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  • Introduction to the Special Issue:The Neurobiology of Peer Victimization
  • Tracy Vaillancourt (bio)

Being the target of goal-directed aggression that is typically repeated and occurring in the context of a power imbalance (Olweus, 1993; Volk, Veenstra, & Espelage, 2017) happens at an alarming rate. Most population-based studies indicate that peer victimization affects about 30% of children and youth worldwide (Nansen et al., 2001; National Academies of Sciences, Engineering, and Medicine [NASEM], 2016; UNICEF, 2013), making it the most common form of aggression that youth are exposed to (Vaillancourt, 2018). Peer victimization has long been thought to be a normal part of growing up; however, recent research on the continuing effects of bullying has challenged this glib viewpoint. Indeed, this research has documented that the negative impact of peer victimization is extensive and enduring—affecting victims well into the future and long after the abuse has stopped (Lereya, Copeland, Costello, & Wolke, 2015; Takizawa, Maughan, & Arseneault, 2014).

Given the severity of this issue (i.e., prevalent and destructive), researchers have focused their attention in earnest on identifying psychosocial risk and protective factors associated with peer victimization with the aim of developing appropriate interventions. This research has [End Page 1] been instrumental in providing insight into the prevalence, individual and contextual correlates, and negative consequences of peer victimization (NASEM, 2016). However, this research has also failed to provide insight into processes and mechanisms, given its descriptive nature (Swearer & Hymel, 2015). Another notable issue with this research is that it has failed to consider how individual and contextual factors interact with biology to confer risk, protection, or resiliency (Vaillancourt, 2018; Vaillancourt, Hymel, & McDougall, 2013; Vaillancourt, Sanderson, Arnold, & McDougall, 2017), which is the focus of this special issue on the neurobiology of peer victimization.

Neurobiology is a broad term used to describe the study of the nervous system that includes the examination of its anatomy, biochemistry, molecular biology, and physiology. Research can include analyzing neural networks, genetics, neurotransmitters, and hormones. In this special issue, the emphasis is on inflammatory biomarkers, autonomic nervous system reactivity, the stress response system, and neural functioning in relation to peer victimization. These areas of inquiry are informed by a rich literature on the biological effects of stress in animals (Lupien, McEwen, Gunnar, & Heim, 2009; Lupien, Ouellet-Morin, Herba, Juster, & McEwen, 2016) and on new evidence in humans that documents how early life adversity such as being maltreated by a caregiver, or living in poverty, can program physiology and behavior (Bick et al., 2012; Lupien et al., 2009, 2016; Shalev & Belsky, 2016).

Being victimized by peers is a toxic stressor that interferes with a person's fundamental need to belong (R. F. Baumeister & Leary, 1995). Toxic stress refers to the "strong and chronic activation of the body's stress system in the absence of a buffering protection to stop the activation" of the stress response system (Lupien et al., 2016, p. 7). Targets of peer abuse perceive the experience as highly stressful (Sharp, 1995), and recent studies on the biological effects of bullying, including those in this special issue, support this viewpoint. Peer victimization has been associated with: (a) changes to the neuroendocrine stress response system (Carney, Hazler, Oh, Hibel, & Granger, 2010; Hansen, Hogh, & Persson, 2011; Hansen et al., 2006; Kliewer, 2006, 2016; Ouellet-Morin, Danese, et al., 2011; Ouellet-Morin, Odgers, et al., 2011; Ouellet-Morin et al., 2013; Vaillancourt et al., 2008), (b) increased inflammation (Copeland et al., 2014; Takizawa, Danese, Maughan, & Arsenault, 2015), and (c) epigenetic alterations such as DNA methylation (Ouellet-Morin et al., 2013). Neuroimaging studies have also shown that the brain processes exclusion by peers as social pain (Rudolph, Miernicki, Troop-Gordon, Davis, & Telzer, 2016; for a review, see Vaillancourt et al., 2013), with similar regions activated [End Page 2] as those associated with physical pain (Eisenberger, 2012; Eisenberger & Lieberman, 2004; Eisenberger, Lieberman, & Williams, 2003).

These biological changes are consistent with what is known about the negative effects of chronic and/or extreme stress on health and well-being (Selye, 1976). Psychological or physical stressors initiate a biological cascade that includes increased heart rate, blood pressure, and the increased production of stress hormones like cortisol. This physiological response helps the organism adapt to stressors...

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