<p>The present study tested three aims regarding the socializing roles of peer victimization and harsh parenting in the development of callous-unemotional (CU) behaviors. First, we investigated whether peer victimization’s promotive effects on the development of CU behaviors extend downward to early childhood and persist above effects of harsh parenting. We then considered whether, consistent with recent theoretical models, the effects of family and peer stressful experiences may be stronger for those who are emotionally (i.e., higher irritability) or physiologically (i.e., higher basal salivary cortisol) sensitive. Aims were tested over one year across the transition from preschool to kindergarten using a community sample (<i>N</i> = 263, <i>M</i>_age = 4.32 years, SD = 0.31 years, 47.7% female). Harsh parenting and CU behaviors were measured using parent report. Irritability and physical and relational victimization were measured using teacher report. Basal cortisol was assessed from saliva samples collected on three consecutive days in the morning. Hypothesized effects of peer victimization were not supported. However, consistent with hypotheses, harsh parenting predicted increases in CU behaviors specifically for youth with high levels of irritability (i.e., &gt; 2.12 SDs from the mean; <i>B</i> = 0.26, <i>p</i> =.05). Finally, lower cortisol directly predicted increased CU behaviors (<i>B</i> = − 0.23, <i>p</i> &lt;.001). Findings provide support for negative emotional reactivity as a moderating factor in the effects of harsh parenting on the development of early childhood CU behaviors, whereas HPA axis hypoactivity may directly increase risk.</p>