This study explores the etiology of self-report delinquent behavior using novel extensions of the twin method. 250 pairs of same-sex adolescent twins are surveyed by questionnaire through Ohio high schools. Zygosity of the twin pairs is diagnosed by physical characteristics (e.g. eyecolor, height) except for uncertain pairs diagnosed by extensive blood typing of genetic markers. Substantive variables include 1) family and school attachments; 2) association with delinquent peers; 3) parental behavior; 4) temperament; 5) the twin relationship; and 6) confessed delinquency and reflect theoretical processes identified by social control, subcultural, and differential association models of delinquent behavior. The analyses involve first using the traditional twin method of comparing identical and fraternal twins to partition variation into genetic and environmental components; several checks of the equal environments assumptions are conducted. As we anticipate only very modest genetic influence, the major aim of the study is to apply extensions of the twin method to environmental influence. In particular, the twin method can reveal whether environmental influence operates primarily within families (making children in a family dissimilar) or between families (making children in a family similar but unlike children in other families). The between family hypothesis corresponds to a model of delinquency which emphasizes family experiences as etiological factors; the within family hypothesis corresponds to a model which emphasizes peer and school experiences, independent of family background, as the main etiological factors. Using both univariate and multivariate analyses, support for both hypotheses is evaluated and implications for delinquency theory are drawn.