1. Transitions to Sexual Intercourse and Substance Use Among South African High School Students Lori-Ann Palen, Edward A. Smith & Linda L. Caldwell The Pennsylvania State University Funded by NIH R01 DA01749, NIH T32 DA017629-01A1 DISCUSSION RESULTS  Of sexual intercourse and substance use, substance use was typically the first risk behavior to be initiated.  Youth were more likely to initiate new risk behaviors during the school year than during the summer months.  There were no significant treatment group differences in engagement in, or transitions to, risk behavior. Details of Model-fitting:  The best-fitting models fit reasonably well: G 2 boys (2031) = 740, G 2 girls (2031) = 497.  The final measurement parameters showed that participants had a high probability of being classified into the correct treatment group (ρ boys =.89, ρ girls =.89) and risk behavior condition (P boys =.91, P girls =.96).  The best-fitting models were the ones in which: the proportion of participants in each risk behavior condition and probabilities of transition between these conditions were equal across treatment groups. the probabilities of transition were different by timing (over school year vs. over summer) but otherwise equal (T1  T2 = T3  T4, T2  T3 = T4  T5). INTRODUCTION  HIV/AIDS is a pressing public health concern for youth in South Africa (MacPhail & Campbell, 2001). Uncovering and addressing population-specific risk factors for sexual behavior may help reduce the transmission of HIV.  Previous cross-sectional research suggests that substance use is linked to both sexual intercourse and certain sexual risk behaviors in this population (e.g., Palen, Smith, Flisher, Caldwell, & Mpofu, 2006).  Longitudinal studies of U.S. adolescents have shown that substance use tends to predict sexual initiation (e.g., Blinn-Pike, Berger, Hewett, & Oleson, 2004; Guo et al., 2005; Santelli et al., 2004).  However, previous research has not examined populations from outside of the U.S., nor has it tested the alternative hypothesis that sexual activity plays a role in the initiation of substance use.  A clearer understanding of the developmental nature of associations between sex and substance use could inform efforts to prevent the transmission of HIV among South African youth. RESEARCH AIMS  Describe patterns of transition to substance use and sexual intercourse among South African high school students.  Determine whether these patterns differ by treatment status and time of year. METHOD PARTICIPANTS  2,416 students from Mitchell’s Plain, South Africa  Participating in a research trial of a classroom- based leisure, life skill, and sexuality education program (42% program, 58% control)  Five assessments: beginning and end of Grades 8 and 9, beginning of Grade 10  Baseline demographics: mean age 14.0 years, 51% female, 86% Coloured (derived from Asian, European and African ancestry) MEASURES  Composite dichotomous substance use variable, indicating whether participants had used alcohol and/or marijuana in their lifetime  Dichotomous variable indicating whether participants had engaged in vaginal sexual intercourse in their lifetime ANALYSIS  Latent transition analysis (LTA; Lanza, Flaherty, & Collins, 2002) using WinLTA 3.1 software (Collins, Lanza, Schafer, & Flaherty, 2002)  Parameters estimated: Measurement precision Proportion of participants in each risk behavior condition (no risk behavior, substance use only, sexual intercourse only, both risk behaviors) Probability of transition between conditions Probabilities of “impossible” transitions (e.g., substance use  no risk behavior) were fixed at zero.  Chi-square difference tests were used to compare competing models of risk behavior.  Preliminary analyses showed that the measurement parameters varied significantly by gender. Consequently, separate models were fit for boys and girls.  These results suggest that researchers and practitioners should give greater attention to potential mechanisms driving the association between substances and the initiation of sexual behavior.  We should also attempt to understand the social and environmental factors underlying school/summer differences in transitions to risk behavior in this population.  While this study provided no evidence of treatment effects on risk behavior transitions, we believe recently- implemented program modifications may result in effects for future cohorts. Figure 1: Proportion of participants in each risk behavior condition at Time 1. Figure 3: Proportion of participants in each risk behavior condition at Time 5. Figure 2: Probability of remaining in previous risk behavior condition.