Bullying victimization is a significant predictor of future psychological health problems,1 yet we have limited understanding of how it affects acute and long-term physical health. The field of stress neuroendocrinology has established a wealth of data, however, demonstrating that early and chronic stress can profoundly and negatively affect neuroendocrine, inflammatory, and metabolic processes via epigenetic programming to increase the risk for obesity, cardiovascular disease, cognitive impairment, and accelerated cellular aging.2–7 Exposure to the stress of bullying thus has significant potential to harm both the psychological and physical health of the individual. Moreover, it is becoming increasingly apparent that the physiological mechanisms through which early-life stress affects endocrine and inflammatory processes may be the very same mechanisms that contribute to the poor psychological health outcomes associated with bullying. Given the inherent vulnerability of the developing child and adolescent to the stress-related insults imposed by bullying victimization, it is imperative that basic research findings be rapidly translated to clinical practice. Early intervention is essential to bolster both physiological and psychological resilience and to minimize negative long-term health implications.8
BULLYING AS A FORM OF CHRONIC STRESS
Stress can be divided into acute and chronic forms, as well as being both positive and negative in valence (eustress and distress, respectively).9 The acute stage is one of protection and facilitates the body’s capacity to respond to the demands of the environment. The immediate reaction of “flight or fight” is activated in order to cope with the situation, and once the stress has subsided, homeostasis returns.10 Under chronic stress, however, homeostatic mechanisms fail, and the individual can experience a state of allostatic overload.10 This chronic stress can eventually induce significant and deleterious health consequences that stem from associated elevations of inflammatory mediators, from deficits in neurotrophic factors, and from metabolic adaptations that facilitate insulin desensitization, lipid storage, and other metabolic syndrome–like outcomes, all working together to alter stress reactivity.8–11 Herein we review the harmful psychological and physiological effects of chronic exposure to bullying stress. Our review of the bullying literature focuses primarily on peer-to-peer bullying in childhood and adolescence; much of the research on bullying has been conducted during these developmental periods and focused on the health implications of peer victimization. This period is also one of developmental vulnerability in which the experience of bully victimization can become biologically embedded to modify the individual’s long-term health trajectory.12 Although beyond the scope of the current review, it is important to acknowledge that parents, teachers, and other adults can also bully children and that, in such instances, the health implications are likewise significant.
Bullying, a classic form of chronic social stress, has been defined as a systematic abuse of power, with aggressive behavior or intentional harm-doing by peers that is carried out repeatedly.13 Bullying can take on different forms, including physical (e.g., hitting and pushing), verbal (e.g., insults and intimidation), indirect (e.g., social manipulation and systematic exclusion), and virtual (e.g., cyberbullying).14 More than 25% of children report being bullied or being on the receiving end of peer victimization, and 10% to 14% of those suffered from chronic peer victimization, defined as frequent exposure during a period of six or more months. There is increasing evidence that chronicity of victimization and also severity15–17 are important mediators of long-term health and academic impact. Not surprisingly, children who experience chronic peer victimization suffer greater long-term psychological impact than those who experience briefer episodes of bullying.18,19 Peer victimization affects both genders, though the bullying methods may differ. For example, boys are, more often than girls, the recipients of direct physical and verbal aggression, whereas girls are, more often than boys, the recipients of more relational and social forms of bullying—although such differences are not always observed.20 In any event, a number of sex-specific psychiatric health outcomes have been reported.21
BULLYING AND RISK FOR PSYCHIATRIC DISORDERS
Data regarding the impact of early risk factors for bullying exposure—be it as a bully, victim, or bully/victim—are rare.22 Evidence suggests, however, that the effects of being bullied are as severe as, or even more severe than, maltreatment in childhood.23 Longitudinal studies such as the recent Tracking Adolescents’ Individual Lives Survey (TRAILS)22 have shown that behavioral, emotional, and motor problems, socioeconomic status, and the experience of family breakup during preschool years are associated with involvement in bullying during late elementary and secondary school. Parenting behavior24 and factors in pregnancy have also been suggested to increase the risk of becoming a target of bullying,25 possibly through effects on stress responses at the physiological and behavioral levels. Vulnerability to bullying may thus arise from risk factors inherently associated with later development of psychiatric illness, and the presence of psychiatric illness is itself also associated with increased incidence of bully victimization in youth.26 It would be nevertheless be wrong to discount the harmful impact that the stress of bullying can have on individuals—which is to exacerbate the preexisting risk for, or early presentation of, a psychiatric illness. In this context, the Virginia Twin Study of Adolescent Behavioral Development, using data from 145 bully-discordant monozygotic juvenile twin pairs, found that bullying victimization was an environmental risk factor for psychiatric disturbance, including anxiety, depression, and suicidal ideation and behavior.27 Data from this prospective study demonstrated that being bullied had a significant environmental impact on childhood social anxiety (odds ratio [OR] = 1.7), separation anxiety (OR = 1.9), and young-adult suicidal ideation (OR = 1.3).27 A shared genetic influence on social anxiety and bullying victimization was also identified,27 which the authors noted was consistent with data suggesting that social anxiety could be both an antecedent to, and a consequence of, bullying. Prospective studies such as the above have been particularly valuable in establishing a clear role for exposure to bullying victimization in later development of mental illness. While retrospective and cross-sectional studies have helped to identify the presence of such a relationship, the directionality cannot be ascertained without longitudinal follow-up post-victimization. For example, retrospective and cross-sectional studies cannot determine whether individuals who develop depression or social anxiety were premorbidly more sensitive to criticism and rejection, and thus were more inclined to interpret earlier interpersonal experiences as bullying victimization.
Elucidating directionality within this relationship is important for understanding the health effects of stress related to bully victimization. Given the vulnerabilities associated with the risk for being affected by bullying behavior, be it as the victim or bully, compounded by the impact of stress in the development of psychopathology, it is not unexpected that depression, anxiety, and some abuse disorders are more common in the victims of chronic bullying. Of significant concern are reports from prospective studies that men who were victims of bullying are at 18 times more risk of suicidality than their non-bullied counterparts, while female victims have nearly 27 times more risk for panic disorders.21,28,29 Bullies, by contrast, have four times more risk for development of antisocial personality disorder.21,28,29 Taken together, such data suggest that significant negative psychiatric outcomes are associated with bullying behavior for both the victims and perpetuators. Factors increasing the risk for experiencing either (or both) can be identified early in life and have serious long-term consequences—ones that may be compounded by the social stress of the bullying process. The implication is that bullying is not only a sociological or biological process but something more complex: a biopsychosocial problem that requires an integrated, longitudinal research approach to identify risk factors and long-term health effects.
BULLYING AND SOMATIC SYMPTOMS
Gini and Pozzoli’s recent meta-analysis30 of the association between bullying and psychosomatic problems found that bullied children and adolescents have a significantly higher risk for psychosomatic problems than non-bullied peers. Commonly reported physical health problems associated with bullying include poor subjective health status, poor appetite, sleep disturbances, headaches, abdominal pain, breathing problems, and fatigue.31–33 This association between bullying, victimization, and somatic symptoms is observed in children as early as four years old (preschool age)34 and has led some researchers in the field to suggest that any recurrent and unexplained somatic symptom may be a warning sign of bullying victimization.35 While many children have somatic symptoms during times of emotional distress or trauma, these data underscore the importance of considering bullying as a differential among potential causes of somatic symptoms in children and adolescents. Together, this now well-established relation between psychological and somatic symptoms may serve to alert mental health and primary care providers to the potential experience of bullying and may help to facilitate the eradication of such harmful treatment in child and adolescent patient populations.36 It is important to note, though, that personal resources, ranging from self-efficacy to the availability of social support, can mitigate the impact of bullying on the development of psychological and somatic complaints,33 indicating that the deleterious effects of bullying do not affect all individuals equally. This finding is consistent with the potential for protective psychological constructs to buffer the physiological impact of stress on the body. Indeed, peer victimization in late childhood or early adolescence has been shown to impair adaptive stress responses,37 which itself may be represent a key, critical mechanism through which the psychological and physiological symptoms develop.
STRESS RESPONSES AND ALLOSTATIC LOAD
Stress, both psychological and physiological, elicits a biological response that, if continued unabated, contributes to allostatic load on the brain and body.38 Allostatic load provides an index of biological “wear and tear” from cumulative exposure to stress.38 Over time, this increasing allostatic load accelerates the wear and tear on the body as a result of chronic exposure to fluctuating or heightened neural or neuroendocrine responses to stress.39 Through the process of increasing allostatic load, the accumulation of health-related risk factors increases an individual’s chances of developing disease.39 This increasing load, along with eventual allostatic overload, is now well established to be implicated in the development of a variety of diseases, including depression, diabetes, and heart disease.9,38,40–45 Allostatic load is also implicated in the acceleration of psychiatric illness progression and poor treatment outcomes.46–56 Research over the last two decades has illuminated the mechanisms through which continued physiological stress burden and accruing allostatic load can accelerate aging and neurodegenerative and disease processes.
The process of increasing allostatic load is one in which the body accrues damage over time, impairing its capacity to maintain and restore homeostasis in the face of future challenges. It is through this process, mediated in part by epigenetic modifications that alter system-wide responses (discussed below), that early adverse experiences, such as chaotic family environments, low socioeconomic status, and experiences of abuse and other forms of interpersonal aggression, have been associated with a variety of poor health outcomes in adolescence and adulthood, including higher rates of chronic illness and death from disease.57–65 Markers for disease risk, such as inflammation and increased blood pressure, are also associated, in adulthood, with the experience of adverse, stressful experiences in childhood.66–74 The impact of early-life stress on long-term health is likely to be further mediated by the potential for early adversity to impair children’s development of skills that foster resilience. Specifically, early-life stressors increase the likelihood that individuals will develop cognitive and interpersonal styles,75,76 as well as coping behaviors,77 that reduce their capacity to cope with stress in the future. These negative effects are further likely to influence the types of environments in which affected children reside later in life, and to increase the probability that individuals will perceive and react to stress in ways that escalate exposure to stressful circumstances.78 The cumulative direct and indirect effects of early-life stress can therefore work together, over time, to enhance the negative health impacts through increased biological vulnerability together with increased rates of sequential exposures to perceived stressors.79–81
Although limited data are available on the impact of bullying on health over the lifespan, the health effects of stress associated with bullying and victimization would presumably follow the same pattern. That is, drawing from the literature on other types of chronic stress in childhood, on allostatic load, and on associated risks for health and mental health problems, it is reasonable to consider chronic peer victimization as capable of generating negative outcomes through similar cascading processes. We posit that the psychological experience of chronic peer victimization activates biological responses and also serves to interfere with health-related behaviors (e.g., sleep, diet, and exercise) that further perturb basic homeostatic mechanisms and compromise long-term psychological and physiological health. This suggestion is in line with studies showing that the long-term psychological impact of peer victimization is mediated by activation of the physiological stress response (i.e., hypothalamic-pituitary-adrenal axis and autonomic nervous system) in response to the anticipation of victimization.82
EPIGENETICS: ENCODING LONG-TERM HEALTH EFFECTS
An individual’s behavioral, physiological, and social conditions collectively influence, and are influenced by, the epigenome.83 Epigenetic marks are sculpted by environmental interactions during development. Functionally, these marks modify the activation of certain genes to shape cellular and system-wide responses upon reexposure to similar stimuli. The most important influences on epigenetic programming include stress, inflammation, and the individual’s metabolic status (integrated metabolic function across multiple bodily systems), and these integrated systems, in turn, functionally co-regulate each other.84 These processes enable early stress exposure to leave a lasting imprint on stress responsivity, associated immune activations, and metabolic efficiency.83 Epigenetics thus represents an important biological mechanism through which social stressors such as bullying can directly and physiologically contribute to the development of disease.85 The number of human studies that have directly assessed the impact of early-life peer bullying on epigenetic profiles is limited,86 though the impact of early-life stress on the epigenome, along with the associated health effects, is well established.87,88 Inherent in this conceptualization is that the disease process involves the integration of social, behavioral, and biological factors—all of which are mediated by, and mediate, epigenetic marks. More research is needed.
BULLYING, INFLAMMATION, AND METABOLIC DYSFUNCTION
Preclinical and clinical evidence demonstrates a link between early-life stress and inflammation.89 While acute inflammatory responses are important for fighting infection and healing processes, chronic inflammation is a significant contributing factor to the development and progression of various serious diseases, including cardiovascular disease.90 Chronic inflammatory states are activated and maintained by environmental stressors as well as by a range of health-risk behaviors, including poor diet, lack of exercise, and sleep disturbance.91,92 One marker of inflammation, C-reactive protein (CRP), has been extensively researched because of its association with cardiovascular risk93,94 and metabolic syndrome.95,96 In line with the stressful nature of peer victimization and the association between early-life stress and chronic inflammation in later life, a growing evidence base indicates that childhood bullying predicts low-grade systemic inflammation into adulthood.29,97 Recent work in this area has shown that involvement in childhood bullying, either as a bully or bully/victim, can predict changes in CRP levels well into adulthood.29 The directional impact, however—relative to those uninvolved in bullying—was different for those two groups. Victimization was associated with greater increases in CRP levels, and bullying was associated with lower increases. These long-term effects held even following adjustment for other related and potentially confounding factors, including body mass index, substance use, childhood physical and mental health status, and exposure to other early-life psychosocial adversities, suggesting that the bullying experience itself was a significant contributor to inflammatory status.29
The link between bullying, stress, and inflammation is supported by data from another recent study demonstrating that childhood bully victims had increased levels of CRP at midlife and higher risk for clinically relevant inflammation (CRP > 3 mg/l).97 Again, these findings remained significant when controlling for potentially confounding variables such as body mass index and psychopathology in childhood, and smoking, diet, and exercise in adulthood. An interesting finding from the same study was that central distribution of fat was more prevalent in individuals who had been bullied.97 Given that central adiposity contributes to an overall inflammatory state,98 it is important to keep in mind the potential link between bullying and metabolic dysfunction. Studies focusing on early-life stress, though not specifically on bullying, suggest that the interaction among stress, inflammation, and metabolic dysfunction are more interdependent than previously appreciated. For example, a longitudinal study demonstrated that early adversity was predictive of higher body mass index, which, in turn, was predictive of higher levels of inflammatory mediators, including CRP.89 While the studies specific to bullying suggest that it has a direct and deleterious physiological impact, it is likely that ongoing stress and maladaptive health behaviors can further compound the negative health consequences of that altered physiology. Given that maladaptive health behaviors alone foreshadow chronic inflammation in young adulthood, they serve as promising target for intervention to reduce long-term health consequences.
COULD BULLYING INCREASE RISK FOR DISEASE?
Given the emerging evidence demonstrating a link between bullying, inflammation, and metabolic dysfunction, along with impact of early-life stress on health-risk behaviors, one might suspect that bullying could increase the risk for chronic lifestyle diseases in which these various processes are directly implicated. Although this association remains to be confirmed longitudinally, and many confounding factors limit retrospective confirmation, bullying by peers appears to have a direct and non-negligible effect on age-related disease risk, independent of other established risk factors.97,99 The biological mechanisms through which childhood bullying affects long-term health are likely mediated via its epigenetic imprint on stress, inflammation, and metabolic systems. More research in this area is urgently needed. Current data nevertheless underscore the importance of investigating bully victimization as a standard component of clinical practice today. Asking about bullying in child and adolescent psychiatry, family medicine, and primary care represents a practical first step toward intervening to prevent traumatic exposure and to reduce the risk for further psychiatric and other morbidities.100,101
To date, the study of bullying has developed in parallel with the study of the physiology of stress, with limited convergence of these two fields. Currently, we do not possess sufficient data to definitively determine whether a direct cause-and-effect relationship exists between childhood bullying and poor long-term health outcomes. Synthesis of these two scientific literatures suggests, however, that chronic peer victimization could have significant physiological and mental health consequences and that cascading processes of the physiological stress response, including chronically elevated levels of inflammation, could play an important role. At this stage, however, our understanding of the relationship is primarily by extrapolating from one field to the other. Future collaboration between the two has the potential to produce findings with rich scientific and clinical implications.
Declaration of interest: Dr. Croarkin has received grant support from Pfizer Inc. and the Brain & Behavior Research Foundation, and in-kind support for equipment and supplies from Neuronetics and AssureRx Health. Dr Tye has received support from Teva Pharmaceuticals and the International Bipolar Foundation.
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