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 might be associated with higher inflammation. In some cultures, affective touch from a friend might signal belongingness; in another, threat. One might sigh at the prospect of collecting the data necessary for ‘intimate knowledge’, but this is an important direction in studying the biology of stress. Interview measures of stress provide context for stressors, which checklists do not (Wethington et al., 1995).

A next step in social safety and threat might consider another parameter: how the relationship is characterized in terms of the interpersonal circumplex and its two axes of dominance and warmth (Gurtman, 2009). Is affective touch more or less effective when provided by a more dominant or more submissive partner? Is there a difference between safety provided by dominance and that provided by warmth and cooperation?

Context is also emphasized in Becker and colleagues’ systematic review of stress reactivity and multitasking (2023). The bottom line of the review is that, where stress reactivity is concerned, multitasking is more than the sum of the two tasks. Multitaskers, take heart! Some people might handle multitasking better than others, and some tasks might lend themselves to effective multitasking better than others. Unfortunately, task difficulty, task characteristics, and individual differences were ‘reported only rarely’ (p. 93).

Heterogeneity in the outcomes disappeared when they were limited to ‘pure’ sympathetic measures or to heart rate. A theoretical model that broadly suggests ‘physiological’ or ‘stress’ reactivity can result in empirical findings that are heterogeneous, in part because the systems measured will be heterogeneous. Generalized reactivity contrasts with the specific focus on systemic inflammation in social safety theory (Slavich et al., 2023) and specific focus on cardiovascular reactivity in motivational intensity theory (Brehm & Self, 1989). As research on multitasking proceeds, a refined theory of what systems should be affected by multitasking and why will be invaluable in guiding future efforts.

One such system is the hemodynamic profile. Howard (2023) reminds us of the subtleties of blood pressure reactivity first identified 70 years ago and that different profiles have different correlates and health consequences. Both the person (neuroticism, rumination) and the situation (active or passive task, task duration) are associated with hemodynamic profiles. As is true of behavior, there may be a lot of variance to be explained by individual differences across task profiles, also known as a person-situation interaction or an if–then profile (Bowers, 1973; Shoda et al., 1994). Again, theory development is important (e.g., Blascovich & Tomaka, 1996).

Howard (2023) also touches on the difference between reactivity (myocardial and/or vascular pattern) and recovery (vascular pattern). Given the link between vascular responding and future cardiovascular disease, perhaps more attention should be given to new directions in the biology of recovery from stress.

O’Riordan and colleagues (2023) address another specific system and a specific profile: blunted cardiovascular response. In doing so, they tackle the difficult question of how to define blunting. Too often, reasoning about blunted or exaggerated stress responses becomes circular. Depending on the results, a response might be labeled blunted if the ‘healthy’ people (psychologically or physically) have higher values or exaggerated if the ‘healthy’ people have lower levels. In this way, it is impossible to falsify a hypothesis about the relationship between ‘health’ and reactivity. As O’Riordan and colleagues put it, ‘Without a designated threshold, researchers are left to arbitrarily determine if responses are blunted and signal poor health, or rather, are simply normative (lower) responses to psychological stress’ (p. 122).

One challenge arises from finding the data necessary to determine what a threshold for blunting should be. Of 30 empirical reports, the mean blunted response could be extracted for only 10. Similarly, only one-third of the authors contacted for Becker and colleagues’ review responded. This response rate is not abnormal (Savage & Vickers, 2009). The Behavioral Medicine Research Council has issued a statement on open science in health psychology and behavioral medicine (Segerstrom et al., 2023). Part of that statement is a call for more open data, a practice that would have greatly enriched the basis for these reviews and their conclusions.

Another challenge comes from the difficulty in thresholding. A threshold can itself be problematic, as it arises from the all-or-none diagnostic practice in the medical model (Widiger, 1992). Furthermore, if one accepts the presence of both exaggerated and blunted responses, quadratic or cubic patterns of reactivity should be tested routinely. In some populations, the least reactive people are not at risk (Brindle et al., 2016). In a quadratic or cubic model, the ‘valley’ could be identified as the point at which exaggeration or blunting begins. If a threshold is needed, a rational decision could be made about how far a response needs to be from this low point to be considered exaggerated or blunted. Again, open data would make an individual-participant synthesis possible, an analysis that would be most informative for future research (Riley et al., 2010).

If these reviews have raised more questions than answers, then they have answered the call to identify new directions in the biology of stress. The hope is that exciting new research follows this special issue.

Disclosure statement

No potential conflict of interest was reported by the author(s).