Answer: C. a criminal biological father
In adoption studies, the method of the adoptee shows similarities with their adoptive and biological parents. Similarities with their biological parents are heritable and as such a result of a genetic effect. Similarities with their adoptive parents are a result of the home environment or shared environment. For criminals that are adopted children, their criminality was traced to their biological parents, which makes it a hereditary factor.
This relationship existing between crime and adoption however does not show evidence of a violent crime; instead, it shows evidence of property crime. Although not 100% defined, the criminality of adopted children has been proven to be greatly influenced by genetic factors. This study was conducted by Sarnoff A. Mednick and Karl O. Christiansen and is one of the classic methods used today for the study of behavioral genetics.