Chiao et al. BMC Public Health 2012, 12:527 http://www.biomedcentral.com/1471-2458/12/527 peer characteristics for each gender. However, because these interactions did not add significantly to the mod- els, they are not included here. All logistic regression models, reported as an adjusted odds ratio (AOR), con- trolled for sample clustering effects. Missing data in the regressions used listwise deletion. Variables used in re- gression models may be highly correlated. We employed analyses of the variance inflation factor (VIF) to assess multicollinearity. VIFs less than 2 were not regarded as a multicollinearity concern. Statistical analysis was per- formed with STATA 11.0 [28]. Results Descriptive findings Table 1 summarizes sample characteristics stratified into two groups: participants who had or had not engaged in premarital sex by age 20. Overall, most characteristics differed markedly between these two groups, and by gender. For instance, sexually active youth were more likely to report cigarette/alcohol use than participants who were sexually abstinent. In each group, male stu- dents reported significantly more risky behaviors than female students. Sexually active youth were also more likely to believe their close friends smoked and drank and engaged in premarital sex, with significantly higher proportions among males than females in each group. In contrast, sexually abstinent youth were more conserva- tive in their premarital sexual attitudes and experienced less stressful life and behavior events. In each group, females reported higher levels of stressful life and behav- ioral events than males. The percentage of sexually ab- stinent 20-year-old college students who engaged in premarital sex by age 22 was significantly higher among males (20%) than females (13%). Relationship between premarital sex and adolescent cigarette/alcohol use Table 2 presents logistic regression models to investigate the effects of adolescent cigarette/alcohol use, perceived peer characteristics, stressful life and behavioral events, and other adolescent covariates on later premarital sex- ual behavior by gender. In order to predict premarital sex during college, the multivariate analyses focused on the sexually abstinent youth at age 20. Model 1 shows adolescent cigarette and alcohol use increased the likeli- hood of engaging in premarital sex. Adolescent cigarette use was significantly related to engaging in premarital sex for males only (AOR = 2.4; 95% CI = 1.4-4.0) while adolescent alcohol was significantly related to engaging in premarital sex for females only (AOR = 2.0; 95% CI = 1.4-4.0). Model 2 adds peer characteristics. Adolescent cigarette use was still significantly associated with higher odds of premarital sex for males only (AOR = 2.0; 95% CI = 1.4-4.0). However, adolescent alcohol use also Page 4 of 10 appeared to be a significant predictor of premarital sex for males and females when peer characteristics were controlled. Model 3 progressively adds variables measuring atti- tudes toward premarital sex, stressful life and behavioral events and other individual/family characteristics to Model 2. Even after controlling for other covariates, ado- lescent cigarette use remained significant in predicting premarital sex for male college students only (AOR = 2.1; 95% CI = 1.2-3.9). In addition, male college students with heavier adolescent drinking were 2.7 times more likely to have premarital sex than abstainers; this result contin- ued to be found among females categorized as light drinkers (AOR = 2.1; 95% CI = 1.2-3.6). For males, most peer effects were not significantly related to premarital sex; the only exception was the male’s perception of close friends’ sexual behavior. Male college students who perceived half or more of their close friends were sexually active by age 20 had a higher odds of having later premarital sex than those male respondents whose close friends were believed to be sexually abstinent, or if they were unaware of their close friends’ sexual activities (AOR = 3.1; 95% CI = 1.6-6.1). Experiencing stressful personal life events was signifi- cantly associated with higher odds of premarital sex dur- ing college for both sexes (AOR = 2.7; 95% CI = 1.6-4.7 for males and AOR = 1.7; 95% CI = 1.001-3.0 for females). With other factors controlled, multivariate ana- lyses revealed college students with “liberal sexual per- missiveness attitudes” were significantly more likely to engage in premarital sex (AOR = 1.3; 95% CI = 0.97-1.9 for males and AOR = 1.6; 95% CI = 1.2-2.3 for females). After adjusting for other covariates, specific family fac- tors associated with a significantly lower likelihood of premarital sex were: having a more highly-educated mother for male college students (AOR = 0.6; 95% CI = 0.3-0.996); and having a lower family income for female college students (AOR = 0.9; 95% CI = 0.9-0.996). Unex- pectedly, significant risk factors for premarital sex during college were “perceived friend support for schoolwork” among females (AOR = 2.9; 95% CI = 1.3-6.6), and “per- ceived friend support for emotional difficulties” among males (AOR = 2.0; 95% CI = 1.1-3.5). Discussion As theories and prior research suggest, our results found cigarette smoking, alcohol use, and premarital sex tend to cluster in youth [2,8-11]. Moreover, our findings demonstrated the influence of adolescent cigarette/alco- hol use on later premarital sex, and showed gender dif- ferences related to self-reported premarital sex during college. Overall, adolescent cigarette/alcohol use was higher among Taiwanese youth who were sexually active by age 20 when compared to their sexually abstinent