Data for the study were obtained from non-familial adoptions (adoption by persons not biologically related to the child) that occurred in Denmark between 1924 and 1947. There were 14,427 such adoptions during that period, 6,700 involving male children and 7,727 involving female children. Court conviction records were obtained for the adoptee, biological parents, and adoptive parents from the office of the police chief in the region in which each subject was born. Accurate and complete registers existed for both the adoptees and their biological and adoptive parents. Social class-related hereditary influences from biological parents could thus be separated from social class-related environmental influences from adoptive parents, and their independent relationships to crime were observed. Genetic factors associated with lower class origins were found to account for a significant portion of crime variance, while controlling for adoptive social class. The social class of the adoptive parents, which is independent of genetic factors, also influences the probability that a child will later engage in criminal behavior. Criminogenic influences of low social class biological origins can be offset by a middle class environment, and the influence of a low social class environment is much less criminogenic for persons born to parents of high social class. While for the male adoptees the environmental social class factors are stronger criminogenic influences than the genetic social class factors, the reverse is true for the female adoptees. Tabular data and 35 footnotes are provided.