The genealogy of mankind displays a remarkable diversity. It is striking that wherever Homo sapiens appeared, other species disappeared. Were our ancestors so much fitter than for example Neanderthals, who were equipped with larger brains, or were they simply ruthless hunters, who were able to overcome constraints to kill their own kind more easily? We know only little about the encounters between groups in prehistoric hunter-gatherer communities. Surveying members of tribal cultures for example in the Amazon region, who had not come into contact with other societies at the time of investigation, suggests that there have always been cultures in which a considerable number of human beings have died primarily at the hands of other men (Chagnon, 1968). Discoveries that date back to the Pleistocene/early Holocene, have confirmed the possibility that our ancestors back in the Stone Age had a violent nature: Lahr and colleagues (2016) reported that ten out of twelve skeletons found at the bank of Lake Turkana in Kenya, showed signs of a violent death at the hands of other humans – thus people were massacring each other as early as the New Stone Age! It is not unreasonable to posit that such aggressive behaviour is governed by phylogenetic elements. Gómez et al. (2016) investigated the percentage of death caused by members of the same species in various mammals and concluded that phylogenetically the percentage of human deaths through interpersonal violence is 2%, a rate, which can also be observed in apes. But of course, the extent of lethal violence dramatically varies throughout the course of history, depending on the various socio-politic organisations of different societies. We nevertheless assert that humans can be put into a mode in which they can and do kill others; and this not strictly in self-defence or to obtain material gain, but rather for the pleasure of the act itself.
Aggression exists in two worlds of feeling
Two essentially different forms of aggression can be described (e.g. Fontaine, 2007; McEllistrem, 2004): Reactive aggression functions to resist a threat. This leads to high levels of arousal with negative affective valence, which is determined by emotions like fear, anger, rage or hostility. Once the threat is overcome, relief is increasingly felt and the physiological arousal diminishes (Fig. 1, left part). A different type of aggression is the one of a ‘predator’. If executed it is often pro-active and intended. It seeks reward, which could be material (prey) or social, in the form of maintaining a reputation or establishing dominance (power). Perhaps anticipation of the expected prey can explain part of the motivation, so that besides fear of negative consequences of the fight there are also positive feelings – but in many cases this is not enough to explain the actual behaviour. Where, for example is the material gain of the first-person shooter game in comparison to more social computer games without any bloodshed? Alternately let us look at hunters in the Stone Age with their often arduous and exhausting endeavours to track an animal, hunt it down, and then finally execute it: The reward is too far away to motivate the hunting behaviour. Rather, the hunt itself must motivate and activate the reward system; deprivation and pain must be met with release of endorphins (Fig. 1, right side). When a cat chases a ball of wool, a football team runs after the ball, or a hunter goes deerstalking, then it is not actually about the wool, the ball or the prey but about the genuine enjoyment of pursuit; in some contexts this becomes the lust for fighting and hunting. However, the relevant culture and society sets the moral boundaries for that lust. Hooligans exceed the threshold of our societally accepted boundaries, although they obey their own set of rules (no tools, no weapons, once someone is on the ground that person is off limits). “Now, the passion to fight for your club and the adrenaline rush that comes with it was unbelievable. But I don’t think that this addiction can be understood …” football hooligan Brown reports in his book Villians (Brown and Brittle, 2006). In his novel Hool Philip Winkler describes this special feeling of anticipation, which sets in prior to the exchange of blows: “as something would start to float in your stomach” (Winkler, 2016). Philosophers, natural scientists and psychologists traditionally characterise the practise of “violence only for the sake of lust” as psychopathological. Someone who is unable or unwilling to comply with this moral taboo is “sick” or “radicalised”. This propensity towards violence is seen as a base instinct, and shrouded in taboo. Although legally judged as particularly heinous, a latent desire is still there in the population and can be easily sparked with the right social and psychological circumstances . Data from diverse contexts show that lust to attack, fight and hunt, including extreme states of “bloodlust”, and “Combat High”, can be potentially awoken in nearly all people, at least when male. We describe this as ‘appetitive aggression’ (Elbert et al., 2010; Weierstall and Elbert, 2011; Elbert et al., 2017), a concept that can be developed from our studies of fighting in conflict regions, from surveys of gang members in townships and from child soldiers and street children. Fighters in every scenario report that after a phase of habituation and transition, in which fighting and killing is perceived as horrendous and accompanied with physical discomfort, repeated acts of violence are felt with an increasingly positive affect: “It has become an urge for me to throw bombs. It is arousing, it is a fine feeling. It is just as nice as shooting someone” (a soldier of the armed forces reports, cited after Neitzel and Welzer, 2011 [our translation]). Such statements are found with a high frequency in all war scenarios, such as in investigations into the genocide in Rwanda: “The more we killed, the more motivated we were to keep doing it. When you can act on your desires without consequence, then it never lets you go. You could recognize us by our dead eyes. … It was an unexpected collective pleasure.” (from Time of the machetes, conversations with perpetrators of the genocide in Rwanda by Hatzfeld, 2005). During our representative interviews with fighters in East Congo, in North Uganda or in Columbia we keep hearing statements like the following: “We were sitting together, my uncle and me, we were talking about our glorious fights and then the need for fighting, the urge came up in us. It could be even at 7 o’clock at night, when it was already dark, that we took the guns then and went to kill.” (Interview by Schauer; from Elbert et al., 2013). Reports of positive affect during hunting, fighting and killing, as well as during preparation for these acts can be found outside of the context of war. For example, appetitive aggression was ubiquitous in our clinical interviews with gang-affiliated youths in the townships of South Africa (Weierstall et al., 2013). The prevalence of appetitive aggression is not limited to a small fraction of persons, in fact the majority of boys join the gangs. Of course, in many cases this can be seen as a situational necessity, but the young men also speak of the thrill of the joint attack and the lust to chase the other gangs.
We argue that capability to hunt in a group and also on your own, that is, the predisposition for appetitive aggression, is biologically determined. It is an intrinsic part of the human behavioural repertoire, and prepared attacks and killings can emerge under certain circumstances, driving the behaviour of whole ethnic groups. Yet, the capability for moral behaviour, for regulation of our reward systems is also genetically determined. The developing child wants to learn moral rules, wants to know what constitutes “fair-play” and what is not permitted. While appetitive aggression is a biologically determined behaviour, regulated by moral constraints, by customs and laws, this control is a lot more complex than the classic Hobbesian opposition of the individual and society: the evolutionarily developed capability for morality and social behaviour manifests itself by early childhood (Tomasello and Vaish, 2013). Thus, the challenge of resolving the conflict between the ambition for cooperative pro-social behaviour and the urge for appetitive aggression begins early in life for both the individual and society.
Every society uses acquired norms to control sexual behaviour, drug consumption and aggression, behaviour of three central areas, activating our reward system. In times of peace, appetitive aggressive behaviour is restricted to athletic competition or entertainment. Even violent computer games become more attractive when they are morally acceptable, when the player has legitimate reason to kill: zombies or a threat to the virtual world obviously have to be fought against, you are even allowed to enjoy this. People not only react in self-defence and anger to threat and attack, but, under certain circumstances (youth, legitimation, group consensus, reward, etc.), can overcome the restraint against acts of violence and gruesome humiliation of another human being. Studies on the largest indigenous tribe in the Amazon region, the Yanomami, whose social norms do not forbid the killing of members of other tribes, reveal that 4 out of 10 men over 25 years had participated in killings and that approximately every third man died a violent death (Chagnon, 1968). Even though one has to bear in mind an implicit colonial bias in Chagnon’s interpretations, it and many other reports across cultures and historical epochs vividly demonstrates for all cultures and peoples, the truth of the following words of Einstein, written to Freud in 1920: “The lust to kill is in our blood. War wipes away the later acquired cultural norms …” (Einstein and Freud, 2005).
Neurobiology of appetitive aggression
As in all types of behaviour, a variety of coordinated brain structures underlie the activation of aggression. These range from the lowest vegetative centres, which for example govern muscular and respiratory functions, to higher functions, such as the assessment of a social threat. There are well established and validated animal models for the area of reactive aggression (Blair, 2004; Panksepp, 1998; Weiger and Bear, 1988). One of the relevant structures is the hypothalamus, which prepares the body for stress reactions, together with the periaqueductal grey, located in the tegmentum. This core complex also modulates the endophinergic (opioid-analogue), efferent pain suppression – an important regulatory mechanism during fighting. It sends efferents to the serotonergic Raphé nuclei and coordinates together with the amygdala fear and flight reflexes (Gregg and Siegel, 2001). The latter probably receives relevant sensory information in part directly from the thalamus in order to react as quickly as possible (LeDoux, 1996). Reactive aggression is part of the defence cascade, a biologically prepared behavioural repertoire with stereotyped behavioural, affective and cognitive reactions to threat. If the stimulus seems too menacing, too sudden, too proximate, the sympathetically dominated fight-and-flight-reaction switches to a parasympathetically dominated tonic immobility with vasovagal syncope, which can progress as far as fainting in order to minimise injury (Schauer and Elbert, 2010). During the transition, phase tonic immobility allows a return to alarm reaction and flight, albeit with a high dual autonomic tone, which includes the risk of cardiac failure.
Animal studies demonstrate the difference between defensive, reactive aggression on the one hand and hunting behaviour on the other hand. Every cat owner can tell the difference between the two from their own pet (Fig. 2). The behaviour can also be triggered by stimulation of different areas in the hypothalamus: the lateral hypothalamus potentiates hunting and appetitive aggression, whereas stimulation of the medial area results in defensive behaviour with reactive aggression. The medial and lateral nuclei of the hypothalamus play an analogue role in other mammals including rats, mice, hamster and primates (summary Haller, 2013).
Hunting behaviour has probably evolved along various evolutionary paths, as suggested by Gómez et al. (2016). This is also the reason why animal models, representing neuronal regulation of aggression, only hold limited transferability to humans, especially when intraspecific aggression is involved, which differs between mammal species in regard to its phylogenetic development (Gómez et al, 2016). We do not know whether humans have an analogous hypothalamic shift mechanism between reactive and appetitive aggression (Fig. 2). Hypothalamic peculiarities, which have been reported in clinically aggressive persons (Koch et al., 2007; Kuhlmann et al., 2013) more likely represent brain anomalies.
Reactive and appetitive aggression in humans is certainly also regulated by a series of higher, flexible functions including cortical regions (Moran et al., 2014). Our studies imply that these two types of aggression are regulated by two distinct networks of connectivity (Fig. 3).
We postulate that dopaminergic pathways are activated for appetitive but not for reactive aggression, similar to activation by sexual stimuli or drug intake. These pathways run from the tegmentum to limbic structures (Nc. accumbens) and frontal cortical areas and thereby signify reward expectation. Accordingly, there must be some form of inhibition, restricting appetitive reactions. Based on studies of reactive aggression, we know that in this context, the ventromedial/orbitofrontal prefrontal cortex as well as the anterior cingulate gyrus (ACC), can inhibit the amygdala and the hypothalamus (e.g. Blair, 2013). The latter in turn affects the tegmentum, in particular the periaqueductal grey. An inhibitory effect of the prefrontal cortex is also likely in appetitive aggression and is supported by our data (Fig. 3). Further, temporoparietal regions are activated, mainly in the right hemisphere, when appetitive aggression is evoked. Overall one can relate appetitive aggression to the systems of reward expecting approaches and reactive aggression to defensive withdrawal and avoidance behaviour. Ultimately, all neuronal structures contribute to those two motivational players, ensuring survival and fitness.
Genetics of appetitive aggressive behaviour
The biological basis of both forms of aggressive responsiveness exists in all humans but is differentially pronounced by genetic variants interlinked with relevant environmental factors as was shown in twin and family studies. As already discussed, regulation of monoaminergic neurotransmitters plays an important role and in fact, polymorphisms of the serotonin transporter (5-HTT) have been linked to aggression, however so far without considering the dichotomy of affect in aggression. In a peaceful environment however, a certain genotype will not differ from one with an aggression-prone predisposition. It is not until placed in an environment that promotes aggressive behaviour or even demands it, that the genotype imfluences phenotypical appearance. For example, it is known that children with differential gene expression of MAO metabolising enzymes show altered control (connectivity) between prefrontal areas and the amygdala. They respond in a more sensitive way to social threats. However, impaired function can only be observed if these children are exposed to disturbing experiences (Buckholtz and Meyer-Lindenberg, 2008; Caspi et al., 2002; for correlation with alcohol consumption see also Heinz et al., 2011). Thus, it is necessary to study development of aggressive behaviour in a context where experiencing violence is a daily struggle. Together with South African colleagues, led by Soraya Seedat, we investigated 295 gang members in the townships of South Africa (Xulu et al., 2015). There, being member of a gang serves as kind of a life insurance, so even those who initially show little enthusiasm have to join the gangs. We divided the persons according to their score on the appetitive aggression scale (Weierstall and Ebert, 2011). The 5-HTT genetic variants in the promotor region (5-HTTLPR) and in intron 2 (STin2) were genotyped and analysed for genetic associations. While appetitive aggression did not show any correlation with the 5-HTTLPR polymorphism, the STin2 variants revealed significant associations: the 10-repeat allele of Stin2 was only detected in persons with high appetitive aggression (Xulu et al., 2015). These findings need to be replicated, but they also suggest that reactive and appetitive readiness for aggression is promoted by genetic variants, which validates their conceptual difference.
Empathy and psychopathy
We have heard testimony of horrendous atrocities in our studies in war regions in e.g. Uganda, Afghanistan, press reports are filled with similar crimes, at the time of writing, those of the IS stand out as particularly gruesome. Across different cultures, humans are tortured, ears and noses are cut off, and genitals are mutilated. We ask ourselves how can humans be so barbarous – do they not have any mercy for their victims? There is an intuitive connection between empathy – meaning compassion and understanding of another’s perspective – and violence. On the one hand, torturing and killing other people requires one to overcome moral thresholds. On the other, the perpetrator cannot put himself in the position of his victim. It is a commonplace belief that psychopaths are unable to feel empathy and therefore can easily kill someone without any emotions involved. However, based on current data, these assumptions cannot be supported. Firstly, the act of killing is never without emotion – it is always affect-laden. But, valence can also be positive (appetitive). Secondly, the statistical correlation between capability for empathy and violence is weak (Vachon et al., 2014). In fact, capability for violence implies the specific regulation of empathy. A typical example is the dehumanisation of the enemy as ‘monkey’ or ‘rat’ (Grossman, 1996; Staub, 2006). During the genocide in Rwanda “only” 1 million ‘cockroaches’, or ‘vermin’ were killed, the German battles at the Eastern front during the second world war were fought against ‘Untermenschen – subhuman beings’. This simplifies warmongering because not only does empathy disappear but it also becomes morally unobjectionable, even necessary to wipe out the vermin, the terrorists, the non-believers.
Between the dehumanisation on the one hand and the biological predisposition for appetitive aggression on the other, it is not only the capability to view the enemy without empathy that emerges, but also the capability to passionately kill him. This attribute is dismissively viewed as abnormal and pathological; only present within sick individuals or psychopaths (Hare and Neumann, 2008). As a matter of fact, altered structures of the reward system in the brain (Buckholtz et al., 2010; Carre et al., 2013) as well as changes in areas activated during empathy (Blair, 2005, 2013; Marsh et al., 2013), imply abnormality. Yet, those changes are not necessarily congenital. A child, who grows up in a violent environment will develop different moral thresholds and at the same time will learn to ignore his empathy. Acquisition of this skill will change brain structure and organisation in the same way other learning processes do (Elbert and Rockstroh, 2004; Elbert et al., 2006). In fact, criminal persons in peaceful environments report a considerable degree of earlier experienced violence when questioned with a check list. Our studies on Burundian ex-combatants revealed that appetitive aggression and therefore lust for violent acts sharply increases in boys who experienced harsh discipline and abuse (e.g. Augsburger et al., 2017). This is not the case for female combatants, who often reported sexual violence (Augsburger et al., 2017). However, combat experience results in a massive increase in appetitive aggression, which reaches parity in male and female combatants (Meyer-Parlapanis et al., 2015). In peaceful societies, women only show one tenth of this value, men only one fourth. These differences between civilian men and women could explain why it is primarily the men who turn into hooligans, who stage fights on the financial market, or who beat each other bloody in martial arts bouts.
“For a real man fighting means everything. When I hear the sound of bullets I wish I was among the combatants. There is this thirst to fight in me. It is like one person’s thirst for a coke. It is only when you drink the coke that your thirst is quenched.” (Ex-combatant, interviewed by Dr. Tobias Hecker in Elbert et al. 2013).
Dr. Thomas Elbert, Dr. James Moran, and Dr. Maggie Schauer (Clinical psychology, University Konstanz and in vivo international) work together with survivors of organized violence in Germany as well as with ex-combatants in crisis and war regions in the field of trauma and aggression. Their research and aid measures are supported by the European Refugee Fund, the European Asylum, Migration and Integration Fund (AMIF), the UN fund for Torture Survivors, the German Research Foundation, the European Research Council (ERC advanced grant) and the International Bank for Reconstruction and Development/The World Bank.
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About the article
Thomas Elbert studied psychology, mathematics and physics at the Universities of Munich and Tübingen. He received his PhD in Tübingen in 1978 and taught there until 1989 with intermittent visiting professorships at Pennsylvania State University and Stanford University. He then became head of a clinical research group in the field of neuroscience at the Medical Faculty of University of Münster. Since 1995 he has been Professor of Clinical Psychology and Behavioural Neuroscience at the University of Konstanz. Together with Prof. Dr. Frank Neuner and Dr. Maggie Schauer he developed the narrative exposure therapy (NET) in order to treat traumatic stress symptoms; NET has also been successfully tested in field studies in crisis regions in Africa and Asia. His studies on the “psychobiology of human readiness for violence and killing” have been funded since 2010 by the German research foundation (Deutsche Forschungsgemeinschaft, DFG) as a Reinhart Koselleck project.
Professor Elbert is Hector fellow, member of the German academy of sciences Leopoldina, the Berlin-Brandenburg Academy of Sciences as well as OCSE Professor in the National Italian Commission for Scientific Qualification and holds honorary professorships at the Université Lumiére in Burundi and the Mbarara University of Science and Technology in Uganda.
James K. Moran
Dr. James K. Moran studied Psychology and Philosophy at Macquarie University in Sydney, Australia, completed his Master’s Degree as a clinical psychologist at the University of Konstanz, and subsequently completed a PhD on the neuropsychological and physiological consequences of traumatic experiences as well as aggression and violence, focusing on members of armed groups. Dr. Moran has investigated these questions both via neuropsychological imaging techniques in laboratory experiments, as well as in the field in war and crisis regions in East Africa.
Dr. Maggie Schauer is a clinical psychologist specializing in trauma-related disorders. She is the director of the Centre of Excellence for Psychotraumatology at the University of Konstanz. Her research projects are focused on multiple and complex traumatization as well as the transgenerational consequences of violence and neglect. Dr. Schauer works with child and adult survivors of organized and domestic violence. In collaboration with Professors Frank Neuner and Thomas Elbert, she developed Narrative Exposure Therapy (NET), an intervention for the treatment of trauma-related disorders after multiple and complex trauma. Dr. Schauer coordinates Therapy and Social Interventions in war and crisis regions, in refugee camps after humanitarian crises and natural disasters, in demobilization projects for child soldiers. She is also active in treating survivors of torture and human rights violations both in their land of origins, and in Germany with refugees and asylum seekers. She is a founding member of vivo international (www.vivo.org), an NGO for the prevention and treatment of traumatic stress. She is a founding member and advisor of the Babyforum, a network of experts specialized in the care of pregnant women, child welfare, and early intervention.
Published Online: 2017-05-16
Published in Print: 2017-05-24