A Comparative Approach to Violence in Animals and Human Societies: An Interview with Professor Caroline Blanchard

For millennia, humanity has been plagued by violence and its consequences: oppression and inequality. Academics, intellectuals, politicians and dissidents have since attempted to understand the causes of these phenomena, with the ultimate goal of promoting peace and forming a sustainable, equal and just society. Theories in economics, political science and psychology have profoundly influenced human kind, though suffering as a result of collective violence, in its myriad forms, is still rampant.

The theory of evolution by Charles Darwin and others has fostered a comparative approach in which animals are often used to deduce information about humans. As such, the disciplines of biology, physiology and medicine have thrived, leading to substantial improvements in human quality of life, health and longevity. However, while individual levels of violence have drastically declined, collective forms have remained consistent.

The field of Behavioral Neuroscience adopts an evolutionary approach to study the brain mechanisms that control behavior. Researchers examine defensive and aggressive behavioral patterns in animal species in order to understand violent expressions in humans.

Caroline Blanchard is a Professor of Psychology/Behavioral Neuroscience at the University of Hawaii and Editor of the Handbook of Anxiety and Fear. She has worked for many years on behavioral and neural systems in defense and aggression and has served as President for the International Behavioral Neuroscience Society and the International Society for Research on Aggression.

In this interview, Blanchard discusses individual and collective forms of aggression and defense in animals and people, their adaptive and maladaptive properties and inherent and learned characteristics as a means of understanding the violence that plagues human society.


Yoav Litvin: What is aggression? Discuss its various manifestations in the animal kingdom.

Blanchard: The ultimate goal of aggression is resource acquisition or maintenance. It is usually, but not always, intraspecific; i.e. directed toward a conspecific – a member of one’s own species. An example of interspecific (as opposed to intraspecific) aggression might be attack by a lion towards hyenas attempting to take over the carcass of an animal the lion killed. Although predation – another example of interspecific attack – is also aimed at gaining a resource (food), the motivations, brain systems, and behaviors in predation appear to be different enough that it is typically not regarded as a form of aggression.

Professor D. Caroline Blanchard

Aggression, or offensive aggression to distinguish it from defensive aggression, is directed towards competitors, and the most common types of disputed resources are territory, dominance position, and access to females in breeding condition; all important for evolutionary ‘success’. Fighting in many species is more likely to test relative strength or size of the combatants than to produce actual tissue damage. The spectacular horns or antlers of many ungulate species help in this testing, by enabling combatants to lock these structures and push or twist, to determine strength. Better yet, if the other guy has much larger antlers than you do, don’t even try!

When one animal (usually male but not always) kills/subdues another in its group, the hierarchical order of the group may change. The winner moves up a slot, if the animal it killed/subdued was higher ranking, or doesn’t change if the opponent was lower in rank.  The hierarchy system may not change, just the occupants of various rungs.  However, if a particularly strong ‘personality’ comes to occupy the top slot or slots, then the overall tone of the group might indeed be expected to change.

This would especially be the case in groups that have a relatively consistent dominance ranking, like (some) baboons, chimpanzees, hyenas or lions in prides. These ‘small group-hierarchical’ mammals are interesting as they tend to be territorial and do show ‘collective aggression’ at times, either invading adjacent territories or being invaded by others. Such aggression typically involves a larger group attacking a smaller group, which is why it is so dangerous to be a very small group under these circumstances, and, why it is such a bad idea to kill one of your own potential co-fighters.


What is the purpose of defense? Discuss its various manifestations in the animal kingdom.

Blanchard: Like aggression, defense is an evolved pattern of behaviors that are functional in the context of a common ‘problem’ situation for most animals. The ‘problem’ is threat or danger to the animal itself, and defensive behaviors function to reduce that danger. Thus, flight removes the animal from danger, while freezing and tonic immobility can reduce the chance of being detected, or of being attacked if you are detected: think of a cat with a mouse, not attacking until the mouse tries to run. One defensive behavior, risk assessment, may be common to almost all threat situations.

Risk assessment – at the intersection of cognition and emotion – involves orientation to, and investigation/analysis of, the threat. The resulting information about both the threat and the situation in which it is encountered typically modulates the choice of the most effective defense against that particular threat, in that situation (e.g. flight is counterproductive if there is no way out), or it may even indicate that there is no threat, promoting a return to non-defensive behavior.


How can you tell the difference between offensive and defensive aggression, i.e. self-defense and aggression? How do these behavioral patterns work to maintain social species, such as humans?

Blanchard: The major area of overlap between aggression and defense is defensive or retaliatory attack in response to either interspecific attack, including predation, or attack from a member of your own species. Defensive attack can be very effective, especially by prey species with weaponry such as sharp teeth, claws, or horns, and predators may be injured or even killed in these encounters.

However, as noted earlier, in highly social species killing a member of one’s own group may reduce the future success of that group in other fights. Thus, structural or behavioral adaptations to limit damage during conspecific attack appear to be common in highly social species. Structural protections for particularly vulnerable body areas are often limited to the gender and developmental stages that incur the intraspecific fighting, such as the beards of adult men or the manes of adult male lions.

An alternative evolutionary strategy for highly social species is behavioral: Conspecific offensive attack may be aimed at a comparatively harmless site on the opponent’s body, where injury will cause pain, potentially enabling the attacker to gain some disputed resource, without great danger of killing the conspecific opponent. Thus, rats, dogs and wolves, among others, tend to direct offensive bites to the back of a conspecific.

A further development in this pattern is that the animal thus attacked tends to protect this target by rolling on its back or facing its attacker: Rolling on the back hides the back and reduces further attack, appearing to provide a ‘submission signal’. Notably, a frightened or hurt animal attacked by a conspecific may bite in return. However, in contrast to the offensive bite, this defensive attack is aimed at particularly damaging areas such as the face of the aggressor.  In such a desperate situation, it is more important to survive than to try to maintain the size of the group!

As this analysis suggests, in animals, aggression and defense are generally very easy to differentiate, both functionally and behaviorally, with the exception of offensive vs. defensive aggression. The confusion for the latter is that defensive aggression is functionally defense, in that it is adaptive in modulating harm from threat or danger, rather than in acquisition of a resource; but superficially it looks more like aggression: At least for the species in which this pattern has been extensively investigated, careful descriptions of eliciting situations, fighting behaviors, and specific target sites for attack can clarify whether a specific behavior is offensive or defensive aggression.


Do humans express similar patterns of defense and aggression? What are some of the differences? How has the evolution of human society shaped these behavioral patterns?

Blanchard: Functionally, both defense and aggression appear to fulfill the same roles in humans as in nonhuman animals: In people, the ‘animal’ defensive behaviors clearly occur in the context of physical threat or danger and are modulated by many of the same contextual circumstances as in nonhuman species.

But there are differences: For one, modern weapons are involved in much if not most, lethal fighting in contemporary society, reducing the efficacy of many defensive behaviors and obscuring target sites and the information they provide as to offensive vs defensive motivations. This is an important, and as yet under-researched issue in self-defense claims with reference to homicide/assault charges.

A view that human aggression is aimed at resource acquisition or maintenance also appears to be valid, albeit with the caveat that human ‘resources’ – while related to those of nonhuman animals – are much more expansive.  Dominance, and its accompaniment, reputation; as well as the most fungible asset, money, are resources with wider application and subtler manifestations and consequences than anything in nonhuman societies. Indeed, the near-universal development in human societies of laws and regulations regarding ownership attests to the centrality and complexity of human resources and their control. These laws and regulations, plus the development of central authorities to enforce them – especially the latter – have substantially reduced individual human lethal fighting over the past 600 years or so (see also here).

In contrast, the lethality of collective aggression, measured as deaths per 100,000 population resulting from wars, has remained relatively steady over the same time frame. Thus, individual lethal violence is declining; collective human violence less so: Although there has been something of a decline since WWII, this period/duration of decline has happened many times over the period surveyed and it is unclear if it represents a true reduction.


What are some of the behavioral manifestations of collective forms of aggression and defense in the animal kingdom?

Blanchard: Collective aggression in nonhuman species, ranging from sea anemones to chimpanzees, is by no means unknown. Moreover, it appears to have the same focal function, resource acquisition, as does individual aggression in both humans and nonhumans. However, the factors determining the magnitude of such collective aggression are not well understood.

Chimpanzees, with a high degree of genetic relatedness to humans, show even higher than human levels of collective conspecific killing while bonobos, with a similar high level of genetic relatedness, show much less. Collective aggression appears to occur in chimpanzees only in some groups, and only as cross-border ‘raids’ in which a patrol (largely but not exclusively male) finds, attacks, and may kill, solitary conspecifics – including females and infants – from other groups. When relatively evenly matched groups encounter each other, the result tends to be a great deal of noise, but avoidance or retreat rather than fighting.

Groups of social animals also show changes in defense. One of these, seen also in people, is a general reduction in defensiveness, in that being part of a group reduces the need for constant vigilance, as other members (including designated ‘sentinels’) may provide warning of danger. This is accompanied, however, by expanded attention to other group members, and presumably by enhancement of recognition of any signals of alarm that they may show.

More active group defenses include the strategies of muskoxen and some cetaceans to gather in a circular formation enclosing more vulnerable group members. All of these are defenses against predators, not conspecifics.

However, a few species, including spotted hyenas and lions, do show collective defenses against both conspecific and non-conspecific attackers over kill, or in response to conspecific invasion of their territories or hunting grounds. What this suggests is that organized group defenses, except against predators, are relatively rare: perhaps reflecting that, as with chimpanzees, group aggression in nonhuman species tends to involve attack by larger groups on smaller: Fear is a wonderful inhibitor of offensive attack. It is also interesting that lions reacting to group attacks on their pride appear to show substantial individual differences in eagerness to join the fray, suggesting a relatively low level of coordination of cooperative defensiveness.

All this reflects that human cultures and societies have an intensity and range of influence on individuals that is beyond anything seen in most nonhuman species.


What is the role of learning in defense and offense?

Blanchard: There are multiple potential roles for learning in both aggression and defense, from relatively simple (e.g. stimuli associated with threat rapidly acquires the ability to elicit defensive behaviors) to cognitively very complex (e.g. aggression is associated with violation of learned belief systems, including “challenge” to “ownership” of specific resources).

Indeed, learning is essential for both defense and aggression with a few exceptions such as painful or loud and sudden stimuli (for defense) and possibly some human and animal expressions/gestures that may elicit unconditioned emotions related to aggression and defense (take toy away from baby). However, a range of highly stressful or traumatizing events, particularly if prolonged, may be associated with epigenetic changes leading to exaggerated or abnormal expression of defense, in particular, and also of aggression. These changes have been documented in species from bees to humans, and may involve neuroendocrine and immune systems, including gut microbiota, as well as the brain.


How do alterations in appropriate levels of aggression and defense contribute to abnormal fear, anxiety, violence and associated disorders?

Blanchard: Both general hyper-defensiveness, and exaggeration of particular defensive behaviors are involved in a range of anxiety/depressive disorders. In particular, risk assessment has been linked to rumination, a condition involving repetitive, negative, self-referential thinking, that is very characteristic of both anxiety and depression and may be largely responsible for the substantial overlap of the two. Repetitive actions such as ‘checking’ in obsessive compulsive disorder may also be linked to a failure of risk assessment processes to determine that a potential danger is not, in fact, dangerous, resulting in failure to return to normal, non-defensive behavior. Specific phobias often, but not always, stem from traumatic experiences involving the particular stimuli that are the focus of the phobic response.

Abnormal aggression is associated with multiple DSM categories of psychopathology, involving both cognitive and mood disorders. The dependence of aggression on cognitive appraisal of situations links to its substantial association with paranoia, and defensive aggression may be involved in a range of cognitive disorders in which perceptions of threat are exaggerated. Depression may sometimes involve blaming others for one’s unhappy state, resulting in irritability or aggressiveness. However, many examples of abnormal aggression appear to be associated with general reductions in inhibitory control of aggressive impulses, sometimes associated with reduced activation of frontal cortex, endocrine regulation, or drug or alcohol intoxication.

Aggressive behaviors in wild and laboratory rats and mice (among others), including some that may serve as models for abnormal aggression, appear to be strongly influenced by both genetic and epigenetic mechanisms: Numerous studies have shown rapid alterations in aggressive behaviors for animals selected for traits such as an absence of biting attack when handled, or changes in the speed and specificity of attack on a conspecific opponent.


Briefly describe a human society that demonstrates low levels of aggression and defense. What conditions enable this sort of development?

Blanchard: One fairly recently touted example was the Pirahã tribe, a group of about 800 individuals living in four villages in the Amazon. They have very strong social prohibitions against any form of coercion; no hierarchical structure, no leaders, and no aggression. However, one of the researchers who has studied them for 30 years told me, when I questioned him after a talk “…but if you get one drunk, he will kill you!”, suggesting that a very tight form of cultural control over aggression, not a lack of aggressive impulses, was the situation. It is also notable that they have very few possessions, no prohibitions about sexual activity, and, due to population size and situation, no reason to even want exclusive territories. So, the major reasons to fight do not seem to be very operative.


Finally, some thoughts on our current political environment and how it fosters violence.

Blanchard: The current global situation of extreme imbalance in individual wealth is not good for anybody, even the ultra-wealthy. It causes discontents that range from individual anger and resentment, to upsets in the hierarchical structures of societies, with these upsets going on to influence international relationships. Another contributing factor to both individual and collective aggression (and their results, individual and collective defense) is the enhanced availability of totally false information, spread as truth through the internet, used to stoke violence. I don’t suspect this will happen anytime soon, but I would like to see some sort of source-requirement for any information disseminated as fact through public media. If these two enormous problems; wealth polarization and dissemination of untruths and propaganda; could be solved, it would at least reduce some of the individual and collective anger – and defensiveness– that is so prevalent in a number of contemporary societies.


Artwork by Jilly Ballistic