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

The Southern California Twin Register is the result of an effort to recruit twins of all ages in the city of Los Angeles and surrounding areas. The register currently includes an ethnically diverse sample of more than 2600 twin pairs. The most recently recruited pairs have been drawn primarily from computerized records of enrollments in local public school districts, and are comparable in sex and ethnic distributions to the general public school population. An ongoing twin study of social and moral development in children, based on 600 pairs of 9-year-old twins from this register, is also described briefly in this paper.