This thesis presented findings relating to the interplay of pubertal timing and psycho-social factors (parent-daughter relationship quality and peer deviance) assessed at age 15 years on substance use (alcohol, cigarette and cannabis use) assessed at age 16 years.
No differences between the pubertal timing groups in substance use at age 16 years were found in the complete case analyses, after adjusting for confounders. However, in the imputed data it was found that late maturing girls drank less alcohol at age 16 years than on-time maturing girls and in addition they also used less cannabis at age 16 years than on-time maturing girls after adjusting for confounders. These findings failed to support the hypothesis that early pubertal timing is associated with increased levels of substance use at age 16 years. However, it is in line with recent research which has also reported the absence of this link in late adolescence (age 15-18 years; Al-Sahab et al., 2012; Kaltiala-Heino et al., 2011; Marklein et al., 2009), which had been found in early/ mid-adolescence (age 9-15 years; Berg-Kelly & Kullander, 1999; Dick et al., 2001; Downing & Bellis, 2009; Ge et al., 2006; Ge et al., 2002; Lanza & Collins, 2002;
Marklein et al., 2009; Michaud et al., 2006).
Taken together, these findings are indicative of a catch-up effect, such that by late adolescence, on-time maturing girls have caught-up with early maturing girls in relation to alcohol, cigarette and cannabis use and late maturing girls have caught-up with early
maturing girls in relation to cigarette use. Such an effect has also been discussed by Marklein and colleagues in 2009, who reported that the effect of early pubertal timing on increased levels of substance use may dissipate by late adolescence when substance use becomes more normative and less deviant. Early maturing girls may also serve as models in relation to substance use for on-time and late maturing peers and thus act as a risk factor for initiation of substance use by their same age peers. However, it needs to be borne in mind that this does not mean that by late adolescence early maturing girls are no longer at increased risk compared to on-time and late maturing girls. Even though early pubertal timing is no longer linked to increased substance use in late adolescence, early maturing girls are still at risk of having a higher chance of becoming substance
dependent. The definition of substance dependence states that one regularly uses a substance over a longer period of time (Triwest, 2013). Early maturing girls initiate substance use at an earlier age than on-time and late maturing girls (Dick et al., 2000; Lanza & Collins, 2002; Westling et al., 2008), therefore they use substances for a longer period of time than on-time and late maturing girls, which places early maturing girls at increased risk of substance dependence. Additionally, early substance initiation is linked to negative health outcomes (i.e., lung and liver damage) also due to a more prolonged duration of substance use (ASH, 2013; NTA, 2013). Based on this it is important to follow-up early maturing girls’ substance use (i.e., their regularity of drinking alcohol) into adulthood.
Although relationships were found between parent-daughter relationship quality or peer deviance and substance use at age 16 years, the effects of peer deviance on substance use at age 16 years showed larger effect size than those of parent-daughter
relationship quality on substance use at age 16 years. This finding supports research by Bahr and colleagues (2005) who stated that there is an increasing drive towards
autonomy from parents during adolescence and that peer behaviour may have a larger effect on adolescents’ substance use in late adolescence (Kandel, 1980; Kandel & Andrews, 1987). More generally, this finding supports Erikson’s theory of identity development, which proposes that adolescents need role models to develop their own sense of identity. Although parents can provide role models, the adolescent may reject them in certain social contexts. This renders peers the main source of identity formation in the adolescent’s life, emphasising the importance of the peer group during adolescence (Erikson, 1950 cited by King, 2004).
I did not find any evidence for the combined effects of pubertal timing and parent- daughter relationship quality at age 15 years on substance use at age 16 years. However, negative parent-daughter relationship quality (low levels of parent-daughter
communication, high levels of parent-daughter conflict and low levels of parental
monitoring) at age 15 years was linked to increased substance use (alcohol, cigarette and cannabis) at age 16 years.
Peer deviance at age 15 years was also linked to increased substance use at age 16 years. Evidence for combined effects of pubertal timing and peer deviance at age 15 years on substance use at age 16 years were found. That is, evidence was found of peer deviance mediating the effect of pubertal timing on substance use at age 16 years. However, after adjusting for a priori selected confounders only the following “indirect- only mediation” effects remained: Early pubertal timing was linked to having more cannabis using friends at age 15 years, which was linked to increased alcohol use at age
16 years; and early pubertal timing was also linked to having delinquent friends at age 15 years, which was linked to increased alcohol use at age 16 years. Additionally, a single moderation effect was found: pubertal timing moderated the effect of the number of alcohol drinking peers at age 15 years on alcohol use at age 16 years. However,
inspecting the separate pubertal timing groups showed that the link between the number of alcohol drinking friends at age 15 years and increased alcohol use at age 16 years was present for all three pubertal timing groups.