Chiao et al. BMC Public Health 2012, 12:527 http://www.biomedcentral.com/1471-2458/12/527 Problem behaviors (e.g., smoking, drinking, drug use, premarital sex) tend to cluster in youth as distinct social behaviors [2,8-11]. If the relationship between cigarette/ alcohol use and subsequent sexual activity can be under- stood, interventions can be designed to efficiently and ef- fectively decrease and/or limit the likelihood of adolescents’ misusing these substances and engaging in premarital sex. Thus far, there is relatively little empirical evidence regarding this link in Asian countries like Taiwan. Mason and colleagues [12] analyzed data gathered from a sample of 808 American youth, followed from age 10 to age 24. Their findings suggest early alcohol use correlates with a higher likelihood of risky sex. However, that study is based on a Western population, and its analyses did not include factors related to social context and peer influence; therefore, additional risk behaviors often associated with adolescent development were not assessed. Using the Theory of Planned Behavior [13-15] and So- cial Learning Theory [16,17], this study explores the as- sociation between adolescent cigarette/alcohol use and premarital sex from a temporal perspective. The Theory of Planned Behavior asserts an adolescent’s intention to engage in premarital sex flows from a youth’s attitude to- ward premarital sex, his/her perception of the subjective (cultural) norms associated with such behavior, and his/ her beliefs regarding his/her ability to engage in sexual activity. How these factors are influenced by relation- ships with peers and stressful life events are also central to the dynamics of when, how, and to what degree youth engage in premarital sex. In addition, Social Learning Theory posits peer group interaction portrays a signifi- cant role in cigarette/alcohol use and subsequent pre- marital sex among college students. In a sample of Taiwanese youth who successfully completed a competi- tive college entrance exam, adolescents report peer pres- sure as a powerful influence in their decision to initiate and experiment with a variety of risky health behaviors [16,18]. Previous research in the Philippines and Taiwan identifies perceived sexual activity by best friends as an influential factor in the initiation of sexual activity in an adolescent’s own life [8,19]. In the present cohort study, we extend our analyses to include the temporal relation- ship between peer pressure and premarital sex during college. Using longitudinal data from the Taiwan Youth Pro- ject, we specifically address this research question: What is the relationship between adolescent cigarette/alcohol use and the likelihood of premarital sex during college, after controlling for premarital sex attitudes, peer influ- ence, life and behavioral events, and family background? The analyses were conducted separately for male and fe- male college youth, due to gender differences suggested by a wide range of existing literature [7,19]. Page 2 of 10 Taiwan provides an interesting cultural context to in- vestigate this question. Traditional Taiwanese culture values conservative social norms such as responsibility, restraint, interdependency and collectivism, suggesting premarital sex is not endorsed, especially among single adults [20]. Taiwanese college students living in this cul- tural context are exposed to a high degree of normative and institutional influence, which leads them to be “well-behaved” and not engage in smoking, drinking, or premarital sex in late adolescence. Over three-fourths of Taiwanese high school graduates enroll in college [21,22]. Studies have shown the school environment serves as a protective factor leading to less deviance, less psychological maladaptation and less premarital sex in Taiwan [8,23]. Even within this context, however, risky behaviors such as cigarette/alcohol use and premarital sex occur, creating a salient public health issue deserving further investigation. Methods The study population The Taiwan Youth Project (TYP) was launched in 2000 and originally included 5,541 junior high students (2,740 girls and 2,801 boys) in northern Taiwan (Taipei city, Taipei county and Yi-Lan county) as the baseline with an annual follow-up through self-administered question- naires and telephone interviews. Data on the sensitive topics of sexual and intimate activities and attitudes were gathered in this cohort since 2004 when youth turned 20 years old [8]. In order to understand sexual behavior after age 20, and to hold constant the docu- mented protective context of being in school, our ana- lytic sample is restricted to never-married college youth, surveyed at age 20 and again at age 22, with complete data on self-reported premarital sex, premarital sexual permissiveness attitudes and other major covariates dis- cussed herein. The inclusion criteria yielded a total study sample of 2,119 college youth. Measures The dependent variable Premarital sex involvement was measured by a self- reported question: “Have you ever had penile-vaginal intercourse?” with a dichotomous response (i.e., “yes” or “no”). This sexual activity item was included in question- naires for youth at both ages 20 and 22. Explanatory variables Late adolescence (18–21 years old) characterizes the peak of sensation seeking associated with an increased likelihood of engaging in physical and social risks, such as risky health behaviors [24]. We defined respondents’ cigarette and alcohol use by age 20 as adolescent risky health behaviors.