Psychosocial stress is a ubiquitous phenomenon in our society. While acute stress responses are necessary and adaptive, excessive activation of neurobiological stress systems can predispose an individual to far-reaching adverse health outcomes. Living in a complex social environment, experiencing stress is not limited to challenges humans face individually. Possibly linked with our capacity for empathy, we also display the tendency to physiologically resonate with others’ stress responses. This recently identified source of stress raises many interesting questions. In comparison to the wealth of studies that have advanced our understanding of sharing others’ affective states, the physiological resonance of stress has only recently begun to be more closely investigated. The aim of the current paper is to review the existing literature surrounding the emerging area of “stress contagion”, “empathic stress” or “stress resonance”, as it has been variably called. After a brief introduction of the concepts of stress and empathy, we discuss several key studies that paved the way for the merging of empathy with the concept of physiological resonance. We then delineate recent empirical studies specifically focusing on the physiological resonance of stress. In the final section of this review, we highlight differences between these studies and discuss the variability in terminology used for what seems to be the same phenomenon. Lastly, potential health implications of chronic empathic stress are presented and possible mechanisms of physiological stress transmission are discussed.
The stress response is a healthy and adaptive function in situations of acute challenge. However, prolonged stress exposure may cause permanent dysregulation of the neurobiological stress systems, thereby leading to wear and tear on the body and brain termed allostatic load (McEwen et al., 2015). The respective physiological changes, specifically within the sympathetic nervous system (SNS) and hypothalamicpituitary-adrenal (HPA) axis, have been linked to detrimental health effects, including cardiovascular, metabolic, autoimmune and mental diseases (Chrousos, 2009; McEwen, 2008; Sapolsky, 2004). While chronic stress can undoubtedly arise from traumatic life events, examining extreme adversity has taken us only so far in understanding why individuals living in less demanding conditions are also susceptible to stress-related conditions. One potential explanation is that allostatic load can accumulate in the presence of non-traumatic but persistent stressors (Almeida, 2005; DeLongis et al., 1982; Lazarus and Folkam, 1984). Above and beyond the daily hassles experienced firsthand, stress can be transmitted between individuals. Understanding this physiological resonance of stress may be a key aspect to explaining how the daily environment, without being traumatic, can significantly impact our health and wellbeing. In the current paper, we review the newly emerging area of physiological stress resonance. Accordingly, studies were selected if they examined physiological resonance in two or more individuals, one of whom was subjected to stress or a stress-like challenge. Studies addressing resonance with other emotional states such as sadness or pain were not included (e.g. Harrison et al., 2006; Hein et al., 2011). Also, research addressing physiological responses to others’ suffering, but without accounting for resonance between the suffering target and empathizing observer, were excluded (e.g. Liew et al., 2003, 2011). Although not all of the summarized work related physiological stress resonance to self-report measures of empathy, the original results were generally discussed in the context of empathy research. Thus, we touch on three independent research fields: stress, empathy, and physiological resonance. While it is beyond the scope of this review to provide a comprehensive theoretical background on each of these fields, a brief synthesis of basic concepts and ideas is given. We begin with an overview of the stress system and a definition of empathy and related terms.