Causal loop diagram
Each causal link in the CLD (Fig. 1) is described in relation to its role as connecting exogenous (section “Exogenous variables: initial conditions”; Figs. 2 and 3) or endogenous variables (section “Endogenous variables: feedback loops”; Figs. 4, 5, 6, 7, 8). The CLD contains two exogenous variables: “uncontrollable childhood and life course stressors” (section “Uncontrollable childhood and life course stressors”); and “socioeconomic resources” (“Socioeconomic resources”). We use the word “uncontrollable” to refer to a person’s assessment of whether their behavioural responses could considerably alter the outcomes of the stressor [1, 58]. As specified above, these were taken to be the initial conditions for the unfolding mechanisms described. The nine remaining variables are endogenous variables, which are involved in five feedback loops. The variable “stimulus” is categorised as an endogenous variable and not as an initial condition, because it is involved in feedback loops. However, an arrow pointing in the direction of the variable “stimulus” was also added to clarify that this variable is contextual and that stimuli can continue to arise as inputs to the system given the accumulation of stressors that is associated with exposure to adverse socioeconomic conditions. Each feedback loop is labelled for its contribution to explaining the dynamics that drive chronic stress in a context of adverse socioeconomic conditions: the feedback loops correspond to (1) progressive deterioration of access to coping resources because of repeated insolvability of stressors (section “Progressive deterioration of access to coping resources because of repeated insolvability of stressors”); (2) perception of stressors as uncontrollable due to learned helplessness (“Perception of stressors as uncontrollable due to learned helplessness”); (3) tax on cognitive bandwidth caused by the stress response and chronic stress (“Tax on cognitive bandwidth caused by the stress response and chronic stress”); (4) stimulation of problem avoidance to provide relief from the stress response and free up cognitive bandwidth (“Stimulation of problem avoidance to provide relief from the stress response and free up cognitive bandwidth”); and (5) susceptibility to appraising stimuli as stressors against a background of chronic stress (“Susceptibility to appraising stimuli as stressors against a background of chronic stress”).
Exogenous variables: initial conditions
Uncontrollable childhood and life course stressors
During primary appraisal, it is established whether a stimulus is assessed as a threat or as a challenge  (Fig. 2). Accordingly, a stressor is a stimulus that is appraised as a threat (link 1/2) . Primary appraisal is in part dependent on past exposure to uncontrollable stressors . It has been reported that exposure to adverse socioeconomic conditions increases the likelihood of exposure to uncontrollable childhood and life course stressors [5, 19, 52].
To understand the effect of past stressor exposure on the primary appraisal of a current stimulus (link 3), the brain has been characterised as a “prediction machine” that derives its primary appraisal not only from the features of the stimulus but also from a person’s “personal memory bank” . This memory bank is marked by previous experiences, where “a history of feeling or being threatened might shift the appraisal of a current stimulus to more of a threat than challenge appraisal” .
Exposure to uncontrollable childhood and life course stressors can also undermine access to coping resources in adulthood (link 4) [19, 35,36,37,38,39,40,41,42,43]. We focused on those four coping resources that we found to be affected by stressor exposure: mastery/self-efficacy/locus of control/fatalism, neuroticism, optimism and self-esteem/self-confidence (definitions provided in Table 1).
Access to each of these coping resources is likely to be negatively affected by uncontrollable childhood and life course stressors [19, 35,36,37,38,39,40,41,42,43]. This may be related to these experiences leaving less opportunities for the development of coping resources. For example, because a general sense of self-efficacy evolves through specific mastery experiences, adverse mastery experiences—where the situation could not be controlled—can undermine self-efficacy . Self-efficacy thus develops through favourable mastery experiences over the life course, implying that adverse mastery experiences are particularly detrimental when self-efficacy has yet to be developed , as is the case for children.
Still, socioeconomic resources in adulthood could facilitate reappraisal of stressors that were previously perceived as uncontrollable (link 5) [20, 21]. Specifically, the extent to which past exposure to such stressors still impacts access to coping resources in adulthood may be modified by socioeconomic resources. Socioeconomic resources may influence whether some level of resolution has been reached as more resources became available over time, even if the stimulus was perceived as an uncontrollable stressor at the moment it occurred. To support this, an analysis of life course pathways leading from adverse childhood experiences towards adult psychological well-being—including distress—showed that social support, as measured by a sense of community, in adulthood can buffer the effect of adverse childhood events on perceived well-being in adulthood .
Secondary appraisal entails assessing whether the resources available can meet the demands raised by a stressor (link 6), which determines whether the stressor is appraised as controllable or uncontrollable [11,12,13] (Fig. 3). In addition to the demands posed by the stressor in question, access to socioeconomic resources (link 7) and coping resources (link 8) also determine the assessment of the stressor’s controllability [11,12,13, 44]. Exposure to adverse socioeconomic conditions is correlated with exposure to stressors with high demands [6, 14]. Moreover, people exposed to such conditions are more likely to have limited access to socioeconomic resources to accommodate these demands  and to have been previously exposed to uncontrollable stressors that have undermined their access to coping resources, as detailed above. This combination makes it more likely that a stressor is assessed as uncontrollable—instigating a stress response (link 9) . Link 6, 7 and 8 together illustrate the balance between exogenous variables (demands posed by a stressor (stemming from a stimulus) (link 6) and access to socioeconomic resources to accommodate those demands (link 7)) and endogenous variables (access to coping resources (link 8)) in determining how a stressor is assessed. For example, whether long working hours result in a stress response may depend on a person’s evaluation of their autonomy in their work environment, dependent on their position within the organisation . Chronic stress can develop via link 9 through the chronicity of exposure to stressors assessed as uncontrollable and correspondingly the repetition of the stress response . As additional stimuli are likely to keep arising over time in a context of adverse socioeconomic conditions, this can result in chronic stress.
Limited access to socioeconomic resources decreases the solvability of stressors (link 10). Solvability of stressors refers to what people can actually do to alter their outcomes and is influenced by access to socioeconomic resources and by possible coping strategies. Firstly, having access to socioeconomic resources increases what is within people’s power to change the outcomes of stressors, increasing the solvability of stressors (link 10). Secondly, the solvability of stressors is affected by which coping strategies are possible given the circumstances (link 11), as is further explained in section “Progressive deterioration of access to coping resources because of repeated insolvability of stressors”.
Endogenous variables: feedback loops
Progressive deterioration of access to coping resources because of repeated insolvability of stressors
We described above that past exposure to uncontrollable stressors, associated with adverse socioeconomic conditions, can undermine access to coping resources [19, 35,36,37,38,39,40,41,42,43] (Fig. 4). Exposure to adverse socioeconomic conditions increases the likelihood of being repeatedly confronted with insolvable stressors. If problem approaching is evoked when a stressor is insolvable, this can undermine access to coping resources as well (link 14) [19, 27, 52]. For example, actively engaging in situations that children are not able to change, such as inter-parental conflict, can result in worse outcomes for them on a behavioural and emotional level, which teaches them not to attempt this strategy when faced with subsequent stressors . This can leave access to coping resources undermined, while access to coping resources affects coping strategies. Undermined access to each of the included coping resources can evoke problem avoidance, while problem approaching may be a consequence of access to the included coping resources (link 12) [27, 45,46,47,48,49].
This mechanism is part of an amplifying feedback loop, as undermined access to coping resources can prevent problem approaching for solvable stressors—a combination which can result in a further undermining of access to coping resources (link 13/14) [19, 20, 27, 36, 50,51,52]. If stressors are solvable, the coping strategy that is induced by undermined access to coping resources, i.e. problem avoidance, can further restrict this access, whereas the coping strategy allowed for by access to coping resources, i.e. problem approaching, can further increase this access (link 14) [19, 27, 52]. A coping strategy thus either increases or decreases access to coping resources, possibly creating a virtuous cycle for those who already have access to coping resources and a vicious cycle for those whose access to coping resources has been undermined in the past. For example, optimism is related to “an active approach mode”, i.e. problem approaching, which facilitates the feeling of accomplishment after a favourable coping experience and in turn reinforces optimism regarding the future (“an ‘I can do it’ attitude”) . Conversely, Aldwin et al. suggest that people who are low in mastery at baseline may have no opportunities for problem approaching, which may lead to reduced “situation-specific mastery”, which in turn may result in even lower “global mastery” as compared to the baseline . In turn, once access to coping resources has been undermined, problem avoidance coping strategies are more likely to be invoked, which can successively further restrict access to coping resources—creating an amplifying feedback loop.
If stressors are insolvable, coping strategies associated with disengagement are actually correlated with lower stress (link 14) [19, 27, 52]. Here, “the impracticality and even the danger of more ‘active’ problem-solving techniques” that more often than not are evident in a context of adverse socioeconomic conditions should be acknowledged .
Perception of stressors as uncontrollable due to learned helplessness
Access to coping resources of people exposed to adverse socioeconomic conditions has more often been undermined by past exposure to uncontrollable stressors, which increases the likelihood of appraising subsequent stressors as uncontrollable (link 8) [11,12,13, 44] (Fig. 5). The secondary appraisal of a stressor as uncontrollable informs which coping strategy is adopted. In particular, if a stressor is assessed as being uncontrollable, it makes less sense to employ a problem approaching coping strategy than if the stressor is assessed as controllable—because an uncontrollable stressor will probably persist regardless of the coping strategy . Accordingly, if a stressor is assessed as uncontrollable, problem avoidance can be the least harmful strategy (link 17) [2, 53]. Despite “active, thoughtful, and creative strategies to cope with the difficult problems” in the face of limited access to socioeconomic resources, problem approaching coping strategies often do not meet their objective due to the circumstances .
We explained that if problem approaching is evoked when a stressor is insolvable, this can undermine access to coping resources over time, which Belle and Doucet describe as “repeated coping failures may then lead to the belief that stress factors cannot be overcome”  (link 13/14) [19, 20, 27, 36, 50,51,52]. As access to coping resources progressively gets more limited, because of the lack of options available for problem approaching, the probability that stressors are assessed as uncontrollable increases. This amplifying feedback loop may gradually result in an increasing number of stressors being appraised as uncontrollable and, consequently, in an overwhelming accumulation of stressors that may have not been assessed as threats in the past but have become threats given the circumstances, making it impossible for all stressors to be addressed.
Low actual control, defined in the literature as low “control that individuals are able to exercise over their living environment through the economic and socioeconomic resources they have at their disposal” , and low perceived control, defined as low control beliefs that can arise from socialisation in environments characterised by adverse socioeconomic conditions, are often separated in explanatory models concerning health inequalities . The relationship between a context of adverse socioeconomic conditions and chronic stress is represented as a linear process, commencing either with what people can actually do or with what they think they can do (as learned by others) to alter the outcomes of a stressor. Looking at this relationship as an amplifying feedback loop can consolidate these two pathways, showing that actual and perceived control are interlinked, where low perceived control does not necessarily originate from socialisation, but can also be empirically deduced from past experiences. It is rational to have “the belief that stress factors cannot be overcome”  if all past experiences under the same circumstances substantiate this belief. This mechanism is indicated in this amplifying feedback loop: low perceived control may stem from undermined access to coping resources, which in turn may be caused by past exposure to uncontrollable stressors and the repeated insolvability of stressors. This suggests that low perceived control can be the result of past experiences of low actual control. The phenomenon emerging from this amplifying feedback loop has been referred to as learned helplessness .
Tax on cognitive bandwidth caused by the stress response and chronic stress
Exposure to adverse socioeconomic conditions is correlated with recurrent exposure to stressors , which can result in the repeated occurrence of the stress response (link 9) , i.e. chronic stress  (Fig. 6). The stress response in turn is accompanied by another amplifying feedback loop. A stress response taxes working memory, leaving less cognitive bandwidth for other tasks (link 18) [22,23,24,25]. Cognitive bandwidth has extensively been researched against the background of scarcity, which is a prominent stressor associated with exposure to adverse socioeconomic conditions. It has been shown that the “internal disruptions” and “involuntary preoccupation” associated with scarcity affect cognitive capacity (i.e. those psychological processes that govern capability to “solve problems, retain information, engage in logical reasoning, and so on”) and executive control (i.e. those psychological processes that govern capability to “manage [ … ] cognitive activities, including planning, attention, and initiating and inhibiting actions”) [23, 25].
An accumulation of stressors, against the background of exposure to adverse socioeconomic conditions [6, 14], can be detrimental because cognitive bandwidth is a finite resource and each stressor takes up cognitive bandwidth [22,23,24,25]. It is important to note that cognitive bandwidth is thus dependent on background processes, but independent of inherent traits  (hence the term “bandwidth”, which in computing refers to “the amount of information that can be sent over a network connection at one time” ). These changes in cognitive capacity and executive control as a function of the stress response can be traced back to the biological underpinnings of the stress response .
A tax on cognitive bandwidth is one of the explanatory factors for why an accumulation of stressors can prevent people from addressing some of the stressors, given the impossibility to deal with them all at once and the trade-off that therefore needs to be made (link 19) [22,23,24,25]. This can result in an amplifying feedback loop where it is impossible to accommodate the demands of current and/or additional stressors due to the fact that they are accumulating, leaving some of the stressors unresolved (link 15/16) [26, 27], which then continue to take up cognitive bandwidth.
Stimulation of problem avoidance to provide relief from the stress response and free up cognitive bandwidth
Exposure to adverse socioeconomic conditions is correlated with recurrent stressor exposure  (Fig. 7). As outlined above, an accumulation of stressors can sustain multiple amplifying feedback loops, presumably operating simultaneously, including a tax on cognitive bandwidth (link 19) [22,23,24,25] resulting from the repeated occurrence of the stress response (link 18) [22,23,24,25].
As has been noted previously by others, problem avoidance is often wrongly presented as a coping strategy that is maladaptive in all circumstances . Specifically, previous empirical research indicates that coping strategies associated with problem avoidance are adaptive for stressors that are temporary and uncontrollable . Analogously, problem approaching is accommodating when a stressor is solvable, but may be maladaptive and even lead to adverse outcomes when it is not . The “impracticality and even the danger of more ‘active’ problem-solving techniques”  in the face of an accumulation of insolvable stressors should be taken into account to recognise that problem avoidance can be the least harmful strategy in a context of adverse socioeconomic conditions in order to provide relief from the stress response (link 20/21) [27, 34], and make cognitive bandwidth available for subsequent stressors that might be solvable.
Susceptibility to appraising stimuli as stressors against a background of chronic stress
People exposed to adverse socioeconomic conditions are more likely to be faced with recurrent stressor exposure  (Fig. 8). If the stress response keeps manifesting, i.e. becomes chronic stress , this may result in changes in primary appraisal of additional stimuli. Particularly, an additional stimulus may be more likely to be perceived as a threat (link 22) .
If the stress response is “sustained or slow to return to baseline”, it is associated with “prolonged anticipation of future events, elevation of affective states such as anxiety and worry, or physiological states of vigilant preparedness, reflected in autonomic nervous system arousal” . While people are experiencing chronic stress driven by a context of adverse socioeconomic conditions, subsequent stress responses might thus be different, where additional stimuli may be more likely to be evaluated as threats. It is however uncertain whether a background of chronic stress always results in additional stimuli being more likely to be appraised as threats. Chronicity of the stress response may, over time, also result in a dampening of the stress response to additional stimuli . Nevertheless, it appears that, at least in some cases, this amplifying feedback loop contributes to chronic stress.
We compared and contrasted two hypothetical scenarios, relating to person A and B, as depicted in Fig. 9. These scenarios are illustrative of how exposure to adverse socioeconomic conditions can affect people in different ways and that the dominant mechanisms leading to chronic stress may differ between people within such a context. Specifically, the CLD, while the structure of the variables and the causal links remains the same, can account for individual variation in two ways. Firstly, the values for the variables relating to a specific person, such as the extent to which they have been exposed to uncontrollable stressors in the past and the precise combination of socioeconomic resources that they have access to, can cause individual variation. Secondly, the strength of the effect of a causal link may differ between people, where for example the negative effect of past exposure to uncontrollable stressors on current access to coping resources may not be equally strong among different people.
In these two scenarios, person A and B are dissimilar in their initial conditions: A was exposed to adverse socioeconomic conditions during childhood and has been exposed to uncontrollable stressors, whereas B is currently exposed to adverse socioeconomic conditions. A has access to socioeconomic resources in adult age, while B has limited access (link 10). Both A and B are affected by a financial setback. They each appraise this stressor as uncontrollable, leading to a stress response (link 9). The cause for this secondary appraisal however differs between A and B—A’s access to coping resources has been undermined by past stressor exposure (link 4, link 8), while B is unable to resolve the stressor because of a context of adverse socioeconomic conditions (link 6, link 7). Both have no options for problem approaching, via different mechanisms: for person A this is because of learned helplessness (link 17, link 13/14), where their “belief that stress factors cannot be overcome”  is grounded in past experiences, whereas person B’s cognitive bandwidth is continually taxed by the financial setback because they do not have the socioeconomic resources to accommodate the demands of this stressor (link 18, link 19). The persistence of the stress response induced by the financial setback causes chronic stress as, given a context of adverse socioeconomic conditions, the stressor is insolvable for both person A and person B (link 15/16).
Rights and permissions
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.