teachers did not
examine the surveys for completeness.
Students answered a two-page questionnaire, which included questions on age, sex and ethnicity (self-
assigned). Because students could choose more than one ethnic group, a priority system was used to
classify any student choosing Māori as such, then any Pacific student as such, followed by any Asian

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student as such, followed by European. Students answered whether their mother, father or best friend
smoked; and whether people were allowed to smoke inside their house.
With regard to their own smoking status, students were asked “Have you ever smoked a cigarette, even
just a few puffs?”, and if they answered “yes”, they were asked “how often do you smoke now?” Those
who answered “no” to both questions were classified as never smokers, while those who answered
“yes” to the first question and “no” to the second were classified as experimenters. Those who
answered “yes” were queried about the frequency of their current smoking (at least once a day, at least
once a week, at least once a month, less often, never).
Susceptibility to future smoking was assessed by asking “Do you think you will smoke a cigarette at
any time during the next year?” Respondents were classified as non-susceptible only if they answered
‘definitely not’. Similar measures of susceptibility have been shown to predict experimentation with
tobacco smoking in previous youth cohort studies.19 20 Students smoking monthly or more often were
asked their age (in years) when they first started smoking monthly (for the years 2003-2005).
The total number of completed questionnaires returned by schools during the 5 year period was
167,488 (30,972 in 2002, 34,812 in 2003, 33,279 in 2004, 34,038 in 2005, and 34,387 in 2006), out of
229,240 on school rolls (73.1% student response). Analyses were restricted to 162,931 students who
were 14 and 15 years old. We further excluded students with missing data for gender (n=509), ethnicity
(n=1283), student smoking status (n=1,291), and parent or best friend smoking status (n=2,211). This
left 157,637 students available for analyses.
All statistical analyses were made using SAS callable SUDAAN (Release 9.0.1, 2005) which corrects
standard errors and confidence intervals for any design effect from clustering of students by school.
The CROSSTAB procedure was use to calculate relative risks, and the MULTILOG procedure was
used to calculate adjusted odds ratios (OR) while adjusting for age, gender and ethnicity, and to test for
interaction. The population attributable risk was calculated by estimating the attributable proportion for
the exposed cases within each exposure category using standard methods.21

Results
The distribution of smoking status, by level of demographic variable and smoking
status of parents and best friend, is shown in Table 1. Age was associated with an
increased risk of smoking, with a higher proportion 15-year-old students distributed in
the smoking categories than 14-year-old students (p<0.0001). Girls were more likely
to be smokers than boys, who had a higher proportion of never smokers (49.3% v.
43.3%, p<0.0001). With regard to ethnicity, smoking levels were highest among
Māori students, followed in order by Pacific, European and Asian (p<0.0001).
Students who lived in a house where smoking was allowed were three times more
likely to be daily smokers than those who did not (20.7% v. 6.3%, p<0.0001).
When students were categorised by the smoking status of their parents and best friend,
student smoking levels were highest among those with both parents and best friend
being smokers (41.1% daily smokers), followed by students who had non-smoking
parents but their best friend smoked (19.1% daily smokers), and by students with
smoking parents but best friend a non-smoker (5.4% daily smokers), while smoking
was lowest among students with neither parents nor best friend being smokers (1.8%
daily smokers). Students were more likely to be exposed to parental smoking (40%)
than best friend smoking (25%).
Table 2 shows the relative risk of never smoking students being susceptible to
smoking in the next year, associated with parental and best friend smoking. Students
of non-smoking parents with a best friend who smoked were most likely to think they
would smoke during the next year (47.9%), followed by students with both parents
and best friend being smokers (41.4%).

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Table 1. Distribution of student tobacco smoking status, by demographic status and parental and best friend smoking

Variable N Never smoked Not smoking now Smokers

Not susceptible Susceptible Experimenters Less than monthly Monthly Weekly Daily
%* % % % % % %
Age (years)
14 85,157 33.3 15.1 26.7 8.1 3.8 3.7 9.2
15 73,068 30.5 13.2 27.3 9.0 4.2 4.3 11.5
Sex
Male 76,914 35.7 13.6 28.9 7.6 3.0 2.9 8.2
Female 80,311 28.4 14.9 25.2 9.4 4.9 5.0 12.2
Ethnicity
European 103,392 32.5 16.6 26.7 9.1 4.2 3.7 7.3
Maori 28,496 16.6 7.8 32.4 9.5 4.6 5.9 23.2
Pacific 10,813 30.3 9.9 30.5 8.1 3.7 4.6 13.1
Asian 14,524 60.1 13.7 15.9 3.3 1.3 1.6 4.1
People smoke in house
Yes 42,459 18.3 10.5 29.6 10.1 5.0 5.9 20.7
No 113,162 37.3 15.7 26.0 7.9 3.6 3.3 6.3
Parents smoke Friend smokes
Yes Yes 22,770 5.7 4.0 20.0 11.7 7.2 10.2 41.1
Yes No 40,349 29.8 13.5 36.5 8.7 3.3 2.7 5.4
No Yes 16,879 10.4 9.6 24.3 15.8 10.1 10.8 19.1
No No 77,231 45.6 18.7 24.6 5.9 2.0 1.3 1.8
*Row percents.

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Table 2. Relative risk, and attributable risk, of a never smoker being susceptible
to smoking in next year.

Parent Best friend Susceptible Relative Risk Attributable Population

smokes smokes Yes No (95%CI)# cases attributable risk+
N (%)* N
Yes Yes 918 (41.4%) 1297 1.45 (1.37–1.53) 285 1.3%
Yes No 5449 (31.2%) 12,041 1.10 (1.06–1.13) 495 2.2%
No Yes 1615 (47.9%) 1755 1.63 (1.57–1.70) 624 2.8%
No No 14,413 (29.0%) 35,219 1.00 – –
Total 22,395 50,312 1404 6.3%
*Percent of total of each parent-friend smoking category; # Adjusted for age, sex and ethnicity; +Attributable cases
/ total number of susceptible.

The effect of parental smoking by itself was weak, with only a 10% relative increase
in the risk of being susceptible (to 31.2%) compared with the reference category of
students with neither parents nor best friend being smokers (29.0% susceptible). The
population attributable risk, which gives the proportion of susceptible students that
can be explained by parental and/or best friend smoking, was only 6.3%, indicating
that susceptibility is explained primarily by other risk factors.
A stronger effect from parental and best friend smoking was seen on the risk of
current non-smokers having ever experimented with cigarettes (Table 3). The relative
risk (RR) of being an experimenter was highest for students with both parents and
best friend being a smoker (RR = 2.01) or with best friend only being a smoker (RR =
1.83). However, the attributable risk value was highest for students with parents only
being smokers (11.9%) because students in this category made up a greater proportion
(29%) of all students not currently smoking compared to the previous two categories
(each 6%). Collectively, 21.7% of current non-smoking students who had ever
experimented with cigarettes could be explained by parental and/or best friend
smoking.
The relative and attributable risks of current student smoking associated with parent
and best friend smoking are shown in Table 4. The general pattern for less than daily
smoking by students was for the effect to be strongest for best friend smoking alone,
followed by both parent and best friend smoking, with parent smoking alone having
the lowest relative risks within each of these student smoking categories. In contrast,
for daily smoking, the effect of both parent and best friend smoking combined (RR =
14.29) was more than the sum of the net effect of parent smoking alone (RR = 2.19)
and best friend alone (RR = 8.25). This interaction was statistically significant
(p<0.0001).
More than half of student daily smokers (53.9%) could be attributed to the combined
effect of parent and best friend smoking. The other important feature of the results in
this table is the progressive increase in the population attributable risk values with
increasing frequency of smoking: from 28.3% for students smoking less than monthly
up to 78.7% for those smoking daily. The pattern in Table 4 occurred within each sex,

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with parental and best friend smoking, separately and together, being significantly
(p<0.01) associated with all frequencies of adolescent smoking (data not shown).

Table 3. Relative risk, and attributable risk, of a non-smoker having ever

experimented with cigarettes

Parent Best friend Experimented Relative Risk Attributable Population

smokes smokes Yes No (95%CI)# cases attributable risk+
N (%)* N
Yes Yes 4553 (66.9%) 2250 2.01 (1.94–2.07) 2288 5.4%
Yes No 14,742 (45.6%) 17,610 1.52 (1.49–1.55) 5043 11.9%
No Yes 4098 (54.7%) 3401 1.83 (1.78–1.89) 1859 4.4%
No No 19,028 (27.6%) 49,858 1.00 - -
Total 42,421 73,119 9190 21.7%
*Percent of total of each parent-friend smoking category; #Adjusted for age, sex and ethnicity; +Attributable cases
/ total number of experimenters.

Table 4. Relative risk and attributable risk of smoking, associated with smoking
by parent and best friend, by frequency of student smoking.

Parent Best friend N (%)* in smoking Relative Risk Attributable Population

smokes smokes category (95% CI)# cases attributable risk+
Smoking < Monthly
Yes Yes 2672 (11.7%) 1.82 (1.72–1.93) 1204 9.0%
Yes No 3519 (8.7%) 1.41 (1.34–1.48) 1023 7.6%
No Yes 2665 (15.8%) 2.44 (2.32–2.58) 1573 11.7%
No No 4589 (5.9%) 1.00 - -
Total 13,445 3800 28.3%
Monthly smoking
Yes Yes 1648 (7.2%) 3.15 (2.90–3.42) 1125 18.0%
Yes No 1344 (3.3%) 1.54 (1.43–1.67) 471 7.5%
No Yes 1707 (10.1%) 4.43 (4.10–4.79) 1322 21.2%
No No 1549 (2.0%) 1.00 - -
Total 6248 2918 46.7%
Weekly smoking
Yes Yes 2321 (10.2%) 5.83 (5.28–6.44) 1923 30.7%
Yes No 1084 (2.7%) 1.72 (1.57–1.89) 454 7.2%
No Yes 1821 (10.8%) 6.69 (6.13–7.30) 1549 24.7%
No No 1035 (1.3%) 1.00 - -
Total 6261 3926 62.7%
Daily smoking
Yes Yes 9361 (41.1%) 14.29 (13.1–15.6) 8706 53.9%
Yes No 2170 (5.4%) 2.19 (2.03–2.36) 1179 7.3%
No Yes 3214 (19.0%) 8.25 (7.57–8.99) 2824 17.5%
No No 1398 (1.8%) 1.00 - -
Total 16,143 12,709 78.7%
* Percent of total of each parent-friend smoking category; #Adjusted for age, sex and ethnicity; +Attributable cases
/ total number of smokers in same smoking frequency category.

Controlling for smoking in the house greatly reduced the relative risks associated with
parental smoking, with this confounding effect weakening with reducing frequency of

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student smoking. For example, compared to students not exposed to parent nor to best
friend smoking, the RR of daily smoking in students exposed to both parent and best
friend smoking decreased from 14.49 shown in Table 4 to 9.51 (95%CI: 8.74–10.36)
with additional adjustment for smoking in the house; while the RR of smoking less
than monthly for the same exposure declined from 1.82 in Table 4 to 1.67 (95%CI:
1.57–1.79) with additional adjustment for smoking in the house.
There was an inverse association between age of starting smoking and frequency of
smoking (Table 5). Students who smoked daily were nearly twice as likely to have
started smoking by the age of 9 years (18.2%) than students smoking weekly (11.8%)
or monthly (10.1%).

Table 5. Distribution of age students started smoking monthly, up to 13 years, by

current frequency of smoking: 2003–2005

Age started smoking monthly (years) Smoking frequency

Daily Weekly Monthly
<9 18.2% 11.8% 10.1%
10 11.1% 9.4% 7.9%
11 14.4% 12.5% 11.4%
12 25.0% 25.7% 25.9%
13 31.3% 40.6% 44.7%
(N) (8030) (2533) (2487)

The increased risk of early initiation in cigarette smoking for daily smokers remained
statistically significant after controlling for demographic variables (p<0.0001). This
finding is consistent with the results in Figure 1 which show that students with both
parents and best friends who smoke have a higher prevalence of smoking at all ages,
followed by students with only their best friend being a smoker, followed by students
with only their parents being smokers, while students with both parents and best
friends who are non-smokers have the lowest smoking prevalences at all ages.

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Figure 2 shows that the proportion of student smokers with a parent who smokes was
higher in their younger years (age < 12 years), when it ranged from 68-72%,
compared with 63% at ages 14 and 15 years.

Discussion
We have shown with data collected in 5 national surveys of New Zealand Year 10
students that the effects of smoking by parents and best friends vary with stage of
adolescent tobacco smoking. The effects were found to be weakest in the earliest
stage along the smoking trajectory, which is susceptibility to smoking among never
smokers (Table 2), and progressively increasing with smoking frequency to be
strongest among daily smokers (Table 4).
The very weak contribution of parental and best friend smoking to smoking
susceptibility among never smokers indicates that other factors, such as possibly risk
taking, family conflict and low self-esteem,8 10 are influential in this first step once
children reach adolescence.
Our results in Table 3 showing a stronger relative risk of experimental smoking from
exposure to friend smoking alone (RR = 1.83), than parental smoking alone (RR =
1.52), is consistent with previous studies which have reported larger relative risks or
mean smoking levels for peer smoking compared with parental.2 6 10 However, our
attributable risk calculations show that parental smoking, because it is more common
than best friend smoking in this age group, is a more important contributor to
experimental smoking, explaining 11.9% by itself, plus 5.4% in combination with
best friend smoking, for a total parental effect of 17.3% (Table 3).
Thus, our results indicate that parental smoking has a key role, and along with peer
smoking, collectively explains about one fifth of smoking experimentation among
adolescents who are not current smokers.

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At the other end of the smoking trajectory, parental and best friend smoking explain a
large proportion of adolescent smoking, varying from 28.3% of less than monthly
smokers up to 78.7% of daily smokers (Table 4). While best friend smoking has
stronger relative risks and attributable risks than parental smoking, the latter is still an
important contributor to adolescent smoking. Parental smoking is most strongly
associated with daily smoking, with the attributable risk calculations showing that it
explains 61.2% of daily smoking either by itself or in combination with best friend
smoking (Table 4). The interaction (or synergistic effect) observed with daily
smoking, whereby the combined effect of parental and best friend smoking (RR =
14.29) is more than the sum of the net effect for parental smoking alone (RR = 2.19)
plus the net effect of best friend smoking alone (RR = 8.25), indicates that the
combined effect of exposure to both parent and peer smoking contributes in part to
daily smoking.
We have previously reported that students with parents who smoke are more likely to
have best friends who smoke.14 The confounding effect from smoking in the home on
the relative risk of daily smoking associated with parent and best friend smoking
emphasizes the importance of the home environment in facilitating adolescent
smoking.14 It is plausible that parents who smoke, by allowing smoking in the home,
for example, create an environment where their children are more likely to interact
with, and befriend, peers who smoke. Thus, part of the parental effect may be
transmitted through peer smokers. Where there is joint exposure, the influence of
parents typically can be expected to precede that of peers, with the consequence that
parental influences are likely to be involved in the types of friends selected by
adolescents.17
The finding of a strong association between parental smoking and daily adolescent
smoking is complemented by the earlier age of starting smoking by students who
smoke daily compared with students who smoke less often (Table 5). This finding is
consistent with previous research.22 Further, the proportion of adolescent smokers
with a parent who smokes is highest for ages <12 years, after which the proportion of
smokers exposed to friend smoking (alone) increases (Figure 2).
Overall, this pattern is consistent with the interpretation that students with parents
who smoke, start smoking earlier than other smokers, so that they are more likely to
be daily smokers in their mid-teens. Further, because many of these students with
smoking parents socialise with other students who smoke, together they drive the
spread of the smoking epidemic among the wider student body.
A major limitation of this study is the cross-sectional design which cannot distinguish
cause and effect. The timing of when parental and peer effects occur can only be
properly studied by cohort studies, which can determine, for example, whether
perceptions by youth that smoking is the norm for children of their age precede the
onset of susceptibility. Moreover, students defined as susceptible or experimenter at
ages 14 and 15 years may not represent the experience of students who are smokers
when they passed through these stages at younger ages.
Another limitation is that our measure of parental smoking did not allow for single
parent and extended family households. However, such measurement error, if random,
is likely to have resulted in under-estimation of the effects associated with parental
smoking. In addition, we did not examine the full range of personal variables

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associated with adolescent smoking (e.g. personality, attitudes, parent attachment),
which could potentially confound the association with parental smoking.
In summary, we have found that smoking by both best friend and also parents explain
a high proportion of adolescent smokers. However, given evidence showing the
limited success of school-based interventions against the effects of peer-smoking,23
our findings support efforts to prevent youth tobacco smoking by targeting parents
who smoke, which may have a double benefit of reducing both adult and adolescent
smoking. In contrast, neither parental nor peer-smoking are related to smoking
susceptibility among adolescent non-smokers, which suggests other factors may be
involved in the progression to smoking once children reach adolescence.
Acknowledgements: The surveys were carried out by Action on Smoking and Health
(ASH), with assistance of the Health Sponsorship Council of New Zealand in 2006.
The New Zealand Ministry of Health provided funds.
Correspondence: Assoc Prof Robert Scragg, Epidemiology & Biostatistics, School
of Population Health, University of Auckland, Private Bag, Auckland, New Zealand.
Fax: +64 (0)9 3737503; Email: r.scragg@auckland.ac.nz
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