In particular, a twin study of the covariation between delinquent behavior and association with delinquent peers is used to demonstrate the value of a behavioral genetic analysis for developing social theory. The phenotypic correlation between self-reported 'delinquency' and the 'delinquency of friends' is apportioned to three theoretical sources: genetic variation; common environmental influences that affect family members equally; and specific environmental influences that affect each individual uniquely. Of the three component sources, genetic factors contributed most to the phenotypic covariation in this study. Although genetic factors are implicated, this result does not mean that delinquency is either a direct result of biological differences or that it is inevitable. Rather, it shows that causal sequences leading to delinquency are traceable to individual differences in genes, so any social causation entails either individual differences in reactions to social processes or differential social reactions to already differing individuals. Implications of behavioral genetic analyses of covariation for theories of delinquency are discussed. (Author abstract)