Human behavior patterns are generally ascribed to an interaction of life experiences and genetic predispositions, but the importance of genetic influences in shaping conduct has often been contested. This debate has been especially intense, and often emotional, in explaining criminal behavior (Sarbin & Miller, 1970). Reluctance to consider genetic factors in crime has had political overtones (Haller, 1968), but it may also reflect the fact that, until recently, the evidence for genetic influences consisted mainly of studies of twins, some of which were methodologically questionable.

Christiansen (1977a) reported on the criminality of a total population of 3,586 twin pairs from a well-defined area of Denmark. He found 52% of the twins concordant for criminal behavior for (male–male) identical twin pairs and 22% concordance for (male–male) fraternal twin pairs. This result suggests that identical twins inherit some biological characteristic (or characteristics) that increases their common risk of being registered for criminal behavior.

It has been pointed out, however, that identical twins are treated more alike than are fraternal twins (Christiansen, 1977b). Thus their greater similarity in criminal behavior may be partly related to their shared experience. This has produced a reluctance to accept in full the genetic implications of twin research. The study of adoptions better separates environmental and genetic effects; if convicted adoptees have a disproportionately high number of convicted biological fathers (given appropriate controls), this would suggest the influence of a genetic factor in criminal behavior.