Participants

The data for this study come from the first five waves of the TRacking Adolescents’ Individual Lives Survey (TRAILS), an ongoing prospective cohort study of Dutch youth focused on the development of mental health from childhood to adulthood (Oldehinkel et al. 2015). Children born between 1989 and 1991 were eligible for inclusion in the study. To this end, all primary schools (N = 135) in five municipalities, including both rural and urban areas in the North of the Netherlands were approached for participation in the study. Thirteen schools refused participation. Parents or guardians received a personal letter containing information about the study and were contacted by telephone to invite the child and parents or guardians to participate. In total, 210 children were excluded from the study because they were unable to participate or because there was no Dutch-, Turkish-, or-Moroccan speaking parent or guardian available. This yielded a final baseline sample of 2230 children (76% response rate) (M age = 11.1, 50.8% girls) (De Winter et al. 2005). Participants were followed from pre-adolescence into emerging adulthood. Retention was good with 96.4% at the second wave (N = 2149, M age = 13.6, 51.2% girls); 81% at the third wave (N = 1816, M age = 16.3, 52% girls); 84% at the fourth wave (N = 1881, M age = 19.1, 52% girls); and 80% at the fifth wave (N = 1778, M age = 22.3, 53% girls) (Huisman et al. 2008; Oldehinkel et al. 2015). Ethics approval for TRAILS was obtained from the National Dutch Ethics Committee Central Committee on Research Involving Human Subjects (#NL38237.042.11).

Measures

For each variable the wave of measurement is noted because not all variables were measured at each wave.

Sexual Identity

Sexual identity was measured using one item that assessed self-identified sexual identity at waves 4 and 5. The question was phrased as follows: “What do you think you are?’ with answer options 1 = Heterosexual, 2 = Homosexual, and 3 = Bisexual. Participants were coded as LGB if they self-identified as homosexual (i.e., lesbian/gay) or bisexual in one or both waves.

Peer Victimization

Peer victimization was measured using one item from the Youth Self-Report (YSR) (Achenbach and Rescorla 2001) measured at waves 1–3 that read as follows: “I am being bullied a lot”. Answering options were 0 = Not at all, 1 = A little or sometimes, and 2 = Clearly or often. Scores on this variable were highly skewed and preliminary analyses indicated that participants who experienced bullying once already reported higher levels of internalizing problems compared to participants who reported no peer victimization at all. Therefore, peer victimization was recoded into a dummy variable distinguishing between participants never reporting peer victimization and participants experiencing peer victimization a little or sometimes” or clearly or often at either wave 1, 2, or 3.

Negative Relationships with Parents

Three constructs were available that measured negative relationships with parents as perceived by adolescents: parental guilt-inducing behaviors, parental angry outbursts, and parental rejection. First, parental guilt-inducing behaviors were operationalized as the mean response to the following statements (assessed for both parents), measured at wave 3: Your father/mother “… avoids you”; “… behaves to you in a silent and cold manner”; “… does not speak to you for long times”. Response options ranged from 0 = Never to 4 = (almost) Always, and the scale was of adequate internal consistency (α = 0.77 for guilt-inducing behavior father; α = 0.74 for guilt-inducing behavior mother).

Second, parental angry outbursts were operationalized as the mean evaluation of the following items (assessed for both parents), measured at wave 3: Your father/mother “… has angry outburst and tells you off”; “… finds it difficult to hide his/her irritations”; “… argues with you and complains about you loudly”. Response options ranged from 0 = Never to 4 = Almost always, and the scale was internally consistent (α = 0.78 for angry outbursts father; α = 0.76 for angry outbursts mother).

Third, parental rejection was measured at wave 4, by means of the EMBU-C (Markus et al. 2003) which includes 4 items for fathers and 4 items for mothers, e.g., “Does your father/mother punish you for minor things?”. Response options ranged from 1 = No, never to 4 = Yes, almost always. The internal consistency of the scale was moderate (α = 0.70 for rejection by the father; α = 0.67 for rejection by the mother). Scores on guilt-inducing behavior, angry outbursts, and rejection displayed by fathers and mothers were strongly correlated (r rejection = 0.58; r angry outbursts = 0.55; r guilt-inducing behaviors = 0.57). Therefore, reports referring to mothers and fathers were combined and the mean response was used. If the participants completed the measure for one parent only, that report was used.

Fear of negative social evaluation

At wave 4, fear of negative social evaluation was measured using a four-item scale (e.g., “I always expect criticism”) that reflects a sense of rejection sensitivity (Tops et al. 2008). The mean score was used and answering options ranged from 1 = Completely false to 4 = Completely true. Cronbach’s alpha was 0.76.

Lack of social support

Social support was measured during wave 3 as part of the Event History Calendar (Caspi et al. 1996). Participants were asked how many close friends they had, with a maximum of seven. For each friend, participants were also asked to indicate the extent to which “this friend helped during hard times in the participant’s life”. Response categories were 1 = Never, 2 = Seldom, 3 = Sometimes, 4 = Often, and 5 = Always. This was reverse coded to reflect a lack of social support. As such, this variable was expected to be associated with sexual identity, minority stress mediators, and outcome variables in the same direction as all other psychological mediation mediators. The sum score of all friends was used. An alternative operationalization based on mean support received did not lead to different results.

Substance use norms peers

Substance use norms of peers were operationalized at wave 3 as the proportion of friends of participants that they believed to use substances. Participants indicated on separate items whether 1 = None to 4 = All of their friends (a) “smoke cigarettes”, (b) “drink alcohol at least weekly”, (c) “get drunk”, or (d) “smoke marijuana”. The mean response on these items was used, which together comprised an internally consistent scale (α = 0.80).

Internalizing problems

Internalizing problems were measured at wave 4 with the internalizing problem behaviors broadband dimension of the Adult Self Report (ASR) (39 items). The ASR is an evaluation of emotional and behavioral problems in the past six months (Achenbach and Rescorla 2001). Participants were asked to rate the items (e.g., “I worry a lot”, “I refuse to talk”, and “I have difficulties to make and keep friends”) on a 3-point scale (0 = Not true, 1 = A little or sometimes true, 2 = Clearly or often true). Cronbach’s alpha was 0.93.

Substance use

Three types of substance use were assessed: cigarette smoking, alcohol use, and marijuana use. Smoking was measured at wave 4, using the following question: “Did you ever smoke cigarettes, even if it was only one cigarette or just a few puffs?”. Response options were 0 = I have never smoked, 1 = I have only smoked once or twice, 2 = I used to smoke, but I quit entirely, 3 = I smoke every now and then, but not every day, 4 = I smoke every day. Responses were dichotomized to distinguish between participants that never smoked or smoked only once or twice (0–1), and participants who smoke or used to smoke (2–4).

Alcohol use was measured at wave 4 as the number of times participants drank alcohol in the past month. Response options ranged between 0 and 40 times or more.

Marijuana use was assessed at wave 4 by asking participants whether they had smoked marijuana in the past year. Response options ranged from 0 = 0 times, to 13 = 40 times or more. Responses were dichotomized such that they distinguished between participants that had never smoked marijuana (0) and participants who had smoked marijuana within the past year (all other options).

Covariates

Gender, age at wave 4, parental socio-economic status and ethnicity were used as control variables. A composite measure of parental socio-economic status was created by adding the z-scores of parental occupational status (ISCO-88), parental education and parental income (Veenstra et al. 2005). Ethnicity was operationalized as a dummy (0 = Ethnic majority; 1 = Ethnic minority background). Participants were coded as having an ethnic minority background when either they or at least one parent was born in a non-Western country.

Similar to a previous TRAILS study (la Roi et al. 2016), it was empirically acknowledged that early childhood adversities might have an impact on the development of mental health later in life. Therefore, the following variables (all parental report) that reflect exposure to early childhood adversities were controlled for: childhood events (e.g., parental divorce, severe illness of one or both parents), parental internalizing problems, and perinatal complications as reported at wave 1. Multivariate analyses furthermore controlled for parents’ wave 2 report of early childhood (age 0–5) stressfulness of life, or long term difficulties (for more details about the instruments: Heininga et al. 2015; la Roi et al. 2016). Lastly, as substance use was among the outcome variables in this study, wave 1 parental past year smoking and alcohol use was controlled for.

Analytic Strategy

Hypotheses were tested by estimating indirect effects in two serial mediation path models, in Mplus 7.4 (Muthén and Muthén 1998–2012). The first model tested for the presence of mechanisms in line with the minority stress framework. This model estimated whether associations between sexual identity and smoking, marijuana use, alcohol use, and internalizing problems were mediated by peer victimization, parental guilt-inducing behaviors, parental angry outbursts, and parental rejection.

In the second model, the presence of mechanisms in line with the psychological mediation framework was tested, that is, whether associations between minority stressors and substance use and internalizing problems were mediated by fear of negative social evaluation, lack of social support, and substance use norms of peers (only for the minority stress – substance use links). Figure 1 depicts both models. Baseline levels of outcomes were not controlled for in path analyses in order to estimate between-person differences. A discussion of the methodological and conceptual consequences of controlling versus not controlling for prior reports of health outcomes is provided in the section on sensitivity analyses below.

Peer victimization, smoking, and marijuana use were operationalized as dichotomous variables and therefore a robust weighted least squares estimator that employs a diagonal weight matrix was used (ESTIMATOR = WLSMV in Mplus) (e.g., Muthén et al. 2016). Within the path analyses, probit regressions were estimated in models with categorical dependent variables, whereas the continuous latent response variable underlying the observed dichotomous peer victimization variable was used in path coefficients in which peer victimization was an explanatory variable (Muthén et al. 2016). Path models included both dichotomous and scale level variables, Therefore, unstandardized effects with continuous mediators and dependent variables being standardized before estimating path analyses were estimated to optimize the interpretability of path coefficients.

Analyses were performed on all participants for whom information on sexual identity was present (n = 1738). In order to prevent loss of cases and potential bias due to missing data in other variables than sexual identity, multiple imputation using chained equations was conducted and 20 imputed datasets created. Predictive mean matching was performed for imputing missing values, using a donor pool of size k = 5 for selecting potential donor responses. Predictive mean matching has been shown to be a robust multiple imputation method for imputing non-normal data. Because donor cases are used, plausible values are imputed and the original data distribution is retained (Kleinke 2017; van Buuren 2012; Vink et al. 2014). This was a suitable imputation method for the study variables, because some of them were skewed or had a limited number of response options. Multiple imputations were performed using the mi impute functionality in Stata, using Stata version 15.1 (StataCorp 2017). In order to further adjust for non-normality, bootstrapped standard errors on 5000 bootstrap samples were used.

Last, the classical false discovery rate method (FDR) was used to take into account multiple testing (Benjamini and Hochberg 1995). This was done as follows: For both mediation models, an FDR-derived significance threshold (set at 0.05) was used for determining the statistical significance of paths of theoretical interest (all lines drawn in Fig. 1). Furthermore, for each dependent variable, an FDR-derived significance threshold was used for determining the statistical significance of path-specific indirect effects.