The twin study literature contains inconsistencies regarding both the estimates of genetic, shared, and nonshared environmental influence on delinquency and sex differences in the etiological influences on delinquency. Additionally, little is known about assortative mating and cultural transmission effects on delinquency. The present study was aimed at examining the etiology of delinquency in adolescence and examining assortative mating and transmission effects. To this end, we applied the social homogamy structural equation model to self-reported delinquency data from 486 families with 17-year-old male and female twins. We expected stronger shared environmental influences on delinquency among girls than boys and stronger environmental than genetic effects on adolescent delinquency in general. The results indicated nonsignificant sex differences in the estimates of genetic and environmental influences on delinquency. The best-fitting model suggested that 18%, 56%, and 26% of the variance in delinquency among both boys and girls is associated with additive genetic, nonshared environmental, and shared environmental factors, respectively. This finding was consistent with our expectation that environmental factors play an important role in adolescent delinquency. Parental cultural effects accounted for a small part of the transmission of delinquency within families while assortative mating was, in part, accounted for by cultural and social background factors. Future work should focus on identifying important environmental factors related to delinquency.