Human aggression is the display of behaviors aimed at harming or destroying the target. The study of aggression and its causes is a complex one as it implies a broad array of triggers, causes, stressors and connections among them and requires an interdisciplinary approach. In analyzing the biological causes of aggression there has been in the past diffidence due to the risk that its simple concept may imply determinism, thus limiting the freedom of the human being. Nonetheless the research has demonstrated that biology influences behavior and that genetics correlates with aggression.
After reviewing the definition of aggression, I will review the biological causes (nature) and the social/relational causes (nurture) and I will attempt to clarify if currently the research has come to a clear answer on what factors determine aggressive behavior.
Definition and description of aggression
Aggression in humans is a hostile or destructive behavior. It comprehends all behaviors aimed at attacking someone either verbally or physically, with the intent to harm the target.
If we look at the animal world, we see that aggression not only is present in all species, but actually it provides an evolutionary advantage: it protects the individual, or at least the continuity of the specie. Animals differ among them in the patterns of behaviors displayed in an attack. Often the aggression is limited to a threat, leading to establish a hierarchy in the animal group, without harming the other individual (Carlson, 2010). This regulation of the aggressive behavior (from actual attack to threat) seems to be an evolutionary advantage of the specie, as it prevents the physical elimination of members of the group. Studies of the neurophysiology of animals show that when aggression is directed towards members of the same species, the aggressive individual displays a high level of arousal and activity of the sympathetic nervous system. This does not happen when an animal is engaged in attacking a member of different species to provide food for self or members of its group. We can say that in this predatory activity, there is no emotion like anger towards the prey, but rather a very “cold” pursue of a goal (Carlson, 2010).
These observations in animals reflect the classification of aggression in humans often found in literature of aggression. Aggressive behaviors are often classified into two categories: impulsive (or affectively evoked, or reactive) or predatory (or premeditated, or proactive, or instrumental). Some authors also make a separate category for the defensive aggression (Coccaro & Siever, 2002): the aggressive behavior is in response to a real threat and its aim is to deduce the threat. If this distinction would be intuitively understood in animals, I consider it misleading when it comes to humans, as it is difficult to distinguish between real and perceived threat, as it occurs for example in a paranoid condition: what can be perceived externally as an unjustified attack, in the mind of the aggressor it is simply an act of defense from a perceived danger. So the distinction between defensive and attacking aggression seems to be simply a moral construct, functional to the decision whether to intervene or not. In other words, whether the displayed aggression is socially acceptable or not. From a psychopathology perspective, aggression is not pathology in itself, but rather a component of several mental disorders, as described in the DSM IV-TR.
A large amount of literature deals with aggressive and violent behavior altogether. We need to clarify that violence is not a synonym of aggression, but rather one of its manifestations: violence is an aggression carried out to physically harm or eliminate the target of the action. A large amount of research in the field of aggression has been conducted right in its extreme form of violence, as the severity of the act in itself offers cases in which the causes behind the behavior are more evident than in situations of less extreme aggression, so that they can be better identified. In this way, studies of criminals and of their cases allowed to better describe reactive and predatory violence: reactive violence implies intense stimulation of the sympathetic nervous system, strong emotions and immediate reaction to the trigger, with the purpose of reducing the threat that the trigger of the aggression is perceived to pose. Reactive violence is usually displaced rapidly after a display of behaviors that announce the imminent violence. On the contrary, predatory violence is not accompanied by sympathetic arousal or perceived emotions and seems to be aimed at a multiplicity of goals, and absence of a perceived threat. It is usually planned in advance and accompanied by rituals, mostly of symbolic meaning, performed privately (Meloy, 2006). Studies of forensic populations indicated that individuals who have engaged in predatory violence are most often diagnosed as psychopath with the use of the Psychopathy Checklist-Revised (PCL-R), while the features of affective violence are among the diagnostic criteria for antisocial and borderline personality disorders, and intermittent explosive disorder (Kokler, Nelson, Meloy & Sandford, 2006).
One additional key difference between predatory and affective violence is that in the former case, the perpetrator displays an increased and focused attention, which allows him to plan and execute all the necessary actions to reach his goal. Criminals who have perpetrated violent acts of affective type seem to have acted on the basis of cognitive dysfunction: the executive functioning seems to be impaired and the subject’s actions are mostly controlled by the activated sympathetic nervous system (Kokler, Nelson, Meloy & Sandford, 2006).
The description of the features of impulsive or predatory aggression offers a very valuable framework to study the dynamics of aggression and its possible treatment or prevention. However, it does not clarify why some individuals engage in forms of aggression, more that other individuals. The analysis of the causes therefore should be directed towards the identification of the factors that makes higher the threshold to aggressive behavior. If we reason according to the frustration-aggression model described below, this would mean to understand and address those factors that make one individual more resilient to frustration compared to another one who instead reacts with aggression. I will review the causes found in literature.
Organic causes (Nature)
Genetic correlation continues to be debated and the findings are still inconclusive. The main debate on the research on the implications of the XYY and XXY chromosome has been focusing of the biases of the initial analysis, as these were made on convicted population and not on the general population. A longitudinal study published in 1999 (Götz, Johnstone & Ratcliffe, 1999) observed that the higher rate of antisocial behavior and conviction of the group with the XYY chromosome was to be attributed more to lowered intelligence than to increased aggression. It is also noticeable that the offences were mainly against property than against people. The finding that the group with the XXY chromosome did not show increased rate of conviction is in accordance with the 36-years long Denver Study (Bender, Linden, & Harmon, 2001), which found lower level of cognitive ability, but overall good social adaptation in the XXY group compared to the control.
Another stream of research to evaluate the genetic influence in aggressive behavior is based on observation of twins, brothers and adoptions. Identical twins appear to be more concordant than non-identical twins for criminality, indicating that some factors leading to criminal behavior are transmitted genetically (Raine, 2002). Also several studies with adoptive children show a genetic correlation with criminality, although some studies, which also analyzed the type of offence, indicated that the increased rate of criminality in adopted children with criminal biological parents was due to petty property crime and not violence (Raine, 2002).
Physiological correlations with aggressive behavior include influence of serotonin, hormones, brain functioning, EEG and Cardiovascular underarousal.
Evidence indicates that serotonin inhibits human aggression. This evidence has been supported by both studies that measured the level of serotoninergic activity and the displayed aggressive behavior, as well as in studies that analyzed the decrease of the aggressive behavior with the use of serotonin agonist drugs (Carlson, 2010).
Evidence that testosterone increases aggression are clear, so that is one of the factors to consider, but its weight in the regulation of aggression is still not defined (Raine, 2002).
A critical role in the inhibition of aggressive behavior is played by the ventromedial prefrontal cortex of the brain. Like in many cases of study of brain functioning, also for the correlation between this portion of the brain and aggression, strong indication is provided by cases of brain lesions: patients with a lesion of the prefrontal cortex seem to have behavioral and decision making difficulties, mainly due to the lack of regulation of emotions that are generated in the amigdala (Carlson, 2010). Functional imaging studies of the brain showed that individuals convicted for murder had a significantly poorer activity of the prefrontal cortex, compared with control group (Raine, 2002). This finding reinforces the ipothesis that the prefrontal cortex plays a major role in control and inhibition of all those behaviors that depend on emotions, like aggression, problem solving, risk taking, all characteristic involved in antisocial disorder. This means that the factors that cause an impairment of the function of this part of the brain are likely to have a correlation with increased aggressive behavior. This has been found true for birth complications, especially specific complications like anoxia and forceps delivery, although never as a unique determinant, but combined with maternal care factors (Liu, 2004).
Another part of the brain that has showed a significantly poorer function in murderers is the left angular gyrus: lower activity of this portion of the brain is correlated to cognitive deficiencies like language or mathematical skills (Raine, 2002). It is likely then that its poorer activity of the left angular gyrus in criminals is not directly related to increased aggression, but rather has led to more difficult living conditions through difficulties in school and employment, thus preparing the conditions for being involved in crime.
A triad of physiological conditions has also been correlated with crime in a prospective study: low heart rate at rest, high presence of slow waves in the EEG, and low skin conductance were -together- correlated to crime in the following ten years (Raine, 2002). As these physiological conditions are observed in condition of under-arousal, the author suggests that a theory of under-arousal in aggression could be explored, as this condition has been related to fearlessness and thrill-seeking, both components of anti-social behavior.
The impact on aggressive behavior of the experiences through which the individual goes in its life, and especially during infancy, childhood and adolescence is described in literature through several models or perspectives. Each one seems to capture one or some aspects of this complex topic of how aggression is learnt, developed or controlled and inhibited.
Social Learning theory (cognitive-behavioral perspective): describes learning as a consequence of imitation, and observational learning. The role of imitation was confirmed in the sequence of experiments that Bandura (Bandura, 1973, quoted in Liu, 2004) conducted to evaluate if children would imitate the aggressive behavior that they just observed performed by others. Observational learning adds to this simple imitation mechanism the observation of the positive consequences of the aggressive behavior (Liu, 2004), leading to a reinforcement of the appropriateness of the aggressive behavior to pursue own goals. This kind of observational learning has been called to describe the negative effects on children of violence in TV or videogames.
Frustration-Aggression Model: describes aggression as a consequence of experience frustration in non-achieving a desired goal. As this theory implies experiencing emotions like frustration and anger, it addresses affective aggression, but does not explain the predatory aggression (Berkowitz, 1989).
Family violence and abuse are conditions in which aggressive behavior can be learned: the child imitates the responses that violent parents are usually demonstrating, while an abused child can as an adult repeat the same abuse on his or her own child, as the abuse cycle theory describes. However, not all children coming from violent or abusive families become criminals as adults, suggesting that many components build the picture of cognitive and emotional development of individuals.
Aggression is intrinsic in animals and in human beings and cannot be eliminated. The brain structures that appear to be directly involved in its regulation belong to a newer -from an evolutionary point of view- part of the brain, the prefrontal cortex. We can thus suggest that the evolution of which the human is the result has considered an advantage to the specie a highly sophisticated mechanism of inhibition of aggressive behaviors.
In reviewing the literature on the mechanisms and causes of aggression a number of factors make more difficult the evaluation of the relative impact of nature or nurture on aggressive behavior (retrospective studies on selected populations, overlapping of definitions that make difficult the generalization of specific cases, small size of the populations in the studies). Nonetheless, a vast amount of literature suggests that aggression is influenced by biological factors and has at least some genetic components. Genetics studies still pose the question of what is actually inherited, because – although familiarity has been found – often the inherited feature seems to not be a direct trigger of aggression, but rather a cognitive feature that impact the development of personality (like reduced intelligence in the XYY chromosome disorder). It is evident therefore that the demonstration of aggressive behavior is the result of a complex sequence of events that involve both biological structures and the environmental forces that shape the cognition and affection and that at the same time are shaped by these evolving functions of the child. At this regards it is worth to mention the doubt that Raine (2002) expresses discussing the studies on adoptive children that analyzed the relationship between inherited aggression and adoptive parental care. For some children who had criminal parents (likelihood of inheriting aggression) and were rejected by the adoptive parents, he wonders whether the rejection was actually due to the behavior of the child, rather that to the personality of the parents, indicating that the child himself produces environmental conditions that may aggravate the likelihood to incur in aggression, in a vicious circle.
Whether biological or environmental factors facilitate the appearance of aggression, one aspect to be considered in striving to find a clear predictor of violent behavior, are the ethical implications. One example is the strong debate triggered in 1999 when the UK Department of Health together with the Home Office issued a paper with a proposal for the management of dangerous people with severe personality disorders (Home Office & Department of Health, 1999, also quoted in Davey, 2008). The debate focused on whether it would be ethically acceptable to convict individuals with feature that clearly point to high risk to public safety (as it could be the diagnosis of psychopathy) if no crime has been committed.
In conclusion, I can say that both nature and nurture work to shape each other and producing in some individual aggression, but that the entity of the respective role is not clear. Considering the number of factors involved in producing aggressive behavior, and the complexity of the interactions among different causes, I would consider it more appropriate to refer to them as “risk factors” rather than causes. This conclusion is reinforced by the observation that none of the factors has been found to unequivocally determine aggressive behavior in every individual affected by that specific factor. If the research on nature or nurture in aggression is conducted with the