This study provides one of the first reports from a large twin study examining the genetic and environmental covariation among childhood psychopathic personality traits and reactive and proactive aggressive behaviors. Our results demonstrated, most importantly, a significant genetic and non-shared environmental correlation among psychopathic personality traits and reactive and proactive aggressive behaviors in children using both caregiver reports and child self-reports. We also found unique genetic and nonshared environmental influences in each of the measures, suggesting some independence among these behaviors and constructs. Furthermore, we also found a significant association between psychopathic personality traits and proactive aggression, but only in child self-reports. This may in part be due to the fact that children may have more insight into their own actions and behaviors (at least more so than their parents).
Common genetic estimates for psychopathic personality traits and aggression ranged from 21% to 46% using caregiver reports and 15% to 26% using the twins’ self-reported data. Bivariate analyses also revealed significant genetic correlations between both psychopathic personality factors with reactive and proactive aggression indicating further that genetic influences may be common to psychopathy and aggression during childhood. This shared influence could arise because of a common set of genes influencing both types of behaviors and in turn may influence the susceptibility to both psychopathic personality traits and aggressive behaviors and thus their covariation. Evidence of genetic influence on these traits, however, does not signify that the children in our sample are not capable of change or that they are impervious to intervention methods. This result falls in line with other twin studies on psychopathic personality traits that have found that a common genetic factor influences psychopathic personality traits and different types of externalizing behaviors in adolescents (Larsson et al., 2006
In comparing shared genetic influences between psychopathic personality traits with different forms of aggression, we found that the genetic correlation between the callous & disinhibited factor and proactive aggression was significantly greater than callous & disinhibited and reactive aggression, but only using the twins’ self-reports. Similarly, the twins’ self-reports also indicated that the genetic correlation between the manipulative & deceitful factor and proactive aggression was significantly greater than with reactive aggression. There results fall in line with what has been previously found phenotypically in the psychopathy and aggression literature (Patrick & Zempolich, 1998
). Previous analyses within this sample have also demonstrated that children might be better reporters of their own aggressive behaviors in that they have better knowledge of their motivation for perpetrating different aggressive acts as compared to other reporters including caregivers (see Baker et al., 2008
; Raine et al., 2006
). Relatedly, we have found a better fit of the two-factor proactive–reactive model to self-report data compared to caregiver or teacher reports, indicating somewhat greater validity and differentiation between these two forms of aggression when assessed by self-report (Baker et al., 2008
Furthermore, it may be difficult for a single rater to provide a complete or thorough picture of a desired behavior. Data collected from multiple informants has been previously found to be more informative (Baker et al., 2006
; Baker et al., 2008
). In general, interrater agreement has been found to be low (Youngstrom, Loeber, & Stouthamer-Loeber, 2000
). Moreover, meta-analyses conducted on informant data demonstrated that a correlation of r
= .28 (p
< .05) was found between different types of informants, and a correlation of r
= .22 (p
< .05) was found between self-reporters and other informants or reporters (Achenbach, McConaughy, & Howell, 1987
). These findings suggest that different raters may be privy to different aspects of a behavior. Additionally, individuals are also likely to behave differently in different situations. For example, parents may be less aware of their children’s proactive aggression than their reactive aggression. It is also possible that parents may not fully know every detail of their children’s behaviors.
Another source of rater disagreement may be rater bias. Rater bias may occur in twin studies because a single informant (mainly the biological mothers) is reporting on both twins (either as being more similar than they actually are or exaggerating the differences; Bartels et al., 2003
; Hudziak et al., 2003
). Additionally, greater shared environmental influences found in parental reports may be indicative of rater bias; however, no shared environmental influences were found for the current study indicating that rater bias may be low.
Moreover, the genetic relation between psychopathic personality traits and aggression was significantly stronger for proactive than reactive aggression, but only in child self-reports. This finding is in line with the prediction that psychopathic personality traits should have more of a core relation with proactive than reactive aggression (Patrick & Zempolich, 1998
). The lack of a significant difference in genetic and environmental correlations for psychopathic personality traits with reactive versus proactive aggression using caregiver reports may stem in part from parents’ inability to distinguish proactive from reactive aggressive behaviors in their children, as previously reported in this study (Baker et al., 2008
We also found a significant nonshared environmental factor commonly influencing psychopathic personality traits and reactive and proactive aggression. It should also be noted that the model used in the present study partitions measurement error away from variance in the latent nonshared environmental factor. When doing this, common nonshared environmental estimates ranged from 15% to 19% in caregiver reports, and 14% to 30% in twins’ self-reports. These nonshared environmental influences in psychopathic personality traits and aggression may involve experiences or circumstances that are unique to psychopathic personality traits or aggression. They may also include influences that are unique to the individual (not shared by his or her cotwin). These may involve the young twins having different friends and influences.
In addition to finding common genetic and environmental factors among psychopathic personality traits and aggression, our study also found substantial and significant specific genetic and nonshared environmental influences, including measurement error, on psychopathic personality traits and reactive and proactive aggression. This implies that there are etiological differences in psychopathic personality traits and aggressive behaviors in our sample of young twins. As the genetic correlations between psychopathic personality traits and aggression were significantly less than 1.0 using both caregiver and twins’ self-reported data, this lends further support to the notion that there are multiple factors that capture psychopathic personality traits and aggression within this sample.
In line with previous research on psychopathic personality traits (Larsson et al., 2006
) and aggressive behavior (Eley, Lichtenstein, & Moffitt, 2003
) we did not find any quantitative sex differences in the covariation among psychopathic personality traits and reactive and proactive aggression using either twins’ self-reported data or the caregiver reports of the twins’ behaviors. Evidence suggests that psychopaths seem to choose to engage in instrumental or proactive aggression and antisocial behavior (Blair, 2007
), and that psychopathy is a relatively good predictor of violence (Hart, 1998
; Salekin et al., 1996
). Others have also suggested that psychopathy is significantly correlated with future violence (Grann, Langstrom, Tengstrom, & Kullgren, 1999
; Hemphill et al., 1998
). Knowledge and understanding of the link between psychopathic personality traits with aggression may greatly aid in the prediction of future antisocial and violent behavior in adult offenders (Salekin et al., 1996
). Research has demonstrated that psychopathic offenders were approximately five times more likely than nonpsychopathic offenders to engage in violent recidivistic behaviors (Serin & Amos, 1995
). The results from the current study extend prior knowledge of the relation between psychopathic personality traits and aggression in examining the genetic overlap of psychopathic personality traits with reactive and proactive aggression. Results demonstrated a similar pattern between psychopathic personality traits and reactive and proactive aggression in this sample. Thus, it may be that it is difficult to differentiate these types of aggressive behaviors at such a young age when using caregiver reports.
Although most research on psychopathy has focused thus far on adults, there is increasing evidence that maintains that psychopathic personality traits are related to aggression much earlier in life (Frick, Bodin, & Barry, 2000
; Lynam, 2002
). For example, several researchers have purported that psychopathic personality traits emerge early in life (in childhood) in the form of “callous–unemotional” traits (Lynam, 1997
). These traits resemble adult psychopathic personality traits and may be associated with serious aggressive behavior and may signal a pattern of antisocial and violent behavior (Dodge, 1991
; Frick, 1998
). The current findings do provide evidence that these traits may be associated early in life. Fully understanding antisocial and aggressive behaviors is crucial for designating a subgroup of antisocial youth who showed more severe aggression and violence, especially youth who were more likely to show both instrumental (e.g., for gain) and reactive (e.g., in response to perceived provocation) aggression (Enebrink, Andershed, & Langstrom, 2005
; Frick et al., 2003
; Kruh et al., 2005
The current findings of significant genetic and environmental covariance between psychopathic personality traits and aggressive behaviors may have implications for future molecular genetic studies. Understanding the covariance among complex behaviors including antisocial and aggressive behaviors may aid in trying to identify susceptibility genes as well as aid in the possible prevention and intervention of externalizing disorders through identification of at-risk children early in life. Moreover, the significant nonshared environmental effects found may also suggest that unique experiences (e.g., influence from peers) may play an important role in the development of these behaviors—perhaps, intervention methods may also want to target and incorporate influence from peers.
Study Strengths and Limitations
The results of the current study should be examined while taking a few limitations into account. First, we may not necessarily be able to generalize the results from our twin study to the general population because there may be something innately different about twins compared with nontwins. However, our twins are no more or less disordered than any other population in terms of conduct or oppositional defiant disorder (Baker et al., 2006
). The examination of psychopathic personality traits in children could also be questioned. Some researchers believe that it might not be developmentally appropriate to diagnose this syndrome in children and adolescents (Vincent & Hart, 2002
); however, studies have shown that psychopathic personality traits can be seen in young children (Lynam, 1997
; Viding et al., 2005
). Moreover, one of the purposes of investigating personality disorders including psychopathic-like traits is to increase the understanding of these traits (and increase the genetic and environmental underpinnings of these traits). Additionally, results from the current study demonstrate the variance proportions associated with psychopathic personality traits and aggressive behaviors as well as the covariation of these two complex constructs; however, as with many behavior genetic studies, they do not inform us about the underpinning biological mechanisms associated with complex behaviors or disorders.
Finally, our study is cross-sectional involving a sample of 9- to 10-year-old twins, and therefore does not consider any developmental changes in the different constructs, or whether one precedes the other. However, targeting youth at such a young preado-lescent age may also be considered a strength and provide insight into early behavior problems.
Conclusions and Future Directions
In conclusion, the results from the current study may aid in the understanding of the etiology of psychopathic personality traits and aggressive behaviors early in life during childhood. Additionally, we found that the twins may be more aware of their motivations for different forms of aggressive behaviors (proactive–reactive) compared to their caregivers who may see the behavior but not necessarily the motivation. This study may further be extended in the future to explore the longitudinal stability or change (both phenotypically and etiologically) of these traits throughout adolescence and adulthood.