7. PROJECT SUMMARY/ABSTRACT Callous-unemotional (CU) traits (i.e., lack of guilt or remorse, deficits in empathy) are a risk factor for the development of persistent, severe levels of externalizing and conduct problems, and are closely associated with the adult construct of psychopathy (Frick et al., 2014). Understanding developmental mechanisms of CU traits is critical for informing prevention efforts with important public health implications. Irritability predicts higher levels of CU traits and psychopathic features across development for some individuals, who are thought to acquire them through negative environmental experiences (e.g., Hawes et al., 2016; Yildirim & Derksen, 2015). Recent developmental models of CU traits have primarily focused on the socializing role of harsh parenting (e.g., Waller & Wagner, 2019), to the neglect of negative peer experiences. Problematic peer relationships, including peer victimization, have been tied to increases in CU traits across development (Fontaine et al., 2018). Irritable children are prone to experience higher levels of peer victimization and rejection (e.g., Evans et al., 2016), which may lead them to believe that others are insensitive to their needs, in turn decreasing their sensitivity to others’ needs (Fontaine et al., 2014), consistent with demonstrated effects of harsh parenting (Waller & Wagner, 2019). In the only known direct test of this model, Barker and Salekin (2012) found that both irritability and victimization predicted increases in one another and in CU traits, although only indirect effects from victimization to CU traits through irritability were significant, contrary to hypotheses (Barker & Salekin, 2012). Despite calls for replication efforts (e.g., Fontaine et al., 2018), this remains the sole longitudinal investigation of these pathways. Furthermore, this study did not distinguish between physical (e.g., being hit) and relational (e.g., being socially excluded) forms of victimization, and therefore may not adequately capture girls’ experiences (Crick & Bigbee, 1998; Ostrov & Kamper, 2015). The present study aims to fill these gaps in the literature by: 1) testing a primary and alternative model of physical and relational peer victimization as socialization agents in relations between irritability and CU traits; 2) examining aggression as a moderator of victimization’s socialization effects, consistent with prior findings that children who are both victimized by their peers and engage in aggressive behavior are at risk for especially negative outcomes (Cook et al., 2010); and 3) considering HPA-axis functioning as a moderator of these associations, as hypoactive HPA-axis functioning has been highlighted as a biological marker of risk associated with CU traits (Hawes et al., 2009). These aims will be tested from middle childhood (i.e., grade 3) to adolescence (i.e., age 15) using data from the NICHD Study of Early Child Care and Youth Development (SECCYD), which includes a large...