Introduction to the special issue 'New directions in the biology of stress'.

‘Stress, in addition to being itself and the result of itself, is also the cause of itself’ (Roberts, 1950, p. 105). This famous quote illustrates three places where stress can be located: outside the person (a stressor, ‘the cause’ of stress itself), the person’s perception (an appraisal, the perception of stress ‘itself’), or in the person’s psychological or physical response (reactivity, the ‘result’ of stress itself) (Epel et al., 2018; Segerstrom & O’Connor, 2012). Furthermore, as Becker and colleagues (2023) point out, ‘‘feeling stressed’ is not the same as ‘being stressed’, i.e., a subjective stress response is not necessarily accompanied by a physiological reaction and vice versa’ (p. 79). Indeed, a response or reaction can have a different physiological profile for different people (individual response stereotypy) (Hinz et al., 1994, 2002). For example, one person might have large changes in heart rate, another, respiration, and yet another, blood pressure. The five papers in this special issue identify phenomena related to stressors, stress, and stress response and propose new directions in how we conceptualize what a stressor is, what reactivity is, and what biological systems are involved. Slavich and colleagues (2023) give an extended overview of Social Safety Theory. The premise of this theory is that social stressors have primacy when it comes to physiological and especially immunological responses because social acceptance has been important for survival. The size and metabolic demands of the brain likely arose from the complexities of pair-bonded relationships and, later, bondedness with others. The benefits of the ‘social brain’ for humans must therefore exceed the costs of maintaining this large and hungry organ (Dunbar & Shultz, 2007). Kidd and colleagues (2023) propose that social touch may be important for communicating social safety, indeed, communicating safety better than the presence of ‘safe’ social partners (Conradi et al., 2020). They introduce us to a new ‘stress’ pathway, c-tactile afferents, which along with usual suspects (oxytocin and cortisol) can blunt physiological responses to stress and promote well-being. Both Social Safety Theory and affective touch have their roots in attachment theory, but they take it in different directions. Social Safety Theory focuses on the why – what stressors and appraisals are likely to lead to health-damaging physiological changes? – whereas affective touch focuses on the how – what is the pathway to brain regions and physiological responses? From both perspectives, what ‘safety’ and ‘threat’mean can vary across attachment orientation, personality, and the relationship between the social partners. How well must we measure social stress, interactions, and circumstances? ‘Intimate knowledge of what actually happened as well as how the individual perceived the situation’ (Slavich et al., 2023, p. 14) may be hard to obtain but necessary to know when a socially safe situation was undermined by perceived social threat and when a socially threatening situation was reappraised as being safe. Knowing the type and degree of sociality is also necessary to know the likely consequences. More social belongingness might be associated with lower systemic inflammation, but more extraversion