As social beings, we rarely spend our time in isolation [1]. Most of our time, we are surrounded by other individuals, making it almost impossible to avoid social interactions. We have to interact with individuals that are well-known to us, like partners and friends, but also with individuals that are less known to us, like colleagues and customers. These interactions are marked by different types of challenges and opportunities, implying that we may experience different levels of stress throughout these interactions [2]. Although interacting with other individuals may help us to cope with stressful experiences [e.g., [3,4]], these interactions may also be stressful for us [e.g., [5,6]]. In particular negative interactions, that is, interactions with individuals that behave unexpectedly or unpredictably in potentially threatening contexts [7], are accompanied by a plethora of stress reactions [8,9]. On the subjective level, we may experience a change in emotion and cognition, like, for example, an increase in negative feelings [e.g., [10,11]] or an increase in attention for negative information [e.g., [12,13]]. On the behavioral level, we may show a change in behavior, like, for example, an increase in agnostic behavior in terms of insults and attacks [e.g., [14,15]] or an increase in affiliative behavior in terms of concessions and compromises [e.g., [16,17]]. On the neurobiological level, we may experience a change in autonomic and endocrine reactivity, like, for example, an increase in cardiovascular activity [e.g., [18,19]] or an increase in glucocorticoid and catecholamine activity [e.g., [20,21]]. Although these stress reactions may help us to cope with other individuals in potentially threatening contexts, they may also put us at risk for several diseases [22]. As psychosocial stress is associated with substantial morbidity and mortality [23], there is a growing interest in biomarkers that indicate whether we show adaptive (i.e., stress-buffering and health-promoting) or maladaptive (i.e., stress-escalating and health-impairing) stress reactions in social contexts. In recent years, heart rate variability (HRV), an index of consecutive changes in heart beat [24], has been considered as a promising biomarker for adaptive behavior during social encounters [25,26]. Our subjective, behavioral and neurobiological responses to other individuals are orchestrated by a network of prefrontal and (para-)limbic brain regions [25–۲۷].