JAD Sloman

This paper explores the well-known overlap of anxiety and depressive symptoms in mood and anxiety disorders. We suggest that the regulation of both negative and positive affects has served important adaptive functions (especially for coping with threats, losses, failures and defeats), and that in some contexts both affect systems require regulation at the same time (e.g. increased anxiety coupled with low positive affect). Here we will focus on how low positive, and high negative affect in the individual experiencing losses and defeats, regulates their competitive and acquisitive behaviors and in some cases may prevent, de-escalate, and possibly terminate on-going agonistic (hierarchical) encounters. When high negative affect (anxiety) and low positive affect (depression) fail to fulfill their adaptive role, they tend to persist and often intensify. This may lead each affect control system to stimulate specific types of anxiety and depressive disorders, exhibiting features reminiscent of the original adaptive function of the behavior. Furthermore, as these different systems tend to operate in a synchronous fashion, the psychiatric syndromes they generate are often comorbid.

Introduction

In this paper, we will show how both anxiety and depression in prehistoric times, and perhaps today, have had the adaptive function of promoting the resolution of agonistic conflict. Price (1967), noting the similarities between depressed patients and the losers of hierarchical encounters among long-tailed macaques, proposed that “states of depression, anxiety and irritability” are the emotional concomitants of behaviors necessary for the maintenance of dominance hierarchies in social groups. This whole-body response to the threat of continuing conflict has come to be termed the “Involuntary Defeat Strategy” (IDS) (Sloman and Gilbert, 2000), “involuntary” because it is elicited automatically and often against a person’s conscious wishes, “defeat” because it is triggered by the recognition of definite or imminent defeat, and “strategy” because there is genotypic underpinning of how an individual is to act in contexts of imminent threat, loss and defeat. The key goal of the IDS, namely to “de-motivate” interest in competition over resources (e.g. mates and food) was broadened by Gilbert (1992) and Nesse (2000) to include the avoidance of any failing enterprise, a response that is essentially the basis of anhedonia. Price and Nesse’s intertwined hypotheses could explain why Major Depression has not been extinguished by natural selection, as well as how our ancient forbearers resolved agonistic conflicts associated with the competition for resources.

There are various evolutionary theories of depression (see Sloman, 2000 and Gilbert (in press)), and we have focused on theories of depression, which involve attachment and social rank systems (Sloman et al 2004). In this paper we will show how a particular form of anxiety could originate from specific mechanisms that played a role in the development and maintenance of social rank.

Price (2003) set the stage for this paper by proposing that anxiety contributes to reconciliation between chimpanzees during the aftermath of a ritualized fight. In this paper we will broaden the discussion by considering possible functional roots of Generalized Anxiety Disorder (GAD), Separation Anxiety, Social Anxiety and Panic Attacks. It is noteworthy that chimpanzees, following conflict, tend to seek comfort from their victorious rival rather than from more senior animals or close relatives (de Waal & Aureli, 1997). Reassurance-seeking following an attack (Goodall, 1988), and reconciliation following conflict (deWaal, 1989) have evolved in some primates as a powerful way of controlling stressful conflict interactions.

The chimpanzee's need for physical contact after he has been threatened or attacked by a superior was illustrated by Figan, aged about 10 years, who was badly pounded by the alpha male, Goliath. Screaming and tense, Figan began cautiously approaching his aggressor who sat with his hair still bristling. Every so often, the desire to flee seemed to almost overcome the adolescent's desire for contact and he turned, as though to retreat. Each time, however, he went on again until eventually he was crouched, flat on the ground in front of Goliath. And there he stayed, still screaming, until Goliath, in response to his submission, began to pat him gently on the back until the screaming gradually subsided. Figan then sat up and moved away quite calmly (Goodall, 1990).

Being comforted by the victorious opponent not only helps to relieve anxiety in the subordinate, but also decreases hostile intent in the dominant individual. Ultimately, if all goes well, the loser’s gratitude for being relieved of his/her anxiety overcomes his/her resentment at being bested and thus essentially enables him/her to submit and accept defeat. It may also be noted, that both the IDS and manifest anxiety communicate to the adversary that there remains “no threat” to them (Price et al 2004). Therefore, mild depression could have inhibited retaliation, while anxiety promoted submission to the former rival during conflict resolution.

Increased negative and decreased positive affect during agonistic social interactions have their counterparts in specific clinical syndromes that may either occur alone or in combination with each other. When an individual competes for a secure place in society (e.g. vying to be chosen as a lover, teammate, or employee) and fails, or when he/she is involved in an agonistic encounter that carries a risk of injury, the triggering of a negative affect serves to warn him/her about the risks involved with continuing the struggle. Conversely, a low positive affect serves to decrease the individual’s motivation to continue to meet the challenge. Both these affects are capable of triggering disengagement and preventing a renewed conflict or struggle from arising. Clarifying the evolutionary function of negative and positive affect sheds light on their interaction and the high co-morbidity of anxiety and depression.

Function of negative and positive affects

In the EEA, negative emotions, such as anxiety, anger and disgust alerted the individual to existing threats and indicated that defensive actions are needed (Nesse, 1999:Gilbert, 2001). They evolved by facilitating the avoidance of similar dangers during our species’ development (Hofer, 1995). These particular emotions may loosely be seen as part of a negative affect (Watson et al, 1995) or threat-defense system (Marks, 1987).

We should note the controversial position of anger as a negative emotion. Price et al. (2004) included anger with exhilaration and assertiveness as an escalatory emotion, associated with the escalation of conflict.Escalation and de-escalation are alternative strategies for use in agonistic interactions with conspecifics, and are therefore a sub-set of the body’s overall defenses of freeze, flight, fight and fright (Barcha, 2004) which are available to deal with predators as well as hostile conspecifics.

In contrast, positive affects evolved in the context of success, safeness and security (Gilbert, 2001). Gilbert (Submitted for publication) differentiated between two types of positive affect: social safeness, that taps feelings that others are warm and supportive, and non-social safeness, that taps being able to enjoy sensory pleasures and not having to strive to achieve. Whereas positive emotions, such as love, desire and pleasure, evolved to motivate, guide, and reward approach-behavior, low positive affect evolved as a mechanism to switch off approach-behavior, disengage, and reduce the individual’s efforts. While positive affects encourage animals to develop and broaden their access to, and control over, resources (Fredickson, 2000), failure and loss usually cause the level of positive affect to dip. If this dip is too intense or prolonged we may diagnose depression.

The Involuntary Defeat Strategy

In many group-living animals the search for essential resources, such as food, and social resources, such as alliances and sexual partners, involves competition between the members of the group. The “Involuntary Defeat Strategy” (IDS) (Sloman, 2000: Sloman et al, 2003) is exquisitely geared towards inhibiting combativeness and thereby making the less able or less powerful individual give up his/her struggle, or failing enterprise, and accept defeat. This act of giving up protects him/her from incurring further injuries and wasting additional efforts and leads the winner to tone down potential hostile intent towards the loser. The IDS is characterized by feelings of helplessness, hopelessness, inferiority (others are better or more powerful than self), inadequacy and tiredness. If the loser resets his/her goal-seeking system to that of a lower ranking individual and essentially accommodates to his/her new position in the group hierarchy, then the IDS may be turned off. Rank is, of course, relative and these contests take place throughout the whole of the social hierarchy – not just between top-ranking individuals and their inferiors. In humans they can also be role-focused so that the same person can be dominant in one role and subordinate in another.

Humans compete in different ways, not only through aggression, but by being competent, esteemed, and valued by others, and by being seen as attractive and desired in relationships. In this context, the IDS operates when individuals compete for social acceptance, support and prestige (Nesse, 1991), but fail to impress others and thus end up feeling rejected. This leads to a more intense triggering of the IDS, which may ultimately manifest as clinical depression. One of the special difficulties that humans have is that they can ruminate and brood on their losses and senses of inadequacy and in this way maintain their IDS.

There are, of course, situations where, even if a person may want to escape, submit or seek help they are not able. For example, he/she may be locked into an abusive marriage where his/her seeking help is seen as a further sign of inferiority. In this context natural defenses are blocked and arrested (Dixon, 1998). Indeed Price and Sloman (1987) directed attention to blocked escape in birds that are caged causing serious disturbance because the birds cannot simply ‘fly away.’ Thus when tendencies to escape, submit, or reconcile are blocked, depression can become more intense. Many elements in modern human environments appear to artificially regulate flight and submissive behavior and increase chances of depression (Gilbert 2001). We will now examine the adaptive origins of various anxiety disorders..

Separation Anxiety and Panic Disorder

When the infant is separated from the mother in the wild, it can be in extreme danger; there are good evolved reasons for the infant’s responses, which initially consist of anxiety commonly coupled with angry protests. The arousal of the attachment system motivates it to seek reunion with the mother. Positive affect systems may simply be temporarily “offline” - ready to come on again after reunion with the mother. However, if no reunion occurs, the infant’s continued protests might attract the attention of predators and lead to exhaustion and getting lost. Clearly, simply accentuating anxiety in this situation would be counterproductive; – rather, the infant’s behavior must be slowed down. This happens via the toning down of positive affect systems so that the infant stops exploring and becomes relatively immobile and disengaged from the environment. Generally, both a high level of anxiety and a low level of positive affect are associated with immobility, staying put, and the successful avoidance of predators. Interestingly, depressive experiences commonly recreate this archetypal desire to hide.

Though it is customary to think of Separation Anxiety as promoting reunion between the young child and the mother, Separation Anxiety may be triggered by agonistic encounters with an attachment figure at all ages, when this engenders a fear of abandonment by that attachment figure. Its function is to promote reconciliation with that attachment figure.

Being accepted by, and receiving help from other members of the group is essential for survival and reproduction. As a result, humans have a built-in sensitivity to cues signaling abandonment and/or rejection. Insecure attachment is associated with a constant need for reassurance that one is loved by others. When failing to receive this reassurance, feelings of rejection are triggered, and one develops heightened rejection sensitivity (Sloman, 2000; Downey et al, 1998). The initial feeling is one of impotent rage, which triggers the IDS and may escalate into a clinical depression. There is a fear that anything he/she does will intensify the rejection experienced and thus enhance the other person’s power to reject.

Certain submissive behaviors have a role in ambivalent attachments from the preschool age onward (Crittenden, 1997) and avoidant attachment also has a subordinate or submissive quality (Hilburn-Cobb, 2004). If primary attachment strategies consistently fail to regulate internal states during stress, the child or adolescent may attempt to salvage attachment security through involuntary submission to the caregiver (Hilburn-Cobb, 2004). The individual seeks sympathy and supports by parading his/her helplessness letting, the subordination system take over from the ineffective attachment system. This strategy is often ineffective and may even backfire.

Having discussed how the IDS regulates competition, as well as what role it plays in clinical depression, we now move on to explore how anxiety can prevent and terminate agonistic encounters and how it manifests in a clinical setting.

Panic Disorder is associated with feelings of intense fear, a sense of immediate danger, and a strong desire to flee. While general anxiety evolved to deal with threats whose nature could not be clearly defined, subtypes of threat focused behaviors such as escape, freezing and submission/appeasement have evolved to deal with particular kinds of threats (Marks and Nesse, 1994), but an erroneous triggering of these alarm systems leads to psychopathology (Stein and Bouwer, 1997). When engaged in conflict with a conspecific and faced with an imminent prospect of defeat, the individual must cope with certain potential types of danger (defeat, injury, death), as well as his/her fear thereof. As a consequence, the prospect of imminent defeat may activate the IDS, whereas specific fears of loss, rejection, injury and death may activate the acute fear (fight/flight/freeze) response (Carver and White, 1994). The fight/flight/freeze response is activated at the outset of the encounter but as the danger of injury or death becomes more apparent, the individual may shift his/her dominant response from fight to flight. However, if flight is blocked, either the anxiety may escalate and manifest as panic, or the IDS may escalate and manifest as depression.

The individual’s vulnerability to developing a disorder, as well as the presence of general life stress plays a role in both panic and depressive disorders. An individual becomes increasingly vulnerable to anxiety and depressive disorders, when he/she perceives social defeats, losses, and the blocking of social goals to be an imminent and real danger. .

Reconciliation, or moving from an adversarial or competitive stance to an affiliative one, serves to assuage anxiety. Given the right circumstances, anxiety can be readily resolved. Much of our evolution occurred during a time when humans lived in small groups and many agonistic encounters took place between individuals and their respective attachment figures. When agonistic challenges occur within a family context, a major conflict might trigger separation anxiety, which, in turn, ultimately helps restore harmony and closeness between the individuals involved.

Social Anxiety and Generalized Anxiety Disorder (GAD)

Social Anxiety is often associated with certain submissive behaviors, such as intense self-monitoring, gaze aversion, and inhibited speech and confident displays. The physical symptoms, as well as the behaviors and cognitions associated with Social Anxiety Disorder, are triggered by the perception of real or imagined threat in a number of specific social situations (Gilbert, 2002). Human social anxiety is often centered on concern about what the individual him- or herself will do that might trigger a negative response from others. Feared responses of this sort may include being ignored, ridicule, rejection, or defeat in the pursuit of social goals. In addition for humans, the fear of receiving a friendly rejection from a sexual partner, or during a job interview can lead to intense anxiety. Indeed, most human social anxiety is related to a fear of being ridiculed and/or rejected by others, rather than a fear of being the target of aggression or the victim of injury.

Symptoms of Social Anxiety Disorder include fear of strangers, fear of being observed, as well as feelings of shame and humiliation. These symptoms reflect the individuals’ perceptions of their place in the social hierarchy. They try to avoid interacting with the socially dominant/confident and tend to interpret every interaction with others as being competitive (needing to impress) in nature. Afflicted individuals often have a pre-disposition to behavioral inhibition (shyness), which can become enhanced due to elevated levels of self-consciousness and the presence of beliefs that others will reject them because of their anxiety (Oakman et al, 2000). They constantly anticipate defeat in the competition for attractiveness and prestige, and are primed to respond in a submissive fashion. Because they engage in social interactions in an inhibited way, they receive feedback that they are “strange” or “uninteresting”, which further reinforces their beliefs that they are unlikely to make a positive impact in the social domain.

People with Social Anxiety Disorder can be described as being in a state of enhanced submissive behavior in the domains of their anxiety, or as having a low threshold for triggering the subordination strategy. One person may be socially anxious when public speaking while another in meeting new people, and another only in sexual encounters.

Generalized Anxiety Disorder is characterized by generalized, non-specific, and non-focused anxiety, which is essentially a state of increased threat sensitivity. The individual thus appears to be in a state of anxious awareness, scanning his/her surroundings, essentially trying to identify potential threats that may or may not actually exist (Szechtman et al, 1998). The resulting anxieties are therefore of a focused and anticipatory nature.

GAD is associated with high levels of uncertainty and doubt, which communicate to the opponent that there is no threat thus reducing the likelihood of conflict escalation. If the opponent does not relent, as for example a continually abusive parent, the child may in desperation seek to reduce his anxiety by becoming overly submissive and may develop Social Anxiety. Generalized Anxiety Disorder however operates over a number of social and nonsocial domains and suggests a more chronic generalized submissive strategy to social engagement focus. Furthermore, although different in many respects, Social Phobia and Depression have in common low positive affect, high negative social comparison, as well as a sense of inferiority, fear and shame in social situations (Gilbert

et al, 2002).

Dysregulation of negative and positive affect manifesting as clinical syndromes

The end of a conflict may, through reconciliation of the two parties, bring relief by decreasing the level of negative affect and/or by increasing the level of positive affect. When there is a failure of reconciliation, or when flight is not possible, the mechanisms associated with low positive affect or high negative affect may go into overdrive and become maladaptive by operating at a greater intensity and/or over a prolonged period of time. For this reason, individuals with Social Anxiety may continue to pursue reconciliation, even when it is inappropriate, people with Panic Disorder are intent on escape, even in the absence of obvious danger, and people with depression find it difficult to assert themselves with people of equal or higher status. On the other hand, the person facing inevitable defeat may remain in ‘fight mode’ because he/she feels a sense of (in)justice,or unfairness. After a conflict monkeys typically avoid each other (and have a home range in which to do that) or reconcile. However, in humans it may be impossible to get away from conflict with an abusive employer or spouse. High expressed emotions, associated with criticism and intrusiveness are closely associated with depression, anxiety and other disorders (Wearden et al, 2000). Sometimes a person’s wish to reconcile is refused; conversely, seeking reconciliation can be seen as a sign of weakness. ‘Stone walling’ in a conflict has been linked to depression and relationship break down (Gotman, 1994).

Conclusion

While depression is best characterized as a “state of low positive affect” and anxiety as a “state of high negative affect”, many patients exhibit mixed states of high negative and low positive affect with the former often preceding the latter. In fact, depression can often follow protracted unresolved anxiety (Wolpe, 1971; Bittner et al, 2004). There is good evidence that both a history of Panic attacks and Social Phobia can pre-date Depressive Disorders (Schneier, 1992). There is also evidence that symptoms of anxiety can wax and wane in conjunction with symptoms of depression, and it is well known that SSRIs, used in the treatment of depression, can also be useful for Anxiety Disorders. This evidence suggests that negative and positive affect systems have complex and important co-regulating influences on each other that may explain the high co morbidity of anxiety and depression.

Another major functional reason for toning down positive affect is to reduce explorative behavior and resource acquisition seeking. This occurs in the context of competing for resources. In territorial and group-living animals constant injuries or mortal battles are maladaptive and so there have evolved various strategies for regulating the degree of engagement in competition and maintaining group stability (Sloman, Gilbert & Hasey, 2003)

This paper explored some of the ways in which understanding the dynamics of social relationships - and in particular the threat posed by abandonment and competitive losses - can recruit a range of difficult emotions and moods. We propose that emotional disorders need to be contextualised within a social framework, where relationships can be sources of soothing and safeness as well as threat. Within this context we suggest that elevated anxiety is related to the anticipation of injuries, losses and defeats in competitive encounters, and blocks to achieving social goals (e.g., making friends or developing sexual relationships) which may escalate and manifest as GAD. In other words, anxiety is a preparatory emotion that occurs at the onset of threatening situations and dissipates when the threat is over. Depression may share this function, but also occurs in the wake of an event, when the person must reset their goal seeking/striving systems and avoid further risks and injuries.

When the individual is anxious but lacks a strategy to end the conflict and continues to be oppressed by his opponent, the anxiety may increase and manifest as GAD. When his/her efforts to submit are not accepted by the adversary, this may trigger both anxiety and the IDS, which subsequently manifests as Social Phobia. When the individual’s inability to escape leaves him/her feeling trapped, this may trigger the flight response, or manifest as Panic Disorder and, when an individual, in a confrontation, is threatened by how that confrontation may impact on the relationship, he/she may experience Separation Anxiety, which generates a need to reconcile with the opponent. In summary, the IDS and anxiety together terminate conflict by triggering submission, the attachment system, when aroused by anxiety, motivates one to seek out an attachment figure; and the panic system triggers flight. Therefore, although out of control negative and low positive affects have a negative impact, they can in moderation have a positive function by reducing conflict and promoting reconciliation when conflict has occurred.

We emphasize the similar but complementary roles of anxiety (high negative affect) and depression (low positive affect in resolving conflict. Both anxiety and depression act as appeasement displays which reassure the opponent that retaliation is not forthcoming, and they both lower the individual’s appetite for retaliation; in addition, depression, by lowering self-esteem and inducing negative self-perception, helps to reconcile the individual to reduced status or abandoned goals; while anxiety, by inducing a need for safety, promotes an affiliative approach to the former opponent and reconciliation with the opponent.

References

Barkow, J.H., 1989.: Darwin, sex and status: Biological approaches to mind and culture. Toronto, University of Toronto Press.

Bracha HS, Ralston TC, Matsukawa JM, Williams AE, Bracha AS (2004) Does "Fight or Flight" Need Updating? Psychosomatics, 45, 448-449.

Bittner, A., Goodwin, RD., Wittchen, H.U., Beesdo, K., Hofler, M., Lieb., R. 2004. What characteristics of primary anxiety disorder predict subsequent major depressive disorder? J. Clin. Psychiatry; 65: 618-626.

Carver, C.S., White, T.L., 1994. Behavioral inhibition, behavioral activation and affective responses to impending reward and punishment: The BIS/BAS scales. J. Pers. Soc. Psychol. 67: 319-333.

Crittenden, P., 1997. Attachment and sexual behavior, in Attachment and Psychopathology. In Atkinson, L., Zucker, K. (Eds.) New York, Guilford Press, pp 47-93.

Davidson, J., Zisook, S., Giller, E., Helms, M., !989. Symptoms of interpersonal sensitivity in depression. Compr Psychiatry. 30: 357-368

de Waal FBM, Aureli,F.1997.Conflict resolution and distress alleviation in monkeys and apes. Annals of the New York Academy of Sciences,807,317-328.

de Waal F.M.B.1989. Peacemaking Among Primates. New York, Penguin.

Dixon AK: Ethological strategies for defense in animals and humans: Their role in some psychiatric disorders. British Journal of Medical Psychology 1998; 71: 417-445

Downey G, Freitas AL, Michaelis B, Khori H: The self-fulfilling prophecy in close relationships: Rejection sensitivity and rejection by romantic partners. J Pers Soc Psychol; 1998: 75, 545-560.

Field, T.M., 2000. Infant massage therapy, in Handbook of Mental Health. Zeanah C.H.J. (Ed.) New York, Guilford Press, pp 494-500

Fredickson, B.L., 2000. Cultivating positive emotions to optimize health and wellbeing.

Prevention & Treatment 3: Article 0001a

Gilbert, P., 1992. The evolution of powerlessness. New York, London, Guilford Press, p 237.

Gilbert, P., 2001. Evolutionary approaches to psychopathology: the role of natural defenses. Aust. N Z. J. Psychiatry 35: 17-27.

Gilbert, P., 2002. Evolutionary approaches to psychopathology and psychotherapy In, Cognitive Psychotherapy: an international quarterly, (Special Edition Evolutionary psychology and psychotherapy) Gilbert, P., (Ed.) 16: 263-294

Gilbert, P., Allan, S., Brough, S., Melley, S., Miles, J.N.V., 2002. Relationship of anhedonia and anxiety to social rank, defeat and entrapment. J. Affect. Disord. 71:141-151.

Gilbert, P., 2004. Depression: A biopsychosocial, integrative, and evolutionary approach, in A Handbook of Science and Practice. Edited by Power M. John Wiley & Sons, p.56. .

Goodall, J., 1990. Through a window: My thirty years with the chimpanzees of Gombe. Boston, MA, Houghton Mifflin Publishing.

Gottman, J., 1994. Why marriages succeed or fail and how you can make yours last. New York, Simon & Schuster.

Hilburn-Cobb, C., 2004. Adolescent psychopathology in terms of multiple behavioral systems: the role of the attachment and controlling strategies and frankly disorganized behavior, in Attachment Issues in Psychopathology and Intervention. Atkinson, L., Goldberg, S., (Eds.) 2004.. Mahwah, NJ, Lawrence Erlbaum Associates, p. 117

Hofer, M.A., 1995. An evolutionary perspective on anxiety, in Anxiety as symptom and signal. Roose, S.P., Glick, R.A., Hillsdale, NJ, Analytic Press.

LeDoux, J., 1996. The Emotional Brain. New York, Simon & Schuster.

Marks, I.M., Nesse, R.M., 1994. Fear and fitness: An evolutionary theory of anxiety disorders. Ethology & Sociobiology 15: 247-261

Marks, I,M., 1987. Fears, Phobias and Rituals: Panic, Anxiety and their Disorders. Oxford, UK, Oxford University Press.

Nesse, R.M., 2000. Is depression an adaptation? Arch. Gen. Psychiatry. 57: 14-20

Nesse, R.N., Williams, G.C., 1995. Why we get sick: The new science of Darwinian Medicine. New York, NY, Random House.

Nesse, R.M., 1999. Proximate and evolutionary studies of anxiety, stress and depression: synergy at the interface. Neuroscience and Biobehavioral Reviews. 23: 899

Nesse, R., 1991. What good is feeling bad? The evolutionary benefits of psychic pain. The Sciences. 30-37

Oakman, J.M., Farvolden, P., Van Ameringen, M., Mancini, C., 2000. Challenges in the treatment of shyness and social phobia, in Shyness: Development, consolidation and change. Crozier, W.R., (Ed.) New York, Routledge. pp 207-226

Panksepp,J., 1998. Affective Neuroscience: The foundations of human and animal emotions. New York, Oxford University Press, pp 212.

Price, J.S., 1967. The dominance hierarchy and the evolution of mental illness. Lancet 7502, 243-246.

Price, J.S., 2003. Evolutionary aspects of anxiety disorders. Dialogues in Clinical Neuroscience 5: 129-137.

Price, J.S., Gardner, R., Erickson, M., 2004. Can depression, anxiety and somatization be understood as appeasement displays? J. Affect. Disord. 79, 1-11.

Price, J.S., Sloman, L., 1987. Depression as yielding behavior: An animal model based on Schjelderup-Ebbe’s pecking order. Ethology and Sociobiology 8, 85s-98s.

Schneier, F.R., Johnson, J., Hornig, C.D., Liebowitz, M.R., Weissman, M.M., 1992. Social phobia: Comorbidity and morbidity in an epidemiologic sample. Arch. Gen. Psychiatry, 49, 282-288.

Shankman, S.A., Klein, D.N., 2003. The relation between depression and anxiety: An evaluation of the tripartite, approach-withdrawal and valence-arousal models. Clin. Psychol. Rev. 23, 605-637

Sloman, L., Gilbert, P., Hasey, G., 2003. The relevance of hierarchies, territories, defeat for depression in humans: Hypotheses and clinical predictions. J. Aff. Dis. 74,107-121

Sloman, L., 2000. How the involuntary defeat strategy relates to depression, in An Evolutionary Approach to Mood Disorders and their Therapy. Sloman, L., Gilbert, P., (Eds.) Mahwah, NJ, Lawrence Erlbaum Associates, pp 219-214.

Sloman, L., Gilbert, P., Hasey, G., 2003. Evolved mechanisms in depression: the role and interaction of attachment and social rank in depression. J. Affect. Disord. 74, 107-121

Sloman, L., 2000. The syndrome of rejection sensitivity: an evolutionary perspective, in Genes on the couch: Explorations in Evolutionary Psychotherapy. Gilbert, P., Bailey, K.G., (Eds.) Hove, Brunner-Routledge. pp 257-275.

Stein, B.J., Bouwer, C., !997. A neuro-evolutionary approach to the anxiety disorders. J. Anxiety Disord.11, 409-429

Szechtman, H., Woody, E., Bowers, K.S., Nahmias, C., 1998. Where the imaginal appears real: a positron emission tomography study of auditory hallucinations. Proc. Natl. Acad. Sci. U S A 95, 1956-1960

Watson, D., Weber, K., Assenheimer, J.S., Clark, L.A., Strauss, M.E., McCormick, R.A., 1995. Testing a tripartite model: 1. Evaluating the convergent and discriminant validity of anxiety and depression symptom scales. J.Abnorm. Psychol. 104, 3-14

Wearden, A.J., Tarrier, N., Barrowclough, C., Zastowny, T.R., Rahil, A.A., 2000. A review of expressed emotion research in health care. Clin. Psychol. Rev. 20, 633-666

Wells, A., 2002. Emotional Disorders and Metacognition: Innovative Cognitive Therapy. Chichester, UK, John Wiley & Sons.

Wolpe, J., 1971. Neurotic depression: Experimental analogue, clinical syndromes and treatment. Am. J. Psychother. 25, 32-368

Woody, S, Rachman, S., 1994. Generalized anxiety disorder (GAD) as an unsuccessful search for safety. Clin. Psychol. Rev. 14, 743-753