FRANKLIN D. MCMILLAN

Best Friends Animal Society, Kanab, Utah, USA

Few concepts exhibit a wider disparity between their importance for well-being and their definitional clarity than distress. Despite its frequent use among researchers and scholars, the term is not only encumbered with a variety of meanings, but many of the meanings directly contradict one another. For example, distress is often used both interchangeably with stress (National Research Council, 2008; Overall, 2019) and distinguished from stress (Committee on Pain and Distress in Laboratory Animals, 1992; Koolhaas et al., 2011), is commonly equated to suffering (National Research Council, 2008; Ledger and Mellor, 2018) but is also distinguished from suffering (DeGrazia, 1996), and has been frequently conceived as something caused by stress (Moberg, 1987) but also considered to be a form of stress (Moberg, 1987).

Historically, few attempts have been made to understand distress on a systematic and scientific basis (Committee on Pain and Distress in Laboratory Animals, 1992) and there is currently no universally accepted definition of distress in any species, nonhuman animal (hereafter animal) or human (Clark, 2007; National Research Council, 2008). A standard and generally representative dictionary definition of distress is ‘extreme anxiety, sorrow, or pain’ (Oxford Dictionaries, 2019a), and numerous medical dictionaries define distress as ‘mental or physical anguish or suffering’ (e.g., Dorland’s Medical Dictionary for Health Consumers, 2007; Saunders Comprehensive Veterinary Dictionary, 2007; The American Heritage Medical Dictionary, 2007).

Descriptions are offered for various characteristics of distress, such as its causes, circumstances surrounding its occurrence, and manifestations. However, in so doing current definitions often fail to distinguish ‘what is distress’ from ‘what indicates distress’. A useful analogy here is gravity. The force of gravity is well-described in terms of the physics and mathematics of its actions between two objects (i.e., it is directly proportional to the product of their masses and inversely proportional to the square of the distance between them), how it explains the orbits of celestial bodies, and its contribution to the nature of black holes. In addition, gravity can be measured with extreme accuracy by determining the strength of attraction between two bodies. But none of this answers the question, what is gravity? This is the current status of understanding distress.

11.1 Current Conceptualizations of Distress

The foremost challenge in developing a unified understanding of distress is the variation in the ways that the concept is characterized. The following are the most common, but not necessarily the only, conceptualizations of distress.

11.1.1 Distress is extreme unpleasantness

This view is in line with the Oxford Dictionaries (2019a) definition, in which distress is defined with terms indicating an unpleasant experience of substantial severity. The key element here is that of intensity, where intensity refers to magnitude and/or duration of the adversity, or, stated differently, the ‘quantity’ of adversity. This conceptualization of distress involves a threshold of unpleasantness, above which is regarded as distress and below which is not. This would exclude minor aversive experiences, such as a twinge of anxiety, a slight pain, a hint of sadness, or minor thirst. This does not mean that there cannot be mild distress, just that even the mild forms exceed the threshold. This view of distress can be found in scientific and daily discourse. This is the view of distress that is most consistent with the notion that distress and suffering are the same thing (to be discussed in Section 11.7).

11.1.2 Distress is any unpleasantness

Distress has also been commonly conceived as unpleasantness of any intensity or severity. In common usage, people often speak of things such as ‘watching the news these days is distressing’ and ‘I don’t want to cause you any distress, so I’ll give this assignment to someone else’. This view of distress is also used in scientific fields; for example, Mosby’s Medical Dictionary (2009) defines distress – without reference to intensity – as ‘an emotional or physical state of pain, sorrow, misery, suffering, or discomfort’. Tomkins (1963) used distress to indicate an affective experience of not greater aversiveness (‘toxicity’), but rather to be a relatively lower intensity affect. In the animal literature, Rushen (1990) wrote that ‘Distress, or aversion, as measured by aversion learning techniques, can be considered as the common, negative emotional component that underlies a diverse range of more specific mental states such as fear, pain, anxiety and frustration’, and Russell and Burch (1959) stated that ‘any degree’ of visceral ill health may be assumed to entail a measurable amount of distress.

11.1.3 Distress is any severity of certain mental disorders

This conceptualization of distress is a more specialized version of the previous view and the one used most often in the context of human health, psychology, and well-being. Here, the term (often spoken of as psychological distress) describes an array of types of psychopathology, of any severity. Cromby et al. (2013) uses the term distress ‘to refer to just the same kinds of phenomena that textbooks of this kind usually call mental illness or psychopathology. We use distress to mean all of the different kinds of difficult or unusual experiences associated with the hundreds of psychiatric diagnoses currently employed.’ Psychological distress is defined by some authors as depression and anxiety (e.g., Kessler, 1979; Mirowsky and Ross, 2003) and by others as also including other states such as burnout, irritability, worrying, problems concentrating, and, more broadly, psychological maladjustment and negative mental health states (Holahan and Moos, 1981; Veit and Ware, 1983; de Haes et al., 1990; Dyrbye et al., 2006).

11.1.4 Distress is a specific, discrete, or basic emotion

Distress has frequently been conceived of as a specific emotion. For example, Izard (1977) wrote that ‘distress is the most common negative emotion’ and that ‘distress and sadness are generally considered synonymous’. This view is often expressed as a list in which distress is included as an emotion comparable to other specific emotions, such as Roseman et al. (1994) describing their experimental design with ‘The emotions studied were fear, sadness, distress, frustration, disgust, dislike, anger, regret, guilt, and shame.’

11.2 Key Considerations in Conceptualizing Distress

Discussions of distress have involved several key issues; the following seem to be among the most fundamental for understanding the concept.

11.2.1 The issue of consciousness

On the basis of the intuitive or commonsense conceptualization, the experiential aspect comprises the very essence of distress, for which consciousness is clearly a requirement. This view is not recent; 60 years ago, Russell and Burch (1959) wrote that ‘It is assumed that to be in a state of distress an animal must be in a certain state of consciousness, which may be eliminated by, for instance, deep general anesthesia.’ More recently, the National Research Council (2008) similarly pointed out that distress could be prevented in laboratory animals by using a general anesthetic and Karas and Silverman (2014) stated that ‘While under general anesthesia an animal cannot consciously perceive the presence of a negative state, and thus distress is prevented by loss of consciousness.’

However, some research has questioned the notion that conscious awareness is essential for distress. Schultheiss and Wirth (2018), for example, have noted that researchers in the fields of biopsychology, neuropsychology, psychopharmacology, and social psychology have reached the conclusion that consciousness may not be a necessary prerequisite for goal-directed, reward-seeking behavior (the connection between distress and motivation will be addressed in Section 11.5).

11.2.2 The intensity/threshold question

As described above, two mutually exclusive conceptualizations of distress hinge on the question of whether or not distress involves intensity and a threshold of any kind. Clearly, distress being equated with any degree of unpleasantness makes the recognition (or diagnosis) of distress easier, as it is either present or not; no measurement is required to establish whether an intensity has reached a particular level. However, this presents its own problems, as exemplified by the array of empirical referents and ‘distress scales’ that have been developed and utilized in humans to determine whether individuals are experiencing psychological distress (reviewed by Ridner, 2004). The necessity of such testing attests to the peculiarity of a person being unaware that she is experiencing distress until informed by the person scoring the test.

11.2.3 The question of whether distress is anything more or different than intense negative affect ¹ (NA): whether distress differs qualitatively or only quantitatively from less intense NA

The nature of distress can be thought of in two ways. The first is that distress is nothing more than intense NA; that is, distress differs quantitatively, but not qualitatively, from less intense NA. In this view, as NA – e.g., fear, nausea, pain, anxiety, loneliness – intensifies, we simply label the experience distress when it becomes highly aversive. An analogy here is the term discomfort. Discomfort has multiple definitions, but one common usage is as a description for any mild NA. A mild itch is a discomfort, as is the first sensation of a filling bladder, the feeling in one’s buttocks from sitting too long in one position, the uneasiness of being in an eerie place, chapped lips, the bright sun as one exits a movie theater, and so on. Yet the discomfort here is not something in and of itself – it is simply a descriptive label applied to any mildly unpleasant feeling and the change in feeling is considered to be ‘increasing discomfort’ – staying simply a descriptor – as the intensity of the unpleasant affect grows. Distress may be the same thing with unpleasant feelings, so that ‘increasing distress’ is (and remains) strictly a descriptive label.

The second possible nature of distress is that it in some way differs qualitatively, not just quantitatively, from less intense NA. In this case, as pain or fear rises in intensity, a new experience is ‘added onto’ the intensifying NA (which, of course, remains present). An analogy here would be a simple alarm system. The type of threat and the affect it elicits (e.g., pain, fear, thirst, loneliness) remain the same, but have increased in magnitude whereby the animal is informed – by the ‘added on’ affective experience of distress – of a state of emergency and need for urgent action.

These two different natures of distress involve the threshold concept discussed above. We can reason that if there is no qualitative difference between distress and intense NA, then there is no threshold. Conversely, if there is a threshold then there is a qualitative difference, as something different than simply increasing intensity of NA emerges.

Related to this question is the idea that distress is severe stress. This is a common notion among the general public as well as the scientific community. For example, Moberg (1987) proclaimed in the simplest terms that the most severe form of stress is distress. This view is also expressed in language that describes stress becoming distress, such as in the National Research Council’s (2008) report ‘Recognition and Alleviation of Distress in Laboratory Animals’, where mention is made of ‘the transition of stress to distress’, and when Wolfensohn et al. (2018) refer to the ‘point where [stress] will develop into distress’.

There is evidence to challenge the idea that distress is severe stress. For example, it is possible for severe stress to occur without distress emerging. One example of this is in humans and animals rendered unconscious by general anesthesia. The stress response to surgery (specifically, the tissue trauma) is characterized by increased secretion of pituitary hormones and activation of the sympathetic nervous system. The increase in plasma concentrations of adrenocorticotropic hormone (ACTH) and cortisol can be measured within minutes of the start of surgery and, in general, the magnitude and duration of the response are proportional to the surgical injury (Desborough, 2000). More evidence of stress mechanisms in the (presumed) absence of consciousness is that pronounced physiological stress responses occur – (presumably) without distress – in many invertebrate species, including mollusks and insects (Adamo, 2012). A second line of evidence is that in sentient animals there are potential threats to homeostasis, such as tumor growth, that will elicit stress responses even though the individual may never be conscious of and hence experience no distress from the threats (Carstens and Moberg, 2000).

11.3 Why is it Important to Clarify our Understanding of Distress?

The absence of a unified definition affects the evaluation of distress and its impact on individual well-being of, in particular, animals in veterinary, scientific, and legislative contexts (National Research Council, 2008). Even the most basic foundations of research are undermined by the polysemous nature of distress. For example, if two separate studies measured the number of times per day people reported experiencing distress (or researchers were identifying distress in animals), without clear definitions of distress there would be no way to tell if the studies were measuring the same thing. In addition, understanding how, when, and why distress arises and subsides is critical to developing methods to prevent its emergence and to facilitate its resolution.

11.4 Limitations of Current Research-based Operational Definitions

Scientific investigations of distress require operational definitions, i.e., definitions that prescribe measurable criteria. In humans this is frequently done by self-reports of experiencing distress; since these methods are unavailable in animals, measurable criteria have included pathological effects (Selye, 1974; Kopin et al., 1988; Clark et al., 1997) and abnormal behavior (Kopin et al., 1988; Clark et al., 1997). Before discussing operational definitions of distress appearing in the literature, some important points need to be made. Because operational definitions are trying to achieve something – measurability – that is not (necessarily) the objective of ‘regular’ definitions, none of these definitions should be deemed ‘right’ or ‘wrong’. Relatedly, scrutinizing the accuracy of operational definitions is fraught with challenges. Consider if an analysis were to suggest that operational definition A could not be true since distress could occur ‘outside’ this definition. A problem is immediately evident in the question, What definition of distress is one using to dispute A? If A defines distress using specific criteria, then distress is, by definition, that which meets these criteria. With that caveat in mind, let us look at some operational definitions.

11.4.1 Distress is the state that occurs when an individual’s ability to cope or adapt to an aversive condition or event is exceeded

One of the most widely accepted themes regarding the nature of distress is that it is based on one’s ability to cope or adapt, and that it emerges once this ability has been exceeded or overcome (Kitchen et al., 1987; Kopin et al., 1988; Committee on Pain and Distress in Laboratory Animals, 1992; Clark et al., 1997; Clark, 2007; National Research Council, 2008). Karas and Silverman (2014) encapsulated this concept in stating that ‘When the effort required to adapt to some stressors, or the aversive nature of the stressor, increases and reaches a point where the animal fails to be able to adapt adequately, this is considered by some authors to represent distress.’ A more specific example is evident where The National Research Council (2008) proposed that the distinguishing feature between stress and distress is that the latter reflects ‘an animal’s ability or inability to cope or adapt to changes in its immediate environment and experience’. Minor variations in wording place the emphasis on the inability to cope or adapt to stressors (Kitchen et al., 1987; Committee on Pain and Distress in Laboratory Animals, 1992; Clark et al., 1997; Clark, 2007; National Research Council, 2008; Karas and Silverman, 2014), the inability to maintain homeostasis (Clark et al., 1997; National Research Council, 2008) or adaptive equilibrium (Committee on Pain and Distress in Laboratory Animals, 1992), or that coping mechanisms are overwhelmed (Moberg, 2000; Clark, 2007).

Most problematic for the linking of distress with the inability to adapt is that this view conceptualizes distress exclusively as a failing/failed – a maladaptive – state, exemplified by the Committee on Pain and Distress in Laboratory Animals’ (1992) statement: ‘Stressors do not pose a threat to the animal as long as it can maintain an adaptive equilibrium. When that is no longer possible, the animal enters a state of distress, in which its behavior and physiology become maladaptive.’ Similarly, Clark et al. (1997) stated that a maladaptive state of distress occurs when an animal fails to adapt, cope, habituate to a challenging stimulus, and ‘Progression into the maladaptive state [of distress] may be due to a severe or prolonged stressor or multiple cumulative stressful insults with deleterious effects on the animal’s welfare.’ This provides a logical explanation as to how the phrase ‘leads to distress’ has come to mean something exclusively bad.

There are several problems with the view that distress is the result of a failed or overwhelmed ability to cope or adapt, evident by examining the three main implications of this view: (i) if adaptation (or the more specific coping) is successful then distress will not be present; (ii) its converse, whenever distress is present the individual is failing (or has failed) to adapt; and (iii) distress cannot occur before adaptation processes have begun to fail. Current evidence seems able to discount all of these implications. Most importantly, distress and successful coping frequently coexist: distress is often present during fully successful adaptation or coping attempts (Committee on Pain and Distress in Laboratory Animals, 1992; Tiefenbacher et al., 2005; National Research Council, 2008). For example, during the course of an illness caused by an infection (such as the influenza virus) to which the body is successfully responding, there can be considerable distress (as anyone who has suffered a bad case of the flu can attest). If distress is (only) the result of failing adaptation, distress should not occur since adaptation is succeeding, albeit slowly. The same is true during the time that the body is successfully healing burned skin: distress from pain is present while the adaptation is proceeding (in fact, one of the main features of burn pain is that its duration often exceeds healing time [Latarjet and Choinère, 1995]).

11.4.2 Distress is the state that exists when stress responses lead to pathologic changes

Numerous literature discussions of distress have suggested that a reliable indicator of distress is the development of pathologic changes. For example, Carstens and Moberg (2000) stated that ‘Certainly the presence of pathologies such as disease, self-mutilation, or death are obvious indicators of distress,’ and the Committee on Pain and Distress in Laboratory Animals (1992) wrote that when pathologic changes, such as gastric ulcers, occur, it can be said that the animal is not only stressed, but distressed. However, upon scrutiny it becomes evident that pathology may not be a reliable indicator of distress. The most concise argument here can be stated as such: pathology can exist without distress and distress can exist without pathology.

First, disease processes may arise and progress wholly outside the individual’s awareness. As noted by Karas and Silverman (2014): ‘a poor physiologic state does not always mean that distress is present: an animal may have cancer or other disease for some time and not be “aware” or affected by its body’s efforts or inability to cope and … from the standpoint of health, it is not (yet) distressed’. The National Research Council (2008) made a similar observation, suggesting that it is possible for an animal to be in a state of poor health that does not impinge on its emotional state and that this condition may even last for some time without the animal’s conscious awareness; the authors used the example of an animal having but being unaware of a life-threatening aneurysm and therefore have no experience of a negative emotional state.

Other types of evidence argue against the idea that distress is the state when pathology develops. One is that the ‘development of pathology’ is not at all a precise and recognizable event. For example, not all adverse stress-induced health changes become manifest immediately; pathologic changes may appear at a much later time in the individual’s life. In addition, pathological changes may consist of an increased susceptibility to or risk for, as opposed to overt expression, of pathology. Evidence from studies of humans (reviewed by Fagundes et al., 2013; Miller et al., 2011) and animals (reviewed by Miller et al., 2011) indicates that early life adversity (ELA) shows strong correlations to the development of psychopathology later in life. For instance, several rodent and primate models of ELA, including those that model maternal separation or loss, abuse, neglect, and social deprivation, have demonstrated that these types of early stressful experiences are associated with long-term alterations in neuroendocrine responsiveness to stress, emotional and behavioral regulation, coping style, cognitive function, quality of social affiliations and relationships, and expression levels of nervous system genes shown to be associated with anxiety and mood disorders (Sanchez et al., 2001; Cohen et al., 2006). In humans, severe stress early in life, such as child abuse, increased the risk of anxiety and depressive mood disorders (Heim et al., 2002; Chapman et al., 2004; Shea et al., 2005) and of developing posttraumatic stress disorder (PTSD) in response to major stressors (Brewin et al., 2000) later in life. In all, there is consistent evidence linking early stress and later physical and mental health disorders in humans and animals, and that in animals this is a causal link (Miller et al., 2011). The key point here is that if adverse health effects (pathology) are used to operationally define distress, delayed effects and increased risk of pathology would both require that the confirmation of current distress could only be made retrospectively. In such cases an individual enduring adversity could not be determined at present to be experiencing distress, even for humans assessing their own mental state. This raises the unusual possibility of a person currently enduring some form of adversity being asked ‘Are you experiencing distress?’ and the person responding, ‘I don’t know. I guess I’ll find out when I’m older’.

11.4.3 Distress is the state that exists when adversity leads to abnormal and maladaptive behaviors

One of the most prominently proposed indicators of distress in animals is the occurrence of abnormal or atypical behavior. For example, the Committee on Pain and Distress in Laboratory Animals (1992) has written that ‘When an animal is experiencing distress, maladaptive behaviors result, varying in range and severity with increasing distress’ and that the appearance of such behaviors marks the point where distress arises.

However, this view also has difficulty standing up to scrutiny. First, as noted by the National Research Council (2008), it is unclear at this time whether any or all abnormal behaviors qualify as indicators of distress, and distress may not manifest itself with recognizable maladaptive behaviors. Second, not all abnormal behavior during severe adversity is pathological or maladaptive; some abnormal behaviors can (and often do) represent homeostatic processes of adapting to the stress (Committee on Pain and Distress in Laboratory Animals, 1992; Tiefenbacher et al., 2005; National Research Council, 2008). Research shows that in some humans and nonhuman primates, even more serious forms of abnormal and self-injurious behavior may function to reduce arousal (Tiefenbacher et al., 2005) and it has been suggested by those who favor the arousal reduction hypothesis as an explanation for stereotypic behavior that such behavior may serve to calm the animal and thereby avoid distress (Mason, 1991). For instance, in rhesus monkeys bouts of self-injurious biting were associated with reduced indices of physiological stress (Novak et al., 2006) and several studies have demonstrated that some individuals exhibiting a high degree of stereotypic behavior appear to cope better than nonstereotypic conspecifics (reviewed by the Committee on Pain and Distress in Laboratory Animals, 1992; Mason and Latham, 2004).

11.4.4 But is distress a negative, maladaptive, failed state?

The definitional characterization of distress as a maladaptive, failed state must be questioned. The evidence in the previous sections indicate that distress is not a uniformly failed state; animals (and people) can successfully cope and recover from this experience. Resolving this question comes by reconceptualizing distress as a state that can have either an adaptive (successful) outcome or a maladaptive (unsuccessful) outcome, to which we will return later.

But if it is a misconception to view distress as indicative of failure to cope and maladaptive outcomes, why did this view become so prominent? The explanation may be in the confusion surrounding the concurrence of events at the point when distress arises. As the intensity of unpleasantness increases in response to an aversive stimulus, the events that occur include abnormal behavior (e.g., extreme, forceful, frantic, disorganized, illogical, counterproductive, harmful, futile), pathological changes, an intensified physiological stress response, and impaired welfare (including the related concepts of well-being, quality of life, and happiness). The temporal relationship among these events gives the impression of causal correlations and the assumption that, for example, distress leads to abnormal behavior and/or a decrease in well-being. However, there is insufficient justification for these assumptions; for example, rather than being an effect of distress, abnormal behavior may emerge along with distress as a mechanism for coping with the distress, and the threatened well-being could arise independently where the co-occurring distress is the organism’s response to the threat, i.e., the distress arises to help alleviate the threat to well-being. With the co-occurrence of distress and harm, it is easy to view distress as the cause of that harm, but once it is recognized that distress often has a successful outcome, then the co-existence of distress and harm is uncoupled and causality is no longer assumed to run from distress to the various adverse changes.

11.5 Homeostasis, Stress, and Distress

Deviations from homeostasis represent a threat to and reduced chances for fitness; hence, animals have evolved effective mechanisms for detecting and correcting such deviations (Panksepp, 1998). The CNS assesses the importance of stimuli to homeostasis and, for those stimuli representing a meaningful threat, organizes and initiates the responses necessary to maintain or restore biological equilibrium (Panksepp, 1998). It is now well recognized that defensive responses on the part of the organism cannot be random or generalized in type and intensity; rather, responses must be both specific and proportional to the stimulus or situation that elicits them (Selye, 1974; Rolls, 2000; National Research Council, 2008). For most threats, the simplest and frequently the most biologically cost-effective response for an animal is to alter its behavior in a way that alleviates the threat (e.g., scratch an itch, curl up when cold, move away from a source of heat, cough to clear an airway irritant, step away from a cliff edge). This may involve only the CNS and voluntary muscle activity, and there may be little or no sympathetic or glucocorticoid response (Moberg, 1985; Clark et al., 1997). When a threat is not alleviated by a specific but minor behavioral response, as when the disturbances are more intense and/or persistent, other behavioral, physiologic, and biochemical responses and arousal mechanisms are required to neutralize the threat: these are the stress responses (Clark et al., 1997).

Expanding on Selye’s (1936) original distinction between selective (and normally adequate) responses to minor challenges and nonselective (emergency) responses to major challenges which can be termed stressors, Day (2005) suggested that stress is the body’s multi-system response to any challenge that overwhelms, or is judged likely to overwhelm, selective homeostatic response mechanisms. A crucial point here is that when stimuli are aversive or otherwise threatening enough to activate a stress response, the primary defense mechanisms are assisted by, but not replaced by, the stress response.

11.6 Affect and Motivation as the Core Elements of Distress

Affective states are considered to be psycho-physiological constructs that vary along three principal dimensions: valence, arousal, and motivational intensity (Harmon-Jones et al., 2013). As expressed concisely by Schultheiss and Wirth (2018), ‘Motivation is, at its very core, about affect … [and] attaining a pleasurable incentive (reward) or at avoiding an aversive disincentive (punishment)’ is its hallmark. Abundant evidence suggests that affective states are proximate mechanisms which serve as motivational guides to facilitate behavior that is, at the ultimate level, beneficial to reproductive fitness and to discourage behavior contrary to these goals (Bindra, 1978; Committee on Pain and Distress in Laboratory Animals, 1992; Barnard and Hurst, 1996; Panksepp, 1998, 2011; Duncan, 2004; Leknes and Tracey, 2008; Edwards, 2010; Harmon-Jones et al., 2012). Additionally, in general, the strength of motivation is correlated with the intensity of the affect, so that as the intensity of affect increases, so too does the intensity of motivation (MacDonald and Shaw, 2005; Leknes and Tracey, 2008; Webster, 2011). Together, these elements construct a function for distress that departs widely from the view of a failed, maladaptive state.

When primary responses are inadequate to address a homeostatic challenge, a stress response is activated. This mechanism, honed over millions of years of natural selection, is highly effective in restoring homeostasis; however, in some cases the stress response generates inadequate changes in the behavioral and physiologic homeostatic systems (National Research Council, 2008). When faced with the possibility that current coping methods are insufficient, the individual has four choices:

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