We found that the smoking behavior of friends and their attitude toward smoking were social factors strongly associated with adolescent cigarette smoking. Another strong force protecting adolescents from smoking was parents attitudes towards smoking, the quantity of time spent with parents, and absence of "serious" verbal conflict between parents and child. Alcohol consumption during the previous 30 days greatly increased the risk of smoking both in the unadjusted and adjusted analysis for both daily and occasional smoking.
In this study demographic factors seem to be of little importance for adolescent smoking, probably reflecting the homogeneity of the Icelandic population and relatively well developed social security system. Parental support, parental control and time spent with parents are all protective factors of adolescent smoking in the crude analysis but only time spent with parents continues to be of importance in the adjustment models. This is consistent with prior knowledge from Iceland and is presently one of the cornerstones in the Icelandic Model of substance use prevention (a detailed paper describing this prevention approach is currently in submission). Moreover, this study supports the notion that the quantity of time parents spend with their children may be a major protective factor for adolescent smoking. But the adolescents' perception that parents are not against adolescent smoking greatly increases the risk of being a daily smoker, independent of other factors (OR = 8.92, CI = 6.04–13.17). However, since this study was carried out with a cross-sectional design, this finding may reflect that parents are perceived to tolerate the smoking of adolescents rather than being a risk factor for smoking initiation. Yet, data from other studies indicate that parental expectations do function as a protective factor for the initiation of adolescent smoking , which is consistent with our preferred interpretation of these findings.
Family conflict has been reported to be strongly related with increased risk of adolescent smoking [22, 23]. The social context of this relationship is not fully understood. Findings in this study indicate that it may particularly be serious verbal conflicts between the adolescent and his/her parents that is of importance, being the only family-conflict variable of the four (with interparental serious verbal arguments, interparental violence, and parent-adolescent violence) that remains significantly related to daily smoking after adjustments for other variables in the analysis. This issue will need further study in the future.
Although mothers' and sibling smoking remained significantly related to adolescent smoking after adjustments, and this relationship has shown to be of importance in previous studies , the impact was relatively small in comparison to other factors with adjusted odds ratio of 1.69 (CI = 1.20–2.40) for mother's smoking and 1.55 (CI = 1.10–2.18) for sibling's smoking. Reporting having friends that smoke increased the risk of being a daily smoker 17-fold when adjusted for other variables in the analysis. This was by far the strongest risk factor in the study after adjusting for other factors. The reason for this high odds ratio is the group division of our dependent variable. Most public health studies on adolescent smoking divide participants into "never" and "ever" smokers. In this study, those who smoke daily are distinguished from those who smoke occasionally and both groups are analyzed against those who do not smoke at all. The present finding nevertheless supports previous research that the smoking of peers is of central importance for adolescent smoking [8, 17, 25]. Recently, however, this notion has been doubted by Arnett  who argues that we need to look more closely into the peer-selection process to fully understand the important role of peer smoking in adolescent smoking.
One problem with interpreting the role of peer influences is that we have no data on the order of initiation to smoking in the peer group and consequently do not know who influenced whom. Notwithstanding, what we can safely say is that birds of a feather tend to flock together when it comes to adolescent tobacco smoking. Also, by including in the model a measure of perceived respect from peers, if one would smoke cigarettes, in addition to the smoking of peers, we may be able to analyze further the influence of the peer group. In this study, if smoking cigarettes was perceived to increase peer respect, the odds of daily smoking was 3.27 in the adjusted analyses, down from an OR = 9.96 in the crude analysis which is probably because of other protective factors in the model.
Finally, higher levels of physical activity and academic achievement were protective factors to the risk of adolescent daily smoking after adjustment. This is consistent with the findings of previous studies . In contrast, alcohol consumption during the last 30 days and having used smoke-free tobacco greatly increase the risk of daily and occasional smoking, underlining the fact that it is often the same individuals who engage in risk behaviors of more than one kind.
Due to the study's cross-sectional design we were not able to draw any firm conclusions regarding causality between the dependent and independent variables. Also, small adjusted odds ratio values in a sample of this size (7,430 respondents) should be interpreted with caution, particularly when the 95% confidence intervals are close to, but do not include, 1.0 .