Interplay of peer deviance and pubertal timing on substance use

cigarette smoking and cannabis using friends and having delinquent friends at age 15 years) combine with pubertal timing to increase risk of substance use in girls, while adjusting for potential confounding factors.

9.8.1.1 Mediation analysis. Several mediation effects were found in the

unadjusted analyses but after adjusting for confounders only two effects of “indirect-only mediation” were found. Although pubertal timing was not associated with substance use at age 16 years, early pubertal timing was associated with having more cannabis using friends and having delinquent friends at age 15 years, which were associated with

increased levels of alcohol use at age 16 years. According to Hayes (2009) and Zhao and colleagues (2010) mediation effects can still be present even though no direct effect is found. Therefore, the results provide evidence for a longitudinal effect of pubertal timing on alcohol use at age 16 years via cannabis using and delinquent peers at age 15 years.

The lack of evidence for a direct effect of pubertal timing on substance use supports previous research in which substance use was also assessed in late adolescence (e.g., Al- Sahab et al., 2012; Copeland et al., 2010; Kaltiala-Heino et al., 2011; Marklein et al., 2009). This, as already mentioned above, might be explained by the theory that early pubertal timing being a risk factor diminishes by late adolescence as on-time and late maturing girls catch-up with regards to substance use and early maturing girls gain cognitive resources, which allow them to exhibit more control over their own behaviour.

9.8.1.2 Moderation analysis. Pubertal timing moderated the effect of the number of alcohol drinking friends (age 15 years) on alcohol use at age 16 years, with the effect being weaker for early than on-time and late maturing girls, even though evidence for the effect still was found across all three pubertal timing groups. A moderation effect was also found by Biehl and colleagues (2007); however they reported that having more alcohol drinking friends predicted increased alcohol use in late adolescence for early maturing girls only. This contrasts with the findings of this study, where having more alcohol drinking friends was associated with increased alcohol use in late adolescence for all girls, with the effect being stronger for on-time and late maturing girls than for early maturing girls. However, as this was the only moderation effect detected, it should be interpreted with care.

9.8.1.3 Interpretation of the findings. Within this chapter I looked at the combined effects of peer deviance at age 15 years and pubertal timing on substance use at age 16 years. It was found that although early pubertal timing was not directly linked to increased alcohol use at age 16 years, early maturing girls had more cannabis using friends and were more likely to have delinquent friends at age 15 years, which in turn

were associated with higher levels of alcohol use at age 16 years. These findings suggest that by late adolescence alcohol and cigarette use are more age normative and therefore more acceptable among adolescents, which might explain why the number of alcohol drinking and cigarette smoking peers does not affect the relationship between pubertal timing and substance use. However, cannabis use and engaging in delinquent behaviour are illegal in the UK and therefore represent an increased level of deviance. Early maturing girls in this study still show the tendency to affiliate with deviant peers (i.e., cannabis using friends and delinquent friends) in late adolescence. This raises the question whether early pubertal timing is a lasting risk factor with regards to choosing deviant individuals as peers; or whether early maturing girls are still associating with the same deviant peers they associated with in early adolescence. However, as this data does not provide information on how long early maturing girls have been affiliating with these peers, further research is warranted to answer this question.

In relation to main effects it was found that late maturing girls had fewer alcohol drinking friends than early maturing girls, fewer cannabis using friends than early and on- time maturing girls and fewer delinquent friends than early maturing girls. As no

differences were found between early and on-time maturing girls, these findings suggest that late pubertal maturation functions as a protective factor with regards to affiliating with deviant peers. This might be explained with the advanced level of emotional and cognitive resources late maturing girls have when they enter puberty. These resources of social and cognitive skills might enable the late adolescent girls to control the reward- sensitive system in the brain, which is activated by the hormonal changes caused by the emergence of puberty (Steinberg, 2010). Therefore, these findings support to some

degree Moffitt’s maturation disparity hypothesis. If the dys-synchrony between physical maturation and emotional and cognitive resources in early maturing girls is responsible for early maturing girls affiliating with deviant peers, late pubertal maturation indeed should be a protective factor against affiliating with deviant peers. This is due to the fact that no such dys-synchrony is present in late maturing girls as by the time their body shows signs of physical maturation they already posses social and cognitive skills.