The study found that genetic factors explained between 40 and 45 percent of the variance in adolescent victimization among the twin pairs. It also found that the shared environment did not explain any of the variance in the adolescent victimization scales. This is consistent with behavioral genetic research that has consistently found very little, if any, variance that can be explained by the shared environment. The nonshared environment accounted for approximately 55 to 60 percent of the variance in adolescent victimization. Further, genetic factors accounted for 64 percent of the variance in repeat victimization. None of the variance in repeat victimization was due to shared environmental factors; 36 percent of the variance in repeat victimization was due to nonshared environmental factors. Add Health is a longitudinal and nationally representative sample of American youths enrolled in grades 7 through 12 during the 1994-95 school years. Three waves of data have been collected thus far. The first round of interviews was conducted in 1994, when approximately 90,000 youths completed self-report surveys at school. The current analyses were restricted to twins only (247 dizygotic same-sex twin pairs and 289 monozygotic twin pairs). A series of questions pertained to the frequency with which adolescents were victimized (e.g., had a knife or gun pulled on them, had been shot, had been cut or stabbed, or had been beaten). Other questions pertained to the presence of low self-control, association with delinquent peers, and involvement in crime and delinquency. 4 tables and 54 references