Chiao et al. BMC Public Health 2012, 12:527 http://www.biomedcentral.com/1471-2458/12/527 Adolescent cigarette and alcohol use was defined by whether never-married 20 year-old college youth self- reported cigarette and/or alcohol use. This information was obtained directly from the questionnaire item, “Have you smoked [cigarettes] in the past week?” and, “Have you drunk [alcohol] in the past month?” Respondents were categorized as “smokers” and “non-smokers.” For the alcohol use item, students were placed in one of three groups: “abstainer” (no alcohol use), “light drinker” (1–2 times per month) and “heavy drinker” (3+ times per month). Peer characteristics included respondent perceptions of three behaviors: perceived sexual behavior of close friends, perceived cigarette use of close friends, and per- ceived alcohol use of close friends. Respondents were asked about the perceived sexual experiences of their close friends with possible responses being “none, less than half, and half or greater” of their close friends hav- ing had premarital sex. They were also asked whether their close friends smoked and the frequency of alcohol use among their close friends. Smoking responses were dichotomized (“smoker” and “non-smoker”) for friends perceived smoking behavior, and close friends’ perceived alcohol use responses were placed into 1 of 3 categories (“abstainer,” “light drinker = 1–2 times in past month,” and “heavy drinker = 3 or more times in past month”). Stressful life, school and behavioral events included 4 major types: a stressful school event within the previous 12 months, a stressful personal event occurring during the previous 12 months, deviant behavior within the pre- vious 12 months, and/or psychological distress. Each stressful school event was dichotomized (0 = no; 1 = yes), indicating whether they had experienced any one of four “stressful” school events: 1) withdrawal from school, 2) not getting along with classmates, 3) took a health- related leave of absence from school, or, 4) had a behav- ioral problem at school. Participants also reported whether they had experienced one or more of the fol- lowing personal stressful life events in the past year: death of a parent, death of a close relative, death of a pet, or ending a relationship with a girlfriend or boy- friend. We categorized the number of stressful personal events experienced into 3 levels: 0, 1, and 2 or more events. The indicator of the “deviant behavior” inde- pendent variable was determined by a self-reported ac- knowledgement of participating in one or more of the following activities: using illegal drugs, stealing, cheating on exams, and destroying public property. In addition, psychological depression was assessed via the Symptom- Checklist-90-Revised (SCL-90-R; [24]). This scale contains 16 items assessing affective, cognitive, and vegetative dimensions of depression. The SCL-90-R Depression scale has been shown to be both reliable and valid [25], and has been used in a previous study of Taiwanese Page 3 of 10 adolescents [23]. The total score ranged from 0 to 64 with Cronbach α at 0.90. Higher scores are associated with greater levels of depressive symptoms. Premarital sexual permissiveness was measured by a 3- item scale asking youth to report their agreement levels regarding premarital kissing, petting, and coitus with a partner for whom they have strong affection. The re- sponse ranged from “strongly agree” to “strongly disagree” on a 6-point scale [26]. As prior research suggested [27], each permissiveness measure was dichotomized to “per- missive” vs. “conservative” attitudes and the three items were summed to produce a permissiveness scale, with scores ranging from 0 to 3. Higher scores are associated with more permissive attitudes toward premarital intimate behaviors. Several covariates were used as potential confounding variables affecting the relationships between the main explanatory variables and premarital sex involvement. Perceived support came from one or both of two sources: perceived friend support and perceived family support. In the TYP survey, respondents were asked to rate the availability of perceived support from both family and friends (separate items), in 3 areas: school work, eco- nomic strain, and personal emotion. Other covariates were family socioeconomic status measured by maternal education (3 categories: junior high school or lower, se- nior high school, and college or higher); family structure (living with both biological parents vs. other); and monthly family income with categorical responses. Statistical Analysis To examine the temporal association between adolescent cigarette/alcohol use and sexual activity during college, we began with bivariate analyses that characterized the sam- ple profile by sexually active college participants by age 20. We then used multivariate logistic regression models that elaborated and progressively adjusted the significance of respondent cigarette and alcohol use on later premarital sex in college. Model 1 elaborated adolescent cigarette and alcohol use to investigate our primary research inter- est. Model 2 added peer characteristics to test for possible confounding of our primary research question. Model 3 continued by including attitudes toward premarital sex, stressful life and behavioral events, and other covariates to determine adjusted effects of adolescent cigarette/alcohol use on subsequent premarital sex. In these multivariate models, we limited the analysis to sex abstainers at age 20 who continued to be enrolled in college (N = 1,777). As mentioned above, males and females are studied separately to explore the potential gender specific associations. In analyses not shown here, we also tested for interactions between peer influence and adolescent cigarette/alcohol use variables, to exam- ine whether these risky behaviors varied by perceived