The Southern California Twin Register at the University of Southern California: III.

The Southern California Twin Register at the University of Southern California (USC) was initiated in 1984 and continues to provide an important resource for studies investigating genetic and environmental influences on human behavior. This article provides an update on the current register and its potential for future twin studies using recruitment through school district databases and voter records. An overview is also provided for an ongoing longitudinal twin study investigating the development of externalizing psychopathology from childhood to young adulthood, the USC Study of Risk Factors for Antisocial Behavior. Characteristics of the twins and their families are presented, including recruitment and participation rates, as well as attrition analyses and a summary of key findings to date.