and psychosocial traits, especially when controlling for confounders (e.g., analysis of longitudinal data; Rose et al., 2004). However, when adjusting for confounders it is important to adjust for confounders, which were assessed at an earlier time point than the predictor variable to avoid that the confounders themselves are affected by the predictor variable (Vansteelandt, 2009; Van der Weele, 2009). Mediation analysis permits a test of whether the direct effect of a predictor variable on an outcome variable can be explained by a mediator lying on the pathway between these two variables. I used mediation analysis to explore whether the relationship between early pubertal timing (predictor variable) on increased adolescent substance use (outcome variable) is explained indirectly by mediators in the psycho-social domain (parents-adolescent relationship quality and peer deviance; see Figure 3). Moderation analysis on the other hand allows examination of whether the effect of a predictor variable (in this case parent-adolescent relationship quality or peer deviance) on an outcome variable (adolescent substance use) is different for early, on-time and late maturing adolescents (that is, pubertal timing is the moderator variable; see Figure 4).
Mediation models help to establish whether prevention programs should address the indirect effect rather than the direct effect. More specifically mediation analyses will
provide information on how the prevention program will work (MacKinnon, 2011). Moderation analyses, on the other hand, identify the risk group to aim the prevention program at. More specifically, moderation analyses provide information on for which group the prevention program will work (MacKinnon, 2011). Consequently, the findings of such models can contribute to increased understanding and ultimately lead to more effective prevention approaches.
Although early pubertal maturation, poor parent-adolescent relationship quality and affiliation with deviant peers have each been identified as individual risk factors for increased substance use in adolescence, to my knowledge only a handful of studies have examined the links between these factors. Negriff and Trickett (2012) reported that the effect of pubertal timing on alcohol and cannabis use at age 13 years was mediated by peers’ alcohol and cannabis use at age 12. That is, early maturers, who subsequently affiliated with alcohol and cannabis using peers at age 12 years, tended to use more alcohol and cannabis at age 13 years. Biehl and colleagues (2007) reported that early maturing girls with more alcohol drinking friends used more alcohol when aged between 12-16 years than early maturing girls with fewer alcohol drinking friends. Marklein and colleagues (2009) hypothesised, that early maturers, who affiliated with cigarette smoking peers, tended to use more alcohol, cigarettes and cannabis than early maturers who affiliated with non-smoking peers. However, they found no evidence that peers’ cigarette use moderated the relationship between pubertal timing and girls’ alcohol, cigarette and cannabis use in a sample aged 11-17 years. This lack of evidence might be due to their relatively small sample size (N=264 girls). Lynne-Landsman and colleagues (2010) tested for moderation and found that early maturing girls in average and high risk
families (based on household resources, conflict and stability of household structure) had higher levels of binge drinking, cigarette and cannabis use at age 14 than early, on-time and late maturing girls in low risk families. Finally, Shelton and van den Bree (2010) reported that pubertal timing moderated the effect of parent-adolescent relationship quality (warmth and closeness) on cigarette use at age range 12-15 years. More specifically the effect of low levels of parent-child warmth and closeness on increased cigarette use was stronger in late maturing girls compared to early and on-time maturing girls.
Taken together, studies have focussed either on the effect of pubertal timing and the family environment on adolescent substance use or on the effect of pubertal timing and the peer environment on adolescent substance use. This does not permit a comparison of the combined effects regarding the family environment with the combined effects regarding the peer environment, as they are mostly based on the analyses of different samples. Additionally, only one study has examined the mediating role of peer deviance on the effect of pubertal timing on substance use (Negriff & Trickett, 2012) and no research appears to have tested the mediating role of the family environment. Two studies have focussed on the moderating role of peer deviance (Biehl et al., 2007; Marklein et al., 2009), and these have reported inconsistent findings. Finally, two studies have focussed on family functioning (Lynne-Landsman et al., 2010; Shelton & van den Bree, 2010), whereby Lynne-Landsmann and colleagues (2010) focussed on the moderating role of low, average and high risk families (based on household resources, conflict and stability of household structure) and Shelton and van den Bree (2010), on the other hand, focussed on the moderating role of pubertal timing. I therefore aimed to contribute to the existing
literature by examining the role of parent-adolescent relationship quality and peer
deviance in the links between pubertal timing and substance use in girls by exploring the evidence for the two different models of interrelationships explained above (i.e.,
mediation and moderation). As mentioned above, insight into possible mediation
pathways or moderating influences (which indicate higher risks for a specific subgroup) can inform prevention strategies. Discrepancies between previous studies may possibly have arisen due to small sample sizes. The present dissertation is based on a sample larger than has been previously used (Lynne-Landsman et al., 2010; Marklein et al., 2009; Negriff & Trickett, 2012; Shelton & van den Bree, 2010), with the exception of one study (Biehl et al., 2007). Examining mediation and moderation effects within one study combines what so far has only been done in separate studies. This provides results derived from analysing one sample and therefore one does not need to take differences in sample characteristics into account. Additionally, I adjusted all analyses for a priori selected confounders (including earlier measures of substance use, parent-child relationship quality and peer behaviour, parental substance use and socioeconomic factors), thus limiting the possibility that these earlier influences explained the effects tested.
Figure 3. Mediation model
Pubertal timing Cigarette use (16Y) Alcohol use (16Y) Cannabis use (16Y) Confounders: Financial difficulties,
parental education, maternal & paternal alcohol, cigarette and cannabis use, alcohol, cigarette and cannabis use (14Y), parent-daughter relationship quality (9, 15Y), peer deviance (10, 15Y). c / c’ b a Parent-daughter relationship quality factors (15Y) Peer deviance (15Y)
Alcohol use (16Y) Cigarette use (16Y) Cannabis use (16Y)
Confounders: Financial difficulties, parental education, maternal & paternal alcohol, cigarette and cannabis use, alcohol, cigarette and cannabis use (14Y), parent-daughter relationship quality (9, 15Y), peer deviance (10, 15Y).
Pubertal timing Parent-daughter
relationship quality factors (15Y) Peer deviance (15Y)