The sample consisted of all 14,427 nonfamilial adoptions in Denmark in the years 1924-47. The register used contains information on the adoptee and both adoptive and biological parents. The study hypothesized that registered criminality in the biological parents would be associated with an increased risk of registered criminal behavior in the offspring. Court convictions were used as an index of criminal involvement. Most of the analyses considered the relationship between parents' criminal convictions and criminal convictions in the adoptees. Since female adoptees had so few convictions, the study focused on the criminal behavior of the male adoptees. The study found a relationship between biological parent criminal convictions and criminal convictions in their adopted-away children. The relationship was particularly strong for chronic criminal adoptees and biological parent offenders. There was no evidence that the type of offense committed by a biological parent was related to the type of convictions for adoptees. A number of potentially confounding variables were considered, but none of these was sufficient to explain the genetic relationship. The authors conclude that some factor is transmitted by convicted parents that increases the likelihood that their children will be convicted for criminal offenses. This is especially true for chronic offenders. The implication is that biological factors are involved in the etiology of at least some criminal behavior. 14 references