Callous-unemotional (CU) traits are described as reduced guilt, lack of empathy, and shallow affect in youths (Frick et al., 1994). These characteristics of CU traits are associated with youths’ antisocial behaviours. This essay discusses the relation between CU traits and violence by assessing their association with conduct disorder (CD) and aggression. Moreover, CU traits might be an antecedent to psychopathy because they are part of affective factor in the two-factor (Hare, 2003) and three-factor models of psychopathy (Cooke & Mitchie, 2001). Hence, this essay also discusses factors that support CU traits as an antecedent to psychopathy which are their stability across the developmental trajectories and neurological basis. In this essay, violence is discussed as a subset of aggression, specifically as its extreme form in line with most social psychologists definition of violence (Bushman & Huesmann, 2010 in Allen & Anderson, 2015). CU traits are associated with aggression because they have been found to highly comorbid with CD in youths (Frick & Viding, 2009). Conduct disorder (CD) is a mental disorder involving high levels of rule-violating behaviour and aggression (APA, 2013). In the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders, CU traits have been added as a specifier for the diagnosis of CD which is indicated by the phrase “with limited prosocial emotions”. A review by Frick and Dickens (2006) found high CU youths across forensic, clinic-referred, and community samples exhibit more violent and severe antisocial behaviours compared to low CU youths. This review provides a compelling evidence for the relation between CU traits and aggression. However, it is worth noting that not all studies in the review specifically investigated the role of CU traits but psychopathic traits in general, suggesting the aggression reported might not be exclusively attributed to CU traits. The relation between CU traits and aggression is strong for both proactive and reactive aggression and direct bullying (Fanti et al., 2008; Viding et al., 2009). Bullying is defined as a subset of aggression which consists of element such as intentional harm (Farrington, 1993). It also has been associated with low levels of prosocial behaviours (Arseneault et al., 2006 in Viding et al., 2009) in line with the characteristics of CU traits. In a study by Viding et al (2009) who investigated direct and indirect bullying in youths, self-report measures of CU traits including conduct problems (CP) were used with 704 of youths aged 11 to 13. Peer-report measures of direct and indirect bullying were also collected. Direct bullying is described as physical contact or verbal encounter while indirect bullying involves more subtle and covert actions (Wolke et al., 2000). Viding et al found that youth with higher levels of CU traits were more likely to be involved in direct bullying (i.e. physical aggression) and this association was above the association between bullying and CP. This finding indicates that CU youths are more likely to engage in physical aggression relative to youth with CP without co-occurring CU traits. Viding et al’s study is a good evidence because they used peer-report of bullying and avoided self-report bias which might have led to an underrating of the association. In another study by Fanti et al (2008), they investigated the association between proactive and reactive aggression in a general student sample. Proactive and reactive aggression are described as an aggression used for dominance and defence respectively (Price and Dodge, 1989 in Fanti et al., 2008). Participants were Greek Cypriot adolescents aged 12 to 18, who filled in questionnaires measuring CU traits, bullying behaviours, and aggression. Findings indicated that youths with high CU traits were more likely to exhibit both reactive and proactive aggression (Fanti et al., 2008). Fanti et al’s (2008) study is clear evidence for the association between CU traits and aggression because it distinguishes between two forms of aggression (i.e. proactive and reactive) which are found to be highly correlated and might misinform results in studies examining aggression in general. They also studied youth population which has not been a focus of research on CU traits which suggests a universality of this association. Overall, there is sufficient evidence for CU traits association with violence and it is clearly indicated by direct bullying, proactive, and reactive aggression showed by CU youths. Severe anti-social behaviours and aggression by CU youths have led to a considerable amount of research conducted on CU traits. This body of research has found that CU traits are stable from youth to adulthood (Frick & White, 2008). One research supporting this is a longitudinal study by Burke et al (2007) in a sample of clinic-referred boys aged 7 to 12. Parent and child assessment was conducted annually from the first year of recruitment until the age of 17. At the age of 18 and 19, interview and Psychopathy Checklist-revised were used to assess psychopathic traits. It was found that CU traits were predictive of adult psychopathy even after controlling for other risk factors such as economic disadvantage and children’s CP (Burke et al., 2007). However, this study involved clinic-referred youths so the results might be skewed by the overrepresentation of children with behavioural disorders. The stability of CU traits is further supported by a longitudinal study with a sample of non-referred youths by Frick et al (2003). The findings showed a high stability of parent-report for the CU scale of Antisocial Process Screening Device (Frick & Hare, 2001) across four time points. In a different study by Lynam et al (2007), they also found similar results based on mother-reported Childhood Psychopathy Scale (Lynam, 1997) assessed at age 13 which was predictive of a possible diagnosis of psychopathy at age 24. Importantly, these results were robust and found even after controlling for other risk factors (e.g. childhood antisocial behaviour). These studies have shown the persistence of CU traits suggesting a possible antecedent to adult psychopathy. Affective traits in psychopathy are argued to have manifested early in childhood accounting for the stability of CU traits from childhood to adulthood. This manifestation is argued to be attributed to similar neurological dysfunction (i.e. amygdala dysfunction) found in CU youths and adults with psychopathy (Birbaumer, 2005). Amygdala has been found to specialise in processing emotional cues which modulate cognition and behaviour (Adolphs, 2003). Its dysfunction results in an emotion recognition impairment and empathy deficiency. Neuroimaging studies on adults with psychopathy have found that these adults exhibited a reduced amygdala activity for affective memory (Kiehl et al., 2001) and fear conditioning tasks (Birbaumer et al., 2005) although they were able to learn the tasks like typical adults. Meanwhile, emotion recognition impairment in CU youths was observed in a study by Jones et al (2009) who compared brain scans of 17 boys with CP and elevated CU traits with a group of typical boys during a blocked presentation of fearful and neutral faces. Results showed boys with CP and elevated CU traits exhibited lower activation of right amygdala relative to a comparison group for fear versus neutral face, indicating that these boys were less responsive and had a problem in recognising others’ emotional cues. This problem of emotion recognition resulted in more aggressive behaviours exhibited by CU youths because they were incapable of appropriately process social cues from others (Blair, 2003a). CU youths also manifest a similar affective empathy deficiency and amygdala dysfunction (Marsh et al., 2008; Schwenk et al., 2012) as shown by adults with psychopathy (Kiehl et al., 2001). This affective empathy deficiency in CU youths is showed by Schwenck et al’s (2012) study with CD boys with high and low CU traits. Schwenk et al presented the boys with short films and assessed how much they were affected by the films. CU boys were found less emotionally affected by the films relative to Autism Spectrum Disorder (ASD) and control group suggesting that these boys have problems with emotional affection. Moreover, the task required good language skills but the boys did not differ regarding IQ so it was an appropriate task for all groups. The affective empathy deficit in CU youths is suggested to have neurological basis based on findings from neuroimaging studies (Marsh et al., 2008). Marsh et al (2008) used functional magnetic resonance imaging on youths while they processed fearful, neutral, and angry faces. It was found that CU youths manifested a reduced amygdala activation relative to healthy and ASD control groups while processing fearful faces (Marsh et al., 2008). Jones et al (2009) conducted a similar study with boys with CP and elevated CU traits and found the same results. Importantly, given the difficulty to keep children still during neuroimaging, both studies found similar results suggesting a neurological basis underlying CU youths’ empathy deficiency and emotion recognition problem. Neurological basis of the CU traits suggests that there is a genetic predisposition of the traits (Viding et al., 2005) and their early manifestation in youths is most likely an antecedent to psychopathy in adulthood. Overall, the stability of CU traits across the developmental trajectories and their similar neurological dysfunctions found in CU youths and adults with psychopathy provide strong evidence for CU traits as an antecedent to psychopathy. However, a few factors can challenge this claim. First, the stability of CU traits is dependent on the developmental routes of CD which is childhood-onset or adolescent-onset (Moffitt & Caspi, 2001). Moffitt and Caspi (2001) argued that childhood-onset CD have similar pathologies with psychopathy and not so for the adolescent-onset CD. This claim is supported by Silverthorn et al’s (2002) study which measured CP symptoms, CU traits, and criminal history of 72 adjudicated adolescents. Nearly all of these adolescents met CD criteria and fairly evenly split between childhood-onset and adolescent-onset to their CD symptoms. Silverthorn et al found that childhood-onset CD adolescents showed elevations of CU traits and poor impulse control relative to less than half of adolescent-onset CD did which was in line with the construct of adult psychopathy (Hart & Hare, 1997). This suggests that CU traits might be a valid antecedent to psychopathy only if it is exhibited in youths with childhood-onset CD. Second, less is known about CU traits as an antecedent to psychopathy for girls because majority of studies on CU traits and psychopathy have focused on males. While the studies aforementioned have provided support for CU traits as an antecedent to psychopathy, the findings cannot be generalised to girls because gender differences are observed in CU traits as it has been observed in adult psychopathy (e.g. Lehmann & Ittel., 2012). For example, in the aforementioned Silverthorn et al’s (2002) study, girls with severe CP were mostly adolescent-onset to CD. Importantly, more than half (67%) of these girls did exhibit similar characteristics as childhood-onset CD boys and thus adult psychopathy (Silverthorn et al., 2012). Therefore, CU traits might be an antecedent to psychopathy dependent on different onsets of CD in boys and girls which are a childhood-onset for boys and adolescent-onset for girls. Gender differences are also found in studies of empathy deficiency in CU youths (Dadds et al., 2009). Dadds et al (2009) collected parent-report of 2760 children aged 3 to 13 on their levels of empathy, CU traits, and antisocial behaviour. They found severe affective empathy deficit across all ages for boys with elevated CU traits but no such findings were reported for girls (Dadds et al., 2009). This finding suggests that affective empathy deficiency which is a characteristic of adult psychopathy is not associated with CU traits in girls. Hence, it is justifiable to claim the similar affective deficiency manifested in CU youths and adults with psychopathy might be an antecedent to psychopathy is exclusive for boys. In conclusion, CU traits might be an antecedent to psychopathy because of their stability from childhood to adulthood and underlying neurological basis. However, evidence suggests that they might be a possible antecedent only if found in childhood-onset CD boys. There is also no clear evidence that CU traits are a possible antecedent to psychopathy in girls.