Understanding and Preventing Violence, Volume 2: Biobehavioral Influences (1994)
Chapter: Danish Adoption Studies
TABLE 5 Prevalence of Registered Criminality in Danish Twins by Sex and Zygosity
| Males |
| Females | ||
Zygosity | Numbera | Percent Criminal |
| Numbera | Percent Criminal |
Identical | 730 | 13.42 |
| 694 | 2.59 |
SS fraternal | 1,400 | 12.29 |
| 1,380 | 2.17 |
OS fraternal | 2,073 | 9.55 |
| 2,073 | 1.47 |
χ2 | 11.00b |
|
| 4.70c |
|
a Total number of individuals. bp < .001. c .10 > p > .05. NOTE: SS = same sex; OS = opposite sex. SOURCE: Data from Cloninger and Gottesman (1987). | |||||
that would increase variance (and hence prevalence) as a function of the magnitude of the imitation and heritability (Carey, 1986; 1992).
Danish Adoption Studies
The Danish adoption studies on registered criminality began with identification of 1,145 male nonfamilial adoptees and a series of matched nonadoptees (hereafter termed ''controls") born in the city and county of Copenhagen from 1927 to 1947. Both adoptees and controls were between ages 30 and 44 when they were followed up through national registers for criminality and psychiatric disorder. Detailed analyses on criminal registration in this sample were done by Hutchings (1972) and reported in Hutchings and Mednick (1977); analyses on the diagnosis of sociopathy were performed by Schulsinger (1972). Two major findings are consistent with a hypothesis of genetic effects. First, despite adoption into households offering a representative standard of living in Denmark, adopted males had almost twice the rate of criminal registration as their controls (16.2 versus 8.9%). Because 30.8 percent of biological fathers had a criminal record, compared to 12.6 percent of adoptive fathers and 11.1 percent of control fathers, the increased rate of criminality among adoptees is consistent with their inheritance
of a greater genetic liability toward crime than controls. (This finding, however, is not necessarily proof of a genetic etiology because adoption per se may have elevated crime rates.) Second, criminality in the biological fathers was correlated with criminality in the adopted sons. Among adoptees whose biological fathers were registered, 22.7 percent were registered for criminal offenses and another 35.7 percent were on record as having committed only minor offenses. Among adoptees whose fathers were not registered, the respective percentages are 13.6 and 33.7 percent. This pattern suggests a heritable effect for more serious crime, but perhaps a lack of one for minor offenses.
Also evident in the data was a significant family environment effect. Given criminal registration in an adoptive father, 27.1 percent of adoptees had criminal offenses and another 37.5 percent had minor offenses. When the adoptive father was not registered, the respective percentages were 14.5 and 33.9 percent, again suggesting important family environmental effects for more serious crime.
The sample of Danish adoptees was later extended to the female adoptees, and to the entire country of Denmark, and reported on by Mednick et al. (1983, 1984). The expanded sample lacked the controls of the earlier sample and used court convictions instead of criminal registration as the index of crime. Results on biologic parent criminality replicated earlier results. The probability of court conviction among adoptees increased almost linearly with the number of court convictions for biological parents. Of importance here was that the effect was evident only for property crime and not for violent offenses. (The precise definition of a violent offense is not offered by the authors.) Curiously, the significant effect of adoptive parent criminality found in the Copenhagen adoptees did not replicate in the extended sample, and no reason was offered for this lack of consistency.
Further analyses of this sample have been provided by Van Dusen et al. (1983a,b), Baker (1986), Baker et al. (1989), and Moffitt (1987). Significant predictors of adoptee convictions include socioeconomic status of both the adoptive and the biological parents, psychiatric history of the biological parents (especially personality disorder and substance abuse), and late placement and number of placements before final adoption. Again, these relationships are significant for nonviolent convictions. Either violent offenses were not analyzed (Baker, 1986; Baker et al., 1989), or the results were largely insignificant (Van Dusen et al., 1983a,b; Moffitt, 1987). An exception was a single analysis by Moffitt, reported in Mednick (1987). Here, violent offenses in male adoptees
