In psychology
Psychology is the study of the mind and behavior. Its immediate goal is to understand individuals and groups by both establishing general principles and researching specific cases. For many, the ultimate goal of psychology is to benefit society...

, as well as other social and behavioral sciences, aggression refers to behavior between members of the same species that is intended to cause humiliation
Humiliation is the abasement of pride, which creates mortification or leads to a state of being humbled or reduced to lowliness or submission. It can be brought about through bullying, intimidation, physical or mental mistreatment or trickery, or by embarrassment if a person is revealed to have...

, pain
Pain is an unpleasant sensation often caused by intense or damaging stimuli such as stubbing a toe, burning a finger, putting iodine on a cut, and bumping the "funny bone."...

, or harm. Ferguson and Beaver (2009) defined aggressive behavior as "Behavior which is intended to increase the social dominance of the organism relative to the dominance position of other organisms" . Predatory or defensive behavior between members of different species is not normally considered "aggression." Aggression takes a variety of forms among humans and can be physical, mental, or verbal. Aggression differs from what is commonly called assertiveness
Assertiveness is a particular mode of communication. Dorland's Medical Dictionary defines assertiveness as:During the second half of the 20th century, assertiveness was increasingly singled out as a behavioral skill taught by many personal development experts, behavior therapists, and cognitive...

, although the terms are often used interchangeably among laypeople, e.g. an aggressive salesperson.


There are two broad categories of aggression. These include hostile, affective, or retaliatory
Revenge is a harmful action against a person or group in response to a grievance, be it real or perceived. It is also called payback, retribution, retaliation or vengeance; it may be characterized, justly or unjustly, as a form of justice.-Function in society:Some societies believe that the...

 aggression and instrumental, predatory, or goal-oriented aggression. Both of these can be classified under Relational aggression. Reactive relational aggression (hostile, affective, retaliatory) is used in response to feeling attacked, threatened, or mad. Usually the person who exhibits this type of aggression feels provoked to do so. Instrumental relational aggression (predatory, goal-oriented) is used in order for an individual to get what they want. Empirical research indicates that there is a critical difference between the two, both psychologically and physiologically.

In morale theories, such as argumentation ethics and the non-aggression principle
Non-aggression principle
The non-aggression principle , or NAP for short, is a moral stance which asserts that aggression is inherently illegitimate...

, physical aggression is distinguished from violence
Violence is the use of physical force to apply a state to others contrary to their wishes. violence, while often a stand-alone issue, is often the culmination of other kinds of conflict, e.g...

. Aggression is considered the initiation of violence. Often, retaliatory violence and defensive violence is not considered aggression, because it is a responsive action.


Like most, or even all behaviours, aggression can be examined in terms of its ability to help an animal reproduce and survive. Animals may use aggression to gain and secure territories, as well as other resources including food, water, and mating opportunities. Researchers have theorized that aggression and the capacity for murder are products of our evolutionary past.

Aggression against outsiders

The most apparent type of aggression is that seen in the interaction between a predator and its prey. An animal defending itself against a predator becomes aggressive in order to survive and to ensure the survival of its offspring
In biology, offspring is the product of reproduction, of a new organism produced by one or more parents.Collective offspring may be known as a brood or progeny in a more general way...

. Because aggression against a much larger enemy or group of enemies would be nearly certain to lead to the death of an animal, animals have developed a good sense of when they are outnumbered. This ability to gauge the strength of other animals gives animals a "fight or flight" response to predators; depending on how strong they gauge the predator to be, animals will either become aggressive or flee.

The need to survive and the viability of cooperative behavior as a survival strategy leads to a phenomenon known as altruism
Altruism is a concern for the welfare of others. It is a traditional virtue in many cultures, and a core aspect of various religious traditions, though the concept of 'others' toward whom concern should be directed can vary among cultures and religions. Altruism is the opposite of...

. An example of an altruistic act is the alarm call that is given when a predator is approaching. While this call will inform the community of a predator's presence, it will also inform the predator of the whereabouts of the animal that gave the alarm call. While this would appear to give the alarm caller an evolutionary disadvantage, it would facilitate the continuation of this animal's genes because its relatives and progeny would be more able to avoid predators.

According to many researchers, predation is not aggression. Cats do not hiss or arch their backs when in pursuit of a rat, and the active areas in their hypothalamuses are more similar to those that reflect hunger than those that reflect aggression.

Aggression within a species

Aggression against conspecifics serves a number of purposes having to do with breeding. One of the most common of these purposes is the establishment of a dominance hierarchy. When certain types of animals are first placed in a common environment, the first thing they do is fight to assert their role in the dominance hierarchy. In general, the more dominant animals will be more aggressive than their subordinates. The majority of conspecific aggression ceases about 24 hours after the introduction of the animals being tested.

There are many different theories that try to explain how males and females developed these different aggressive tendencies. One theory states that in species where one sex makes a higher parental investment than the other, the higher investing sex is a resource for which the other sex competes: in the majority of species, females are the higher investing sex. It also holds that reproductive success is cardinal to the perpetuation of an organism's lineage and hereditary characteristics. For males, it is of crucial importance to establish dominance and resource holding to obtain reproductive opportunities in order to pass on their genetics. Unlike females, whose reproductive success is constrained by long gestation
Gestation is the carrying of an embryo or fetus inside a female viviparous animal. Mammals during pregnancy can have one or more gestations at the same time ....

 and lactation
Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process occurs in all female mammals, however it predates mammals. In humans the process of feeding milk is called breastfeeding or nursing...

 periods (at least in mammals), male reproductive success is constrained by the number of partners they can mate with. As a result, males employ physical aggression more often than females; they take more risks in order to compete with other males and gain an elevation of status. Males even go as far as killing one another, although this is rare. Males demonstrate less concern for their physical welfare in such competitions. In contrast, females compete with one another for resources, which can be converted to offspring. The establishment of dominance is more costly for females than for males and females have less to gain from achieving status. The female presence is more critical to the offspring's survival and hence her reproductive success than is the father's. It is only logical then that the health and wellbeing of females would cause them to use less aggressive, low-risk, and indirect strategies to acquire resources. As a result, in the majority of female–female conflicts, females rarely inflict serious damage to one another over resources. When translated to humans, these facts suggest that women should be expected to show less evidence of dominance hierarchies than men do. In society, aggression in boys becomes increasingly motivated by issues of social status and self-esteem, which are usually decided by varying degrees of aggressive reactivity to personal challenge. Aggression in girls, focusing mainly on resource acquisition and not status, is more likely to take less physically dangerous and more covert forms of indirect aggression. There are, however, extensive critiques of the use of animal behavior to explain human behavior and the application of evolutionary explanations of contemporary human behavior.

In humans

Although humans share aspects of aggression with non-human animals, they differ from most of them in the complexity of their aggression because of factors such as culture, morals, and social situations. A wide variety of studies have been done on these situations. Aggression in humans can be assessed by using aggression scales such as the MOAS (Modified Overt Aggression Scale).


Culture is a distinctly human factor that plays a role in aggression. Kung Bushmen were described as the "harmless people" by Elizabeth Marshall Thomas (1958). Other researchers, however, have countered this point of view, calculating that the homicide rate among Bush[wo]men is actually higher than that of developed societies (Keeley, 1996). Lawrence Keeley argues that the "peaceful native american" is a myth that is unsupported by the bulk of anthropological and archeological evidence. Hunter–gatherer societies do not have possessions to fight over, but they may still come to conflict over status and mating opportunities.

Empirical research
Empirical research is a way of gaining knowledge by means of direct and indirect observation or experience. Empirical evidence can be analyzed quantitatively or qualitatively...

 cross-cultural research has found differences in the level of aggression between cultures. In one study, American men resorted to physical aggression more readily than Japanese or Spanish men, whereas Japanese men preferred direct verbal conflict more than their American and Spanish counterparts (Andreu et al. 1998). Within American culture, southerners
Southern United States
The Southern United States—commonly referred to as the American South, Dixie, or simply the South—constitutes a large distinctive area in the southeastern and south-central United States...

 were shown to become more aroused and to respond more aggressively than northerners when affronted (Bowdle et al. 1996). There is also a higher homicide rate among young white southern men than among white northern men in the United States (Nisbett 1993).

A person's beliefs about the social acceptability of an aggressive act (termed " beliefs") are major predictors of their behavior. Today, some polls indicate that 63% of Jewish Israelis consider their country's Arab citizens a "security and demographic threat to the state." Roughly 18% said they felt hatred when they heard someone speaking Arabic, and 34% agreed with the statement that "Arab culture is inferior to Israeli culture." Various social scientists and political leaders suggest that increasing interpersonal aggression against Palestinians as well as Arabs in Israel is due to increasing tolerance for public racism against Arabs. beliefs may partially explain cultural differences in aggression towards certain groups. As these beliefs are readily changeable through intervention, targeting normative beliefs may be a way to decrease aggression in certain individuals.


Some scholars believe that behaviors like aggression may be partially learned by watching and imitating the behavior of others. Some scholars have concluded that media may have some small effects on aggression (Aronson, Wilson, & Akert, 2005) although increasing research is now questioning this view (see Freedman, 2002; Ferguson, 2010). For instance recent long-term outcome study of youth found no long-term relationship between playing violent video game and youth violence or bullying. In addition results from another study suggest there is a smaller effect of violent video games on aggression than has been found with television violence on aggression. This effect is positively associated with type of game violence and negatively related to time spent playing the games. The author concluded that insufficient evidence exists to link video game violence with aggression.
Scholars are continuing to examine the role of violent media in promoting aggression. As of yet, no consensus has been reached.

Situational factors

Alcohol impairs judgment, making people much less cautious than they usually are (MacDonald et al. 1996). It also disrupts the way information is processed (Bushman 1993, 1997; Bushman & Cooper 1990). A drunk person is much more likely to view an accidental event as a purposeful one, and therefore act more aggressively to that.

Pain and discomfort also increase aggression. Even the simple act of placing one's hands in hot water can cause an aggressive response. Hot temperatures have been implicated as a factor in a number of studies. One study completed in the midst of the civil rights movement found that riots were more likely on hotter days than cooler ones (Carlsmith & Anderson 1979). Students were found to be more aggressive and irritable after taking a test in a hot classroom (Anderson et al. 1996, Rule, et al. 1987). Drivers in cars without air conditioning were also found to be more likely to honk their horns (Kenrick & MacFarlane 1986).

Frustration is another major cause of aggression. The Frustration aggression theory
Frustration aggression theory
Frustration-aggression hypothesis is a theory of aggression proposed by John Dollard, Neal E. Miller et al. in 1939, and further developed by Miller, Roger Barker et al. in 1941 and Leonard Berkowitz in 1969.-Further reading:...

 states that aggression increases if a person feels that he or she is being blocked from achieving a goal (Aronson et al. 2005). One study found that the closeness to the goal makes a difference. The study examined people waiting in line and concluded that the 2nd person was more aggressive than the 12th one when someone cut in line (Harris 1974). Unexpected frustration may be another factor. In a separate study, a group of students were collecting donations over the phone. Some of them were told that the people they would call would be generous and the collection would be very successful. The other group was given no expectations. The group with high expectations was much more upset and became more aggressive when no one was pledging (Kulik & Brown 1979).

There is some evidence to suggest that the presence of violent objects such as a gun can trigger aggression. In a study done by Leonard Berkowitz
Leonard Berkowitz
Leonard Berkowitz is an American social psychologist best known for his research on human aggression. He originated the Cognitive Neoassociation Model of aggressive behavior, which was created to help explain instances of aggression that the Frustration-Aggression Hypothesis could not account for....

 and Anthony Le Page (1967), college students were made angry and then left in the presence of a gun or badminton racket. They were then led to believe they were delivering electric shocks to another student, as in the Milgram experiment
Milgram experiment
The Milgram experiment on obedience to authority figures was a series of notable social psychology experiments conducted by Yale University psychologist Stanley Milgram, which measured the willingness of study participants to obey an authority figure who instructed them to perform acts that...

. Those who had been in the presence of the gun administered more shocks. It is possible that a violence-related stimulus increases the likelihood of aggressive cognitions by activating the semantic network
Semantic network
A semantic network is a network which represents semantic relations among concepts. This is often used as a form of knowledge representation. It is a directed or undirected graph consisting of vertices, which represent concepts, and edges.- History :...


A new proposal links military experience to anger and aggression, developing aggressive reactions and investigating these effects on those possessing the traits of a serial killer. Castle and Hensley state, "The military provides the social context where servicemen learn aggression, violence, and murder." Post-traumatic stress disorder
Post-traumatic stress disorder
Posttraumaticstress disorder is a severe anxiety disorder that can develop after exposure to any event that results in psychological trauma. This event may involve the threat of death to oneself or to someone else, or to one's own or someone else's physical, sexual, or psychological integrity,...

 (PTSD) is also a serious issue in the military, also believed to lead to aggression in soldiers who are suffering from what they witnessed in battle. They come back to the civilian world and are still haunted by flashbacks and nightmares, causing severe stress. In addition, those who are inclined toward serial killing find their violent impulses reinforced and refined, possibly creating more effective murderers. These controversial studies are far from definitive and require further investigation.


Gender is a factor that plays a role in both human and animal aggression. Males are historically believed to be generally more physically aggressive than females (Coie & Dodge 1997, Maccoby & Jacklin 1974), and men commit the vast majority of murders (Buss 2005). This is one of the most robust and reliable behavioral sex differences, and it has been found across many different age groups and cultures. There is evidence that males are quicker to aggression (Frey et al. 2003) and more likely than females to express their aggression physically (Bjorkqvist et al. 1994). When considering indirect forms of non-violent aggression, such as relational aggression
Relational aggression
Relational aggression, also known as covert aggression or covert bullying, is a type of aggression in which harm is caused through damage to relationships or social status within a group rather than by means of actual or threatened physical violence...

 and social rejection
Social rejection
Social rejection occurs when an individual is deliberately excluded from a social relationship or social interaction. The topic includes both interpersonal rejection and romantic rejection. A person can be rejected on an individual basis or by an entire group of people...

, some scientists argue that females can be quite aggressive although female aggression is rarely expressed physically (Archer, 2004; Card, Stucky, Sawalani, & Little, 2008).

Although females are less likely to initiate physical violence, they can express aggression by using a variety of non-physical means. Exactly which method women use to express aggression is something that varies from culture to culture. On Bellona Island
Bellona Island
Bellona Island is an island of the Rennell and Bellona Province, Solomon Islands. Its length is about 10 km and its average width 2.5 km. Its area is about 17 km². It is almost totally surrounded by 30–70 m high cliffs, consisting primarily of raised coral...

, a culture based on male dominance and physical violence
Violence is the use of physical force to apply a state to others contrary to their wishes. violence, while often a stand-alone issue, is often the culmination of other kinds of conflict, e.g...

, women tend to get into conflicts with other women more frequently than with men. When in conflict with males, instead of using physical means, they make up songs mocking the man, which spread across the island and humiliate him. If a woman wanted to kill a man, she would either convince her male relatives to kill him or hire an assassin. Although these two methods involve physical violence, both are forms of indirect aggression, since the aggressor herself avoids getting directly involved or putting herself in immediate physical danger.

Evolutionary psychology
Evolutionary psychology
Evolutionary psychology is an approach in the social and natural sciences that examines psychological traits such as memory, perception, and language from a modern evolutionary perspective. It seeks to identify which human psychological traits are evolved adaptations, that is, the functional...

 has proposed several evolutionary explanations for gender differences in aggressiveness. Males can increase their reproductive success by polygyny
Polygyny is a form of marriage in which a man has two or more wives at the same time. In countries where the practice is illegal, the man is referred to as a bigamist or a polygamist...

 which will lead the competition with other males over females. If the mother died this may have had more serious consequences for a child than if the father died in the ancestral environment since there is a tendency for greater parental investment
Parental investment
In evolutionary biology, parental investment is any parental expenditure that benefits one offspring at a cost to parents' ability to invest in other components of fitness...

s and caring for children by females than by males. Greater caring for children also leads to difficulty leaving them in order to either fight or flee. Anne Campbell writes that females may thus avoid direct physical aggressiveness and instead use strategies such as "friendship termination, gossiping, ostracism, and stigmatization".

In children

The frequency of physical aggression in humans peaks at around 2–3 years of age. It then declines gradually on average. These observations suggest that physical aggression is mostly not a learned behavior and that development provides opportunities for the learning of self-regulation. However, a small subset of children fails to acquire the necessary self-regulatory abilities and tends to show atypical levels of physical aggression across development. These may be at risk for later violent behavior.

What is typically expected of children?
  • Young children preparing to enter kindergarten need to develop the socially important skill of being assertive. Examples of assertiveness include asking others for information, initiating conversation, or being able to respond to peer pressure
    Peer pressure
    Peer pressure refers to the influence exerted by a peer group in encouraging a person to change his or her attitudes, values, or behavior in order to conform to group norms. Social groups affected include membership groups, when the individual is "formally" a member , or a social clique...

  • In contrast, some young children use aggressive behavior, such as hitting or biting, as a form of communication.
  • Aggressive behavior can impede learning as a skill deficit, while assertive behavior can facilitate learning. However, with young children, aggressive behavior is developmentally appropriate and can lead to opportunities of building conflict resolution and communication skills.
  • By school age, children should learn more socially appropriate forms of communicating such as expressing themselves through verbal or written language; if they have not, this behavior may signify a disability or developmental delay

What triggers aggressive behavior in children?
  • Physical fear of others
    A phobia is a type of anxiety disorder, usually defined as a persistent fear of an object or situation in which the sufferer commits to great lengths in avoiding, typically disproportional to the actual danger posed, often being recognized as irrational...

  • Family difficulties
    Dysfunctional family
    A dysfunctional family is a family in which conflict, misbehavior, and often abuse on the part of individual members occur continually and regularly, leading other members to accommodate such actions. Children sometimes grow up in such families with the understanding that such an arrangement is...

  • Learning
    Learning is acquiring new or modifying existing knowledge, behaviors, skills, values, or preferences and may involve synthesizing different types of information. The ability to learn is possessed by humans, animals and some machines. Progress over time tends to follow learning curves.Human learning...

    , neurological
    Neurology is a medical specialty dealing with disorders of the nervous system. Specifically, it deals with the diagnosis and treatment of all categories of disease involving the central, peripheral, and autonomic nervous systems, including their coverings, blood vessels, and all effector tissue,...

    , or conduct/behavior disorders
    Conduct disorder
    Conduct disorder is psychological disorder diagnosed in childhood that presents itself through a repetitive and persistent pattern of behavior in which the basic rights of others or major age-appropriate norms are violated...

  • Psychological trauma
    Psychological trauma
    Psychological trauma is a type of damage to the psyche that occurs as a result of a traumatic event...

Corporal punishment
Corporal punishment
Corporal punishment is a form of physical punishment that involves the deliberate infliction of pain as retribution for an offence, or for the purpose of disciplining or reforming a wrongdoer, or to deter attitudes or behaviour deemed unacceptable...

 such as spanking
Spanking refers to the act of striking the buttocks of another person to cause temporary pain without producing physical injury. It generally involves one person striking the buttocks of another person with an open hand. When an open hand is used, spanking is referred to in some countries as...

 increases subsequent aggression in children.

The Bobo doll experiment
Bobo doll experiment
The Bobo doll experiment was the name of two experiments conducted by Albert Bandura in 1961 and 1963 studying patterns of behavior associated with aggression....

 was conducted by Albert Bandura in 1961. In this work, Bandura found that children exposed to an aggressive adult model acted more aggressively than those who were exposed to a nonaggressive adult model. This experiment suggests that anyone who comes in contact with and interacts with children can have an impact on the way they react and handle situations.

Summaries of best practice recommendations
  1. American Academy of Pediatrics
    American Academy of Pediatrics
    The American Academy of Pediatrics is the major professional association of pediatricians in the United States. The AAP was founded in 1930 by 35 pediatricians to address pediatric healthcare standards. It currently has 60,000 members in primary care and sub-specialist areas...

    : "Set firm, consistent limits to help children self monitor emotions and behavior; make sure all care takers agree to the same limits. Provide examples of effective and socially acceptable ways of managing anger; be careful not to reinforce aggression with aggressive forms of punishment. Also, model acceptable behavior as a caretaker by managing your own temper. Remember that occasional outbursts are normal. If aggressive behavior continues for more than a few weeks, consult a pediatrician or mental health professional."
  2. National Association of School Psychologists
    National Association of School Psychologists
    The National Association of School Psychologists is the major national professional organization for school psychologists in the United States...

    : "Overly aggressive behavior can signify a social skills deficit; direct instruction, modeling, and coaching can help children acquire the skill of assertion, which as a replacement behavior may help prevent aggressive behavior."


Aggression is directed to and often originates from outside stimuli, but has a very distinct internal character. Using various techniques and experiments, scientists have been able to explore the relationships between various parts of the body and aggression.

In the brain

Many researchers focus on the brain to explain aggression. The areas involved in aggression in mammals include the amygdala, hypothalamus, prefrontal cortex, cingulate cortex, hippocampus, septal nuclei, and periaqueductal grey of the midbrain. Because of the difficulties in determining the intentions of animals, aggression is defined in neuroscience research as behavior directed at an object or animal which results in damage or harm to that object or animal.

In many animals, aggression is controlled by pheromone
A pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual...

s. In mice, Major urinary proteins
Major urinary proteins
Major urinary proteins , also known as α2u-globulins, are a subfamily of proteins found in abundance in the urine and other secretions of many animals. Mups provide a small range of identifying information about the donor animal, when detected by the vomeronasal organ of the receiving animal. They...

 (Mups) have been demonstrated to promote innate aggressive behavior in males. Mups were demonstrated to activate olfactory sensory neurons in the vomeronasal organ
Vomeronasal organ
The vomeronasal organ , or Jacobson's organ, is an auxiliary olfactory sense organ that is found in many animals. It was discovered by Frederik Ruysch and later by Ludwig Jacobson in 1813....

 (VNO), a subsystem of the nose known to detect pheromones via specific sensory receptor
Sensory receptor
In a sensory system, a sensory receptor is a sensory nerve ending that responds to a stimulus in the internal or external environment of an organism...

s, of mice and rats.

The hypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...

 and periaqueductal gray
Periaqueductal gray
Periaqueductal gray is the gray matter located around the cerebral aqueduct within the tegmentum of the midbrain. It plays a role in the descending modulation of pain and in defensive behaviour...

 of the midbrain are the most critical areas controlling aggression in mammals, as shown in studies on cats, rats, and monkeys. These brain areas control the expression of all the behavioral and autonomic components of aggression in these species, including vocalization. They have direct connections with both the brainstem nuclei controlling these functions and areas such as the amygdala and prefrontal cortex.

Electrical stimulation of the hypothalamus causes aggressive behavior the hypothalamus expresses receptors that help determine aggression levels based on their interactions with the neurotransmitters serotonin and vasopressin.

The amygdala
The ' are almond-shaped groups of nuclei located deep within the medial temporal lobes of the brain in complex vertebrates, including humans. Shown in research to perform a primary role in the processing and memory of emotional reactions, the amygdalae are considered part of the limbic system.-...

 is also critically involved in aggression. Stimulation of the amygdala results in augmented aggressive behavior in hamsters, while lesions of an evolutionarily homologous
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...

 area in the lizard greatly reduce competitive drive and aggression (Bauman et al. 2006). Several experiments in attack-primed Syrian Golden Hamsters support the claim of the amygdala being involved in control of aggression. Using expression of c-fos
In the field of molecular biology and Genetics, c-Fos is a protein encoded by the FOS gene.-Structure and function:c-Fos is a cellular proto-oncogene belonging to the immediate early gene family of transcription factors. c-Fos has a leucine-zipper DNA binding domain, and a transactivation domain at...

 as a neuroanatomically localized marker of activity, the neural circuitry involved in the state of "attack readiness" in attack-primed hamsters was studied. The results showed that certain structures of the amygdala were involved in aggressiveness: the medial nucleus and the cortical nuclei showed distinct differences in involvement as compared to other structures such as the lateral and basolateral nuclei and central nucleus of the amygdala, which were not associated with any substantial changes in aggressiveness. In addition, c-fos expression was found most clearly in the most dorsal and caudal aspects of the corticomedial amygdala (CMA). In the same study, it was also shown that lesions of the CMA significantly reduced the number of aggressive behaviors. Eight of eleven subjects failed to attack. Also a correlation between lesion site and attack latency was determined: the more anterior the lesion, the longer mean elapsed time to the aggressive behavior.

The prefrontal cortex
Prefrontal cortex
The prefrontal cortex is the anterior part of the frontal lobes of the brain, lying in front of the motor and premotor areas.This brain region has been implicated in planning complex cognitive behaviors, personality expression, decision making and moderating correct social behavior...

 (PFC) has been implicated in aggressive psychopathology. Reduced activity of the prefrontal cortex, in particular its medial and orbitofrontal portions, has been associated with violent/antisocial aggression. Specifically, regulation of the levels of the neurotransmitter serotonin in the PFC has been connected with a particular type of pathological aggression, induced by subjecting genetically predisposed, aggressive, wild-type mice to repeated winning experience; the male mice selected from aggressive lines had lower serotonin tissue levels in the PFC than the low-aggressive lines in this study.


Various neurotransmitters and hormones have been shown to correlate with aggressive behavior. The most often mentioned of these is the hormone testosterone
Testosterone is a steroid hormone from the androgen group and is found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is primarily secreted in the testes of males and the ovaries of females, although small amounts are also secreted by the adrenal glands...


Many studies involving testosterone and aggression revolve around the study of birds. It is thought that testosterone plays an integral part of the territorial behavior within bird species. In particular, scientists are focusing on the fluctuation of testosterone which is mitigated by luteinizing hormone
Luteinizing hormone
Luteinizing hormone is a hormone produced by the anterior pituitary gland. In females, an acute rise of LH called the LH surge triggers ovulation and development of the corpus luteum. In males, where LH had also been called interstitial cell-stimulating hormone , it stimulates Leydig cell...

 (LH) during different seasons. Generally, mating behavior is demonstrated in the spring and accordingly, male birds show a sharp increase in LH as well as testosterone during this time. As referenced below, this acute rise in LH and testosterone can be attributed to the increased need for aggressive behaviors. The first need for aggressive behavior comes from the drive to establish territory. This typically occurs within the first few weeks of mating season. The second need for aggression occurs after the first clutch of eggs have been laid. The male, not only needs to guard the eggs, but also to guard his sexually receptive mate from other potential suitors. Thus, the male adopts an “alpha male status” when acquiring territory as well as during the egg laying period. This alpha male status, as mentioned before, comes from the significant increase of testosterone that occurs during the mating season. Further evidence of LH and testosterone mitigating aggression in bird species comes from studies on bird species such as the Song Sparrow
Song Sparrow
The Song Sparrow is a medium-sized American sparrow.Adults have brown upperparts with dark streaks on the back and are white underneath with dark streaking and a dark brown spot in the middle of the breast. They have a brown cap and a long brown rounded tail. Their face is grey with a streak...

 and the European Blackbird who build highly accessible refuges, known as open cup nests.

Because open cup nests can essentially be built anywhere, there is little competition when it comes to nest building sites. Accordingly, both the song sparrow and the European Blackbird do not show an increase in luteinizing hormone or testosterone during territory acquisition .
It should be duly noted however, that not all species of birds show increase levels of testosterone and LH during aggressive behavior. In a landmark study, it was found that Male Western Screech owls, when exposed to another male during the non-mating season showed aggressive behavior without the increase in LH and testosterone. However, when the owls were put in a situation that warranted aggressive behavior during the mating season, there was a large spike in LH and testosterone during the aggressive act. This suggests that the mechanisms of aggressive behavior during the mating and non-mating seasons are independent of each other or perhaps the increase in testosterone somehow increases the aggressive response during the mating season .

The idea that aggression is regulated by different mechanisms in the breeding and non-breeding season is interesting and many studies are being focused on this duality. Estradiol
Estradiol is a sex hormone. Estradiol is abbreviated E2 as it has 2 hydroxyl groups in its molecular structure. Estrone has 1 and estriol has 3 . Estradiol is about 10 times as potent as estrone and about 80 times as potent as estriol in its estrogenic effect...

 (E2), a type of non gonadal estrogen
Estrogens , oestrogens , or œstrogens, are a group of compounds named for their importance in the estrous cycle of humans and other animals. They are the primary female sex hormones. Natural estrogens are steroid hormones, while some synthetic ones are non-steroidal...

, seems to play a key role in regulating aggressive behavior during the non-mating season in several species of birds. As previously noted, many bird species during the non-mating season have low testosterone levels yet still manage to display aggression. As a primary example, when the Washington State Song Sparrow, a bird which shows fairly high levels of aggression during non-mating season despite low testosterone, is exposed to Fadrozole
Fadrozole is an aromatase inhibitor that has been introduced in Japan for the treatment of breast cancer.It is selective....

, an aromatase
Aromatase is an enzyme responsible for a key step in the biosynthesis of estrogens. It is a member of the cytochrome P450 superfamily , which are monooxygenases that catalyze many reactions involved in steroidogenesis. In particular, aromatase is responsible for the aromatization of androgens into...

 inhibitor, the levels of aggression are greatly decreased. When the E2 was replaced, the aggressive behaviors reestablished themselves thus confirming that E2 governs aggressive behavior during the non-mating season. It is unknown however if this is just specific to birds, or if this extends to other animal species.

These examples all culminate in a model known as the challenge hypothesis which focuses on the role of testosterone on aggression in the breeding season. It must be noted that the challenge hypothesis most likely cannot be applied to the non-breeding season since, like mentioned above, there is most likely a mechanism independent of testosterone governing aggression in the non-mating season. Perhaps if more sufficient evidence is found, the challenge hypothesis can be expanded to include other hormones and thus will be able to be applied to the non-breeding season as well.

Challenge Hypothesis

The “Challenge Hypothesis” describes the correlation between plasma testosterone levels and aggressive male-male interactions related to reproduction of various vertebrate species. It was first defined to describe the relationship between aggressive behavior and A sigmoidal
Sigmoidal or sigmoid, literally means S-shaped and can refer to:* Sigmoid function* Sigmoidal artery* Sigmoid colon...

 relationship between testosterone plasma levels and male-male aggression is observed under the challenge hypothesis when the birds’ testosterone levels were above seasonal breeding testosterone baseline levels. If birds remained at the seasonal breeding baseline levels during the breeding season, then there is not a significant difference observed in male-male aggression. In addition, there is a negative, sigmoidal relationship between testosterone levels in the birds and the amount of parental care provided when parents are above the seasonal breeding testosterone baseline levels. As such, the relationship between testosterone plasma levels and male-male aggression is context-specific to the species. Figure 2 and 3 describe the relationships observed of many single- or double-brooded bird species, from male western gulls to male turkeys.

Upon its recognition, the challenge hypothesis has been used to describe the testosterone levels in other species to certain social stimuli. The challenge hypothesis correctly predicts the testosterone influence on aggressive male-male interactions between male Northern Fence Lizards. This study reinforced the challenge hypothesis by showing rapid changes in aggressive behaviors of the lizards do not correlate with testosterone concentrations. Yet, over the mating season, the intensity of the behavior and the levels of testosterone levels yielded a positive correlation. Research has also shown the challenge hypothesis applies to specific monogamous fish species, with a greater correlation in species with stronger pair bonding.

In addition, the challenge hypothesis has been adapted to primates species. In 2004, Martin N. Muller and Richard W. Wragham applied a modified challenge hypothesis to chimpanzees. Similar to the original hypothesis, they predicted that there would be increased male-male aggressive interaction when a receptive and fertile female chimpanzee was present. Muller and Wragham also correctly predicted the testosterone levels of more dominant chimpanzees to be higher as compared to lower status chimpanzees. Therefore, chimpanzees significantly increased both testosterone levels and aggressive male-male interactions when receptive and fertile females presented sexual swellings. Currently, no research has specified a relationship between the modified challenge hypothesis and human behavior, yet, many testosterone/human behavior studies support the modified hypothesis applying to human primates.

Aggression and Testosterone Levels in Humans

Scientists have for a long time been interested in the relationship between testosterone and aggressive behavior. In most species, males are more aggressive than females. Castration
Castration is any action, surgical, chemical, or otherwise, by which a male loses the functions of the testicles or a female loses the functions of the ovaries.-Humans:...

 of males usually has a pacifying effect on aggressive behavior in males. Research on the relationship between testosterone and aggression is difficult since the only reliable measurement of brain testosterone is by a lumbar puncture
Lumbar puncture
A lumbar puncture is a diagnostic and at times therapeutic procedure that is performed in order to collect a sample of cerebrospinal fluid for biochemical, microbiological, and cytological analysis, or very rarely as a treatment to relieve increased intracranial pressure.-Indications:The...

 which is not done for research purposes. Studies therefore have often instead used more unreliable measurements from blood or saliva.

In humans, males engage in crime and especially violent crime more than females. The involvement in crime usually rises in the early teens to mid teens which happen at the same time as testosterone levels rise. The Handbook of Crime Correlates, a review of crime studies, states most studies support a link between adult criminality and testosterone although the relationship is modest if examined separately for each sex. However, nearly all studies of juvenile delinquency and testosterone are not significant. In adulthood, it is clear that testosterone is not related to any consistent methods of measuring aggression on personality scales, but several studies of the concentration of blood testosterone of convicted male criminals who committed violent crimes compared to males without a criminal record or who committed non-aggressive crimes revealed in most cases that men who were judged aggressive/dominant had higher blood concentrations of testosterone than controls. Interestingly, testosterone levels in female criminals versus females without a criminal record mirror those of males: testosterone levels are higher in women who commit aggressive crimes or are deemed aggressive by their peers than non-aggressive females.

Most studies have also found testosterone to be associated with behaviors or personality traits linked with criminality such as antisocial behavior
Antisocial personality disorder
Antisocial personality disorder is described by the American Psychiatric Association's Diagnostic and Statistical Manual, fourth edition , as an Axis II personality disorder characterized by "...a pervasive pattern of disregard for, and violation of, the rights of others that begins in childhood...

 and alcoholism
Alcoholism is a broad term for problems with alcohol, and is generally used to mean compulsive and uncontrolled consumption of alcoholic beverages, usually to the detriment of the drinker's health, personal relationships, and social standing...

. Many studies have also been done on the relationship between more general aggressive behavior/feelings and testosterone. About half the studies have found a relationship and about half no relationship. In one source, it was noted that concentration of testosterone most clearly correlated with aggressive responses involving provocation. However, a correlation between testosterone levels and aggression does not prove a causal role for testosterone.

Studies of testosterone levels of male athletes before and after a competition revealed that testosterone levels rise shortly before their matches, as if in anticipation of the competition, and are dependent on the outcome of the event: testosterone levels of winners are high relative to those of losers. No specific response of testosterone levels to competition was observed in female athletes, although a mood difference was noted. Testosterone has been shown to correlate with aggressive behavior in mice. In addition, some experiments have failed to find a relationship between testosterone levels and aggression in humans.

The possible correlation between testosterone and aggression could explain the "roid rage" that can result from anabolic steroid
Anabolic steroid
Anabolic steroids, technically known as anabolic-androgen steroids or colloquially simply as "steroids", are drugs that mimic the effects of testosterone and dihydrotestosterone in the body. They increase protein synthesis within cells, which results in the buildup of cellular tissue ,...

 use, although an effect of abnormally high levels of steroids does not prove an effect at physiological levels.

A line of research has focused more on the effects of circulating testosterone on the nervous system mediated by local metabolism within the brain. Testosterone can be metabolized to 17b-estradiol by the enzyme aromatase
Aromatase is an enzyme responsible for a key step in the biosynthesis of estrogens. It is a member of the cytochrome P450 superfamily , which are monooxygenases that catalyze many reactions involved in steroidogenesis. In particular, aromatase is responsible for the aromatization of androgens into...

 or to 5a-dihydrotestosterone by 5a-reductase. Aromatase is highly expressed in regions involved in the regulation of aggressive behavior, such as the amygdala and hypothalamus. In studies using genetic knock-out techniques in inbred mice, male mice that lacked a functional aromatase enzyme displayed a marked reduction in aggression. Long-term treatment of these mice with estradiol partially restored aggressive behavior, suggesting that the neural conversion of circulating testosterone to estradiol and its effect on estrogen receptors affects inter-male aggression. Also, two different estrogen receptors, ERa and ERb, have been identified as having the ability to exert different effects on aggression. In studies using estrogen receptor knockout mice, individuals lacking a functional ERa displayed markedly reduced inter-male aggression while male mice that lacked a functional ERb exhibited normal or slightly elevated levels of aggressive behavior. These results imply that ERa facilitates male–male aggression, whereas ERb may inhibit aggression. However, different strains of mice show the opposite pattern in that aromatase activity is negatively correlated with aggressive behavior. Also, in a different strain of mice the behavioral effect of estradiol
Estradiol is a sex hormone. Estradiol is abbreviated E2 as it has 2 hydroxyl groups in its molecular structure. Estrone has 1 and estriol has 3 . Estradiol is about 10 times as potent as estrone and about 80 times as potent as estriol in its estrogenic effect...

 is dependent on daylength: under long days (16 h of light) estradiol reduces aggression, and under short days (8 h of light) estradiol rapidly increases aggression.

Possible Neuronal Mechanisms

Many studies have investigated various brain areas to find out possible neuronal mechanisms through which testosterone influences aggressive behavior. One hypothesis is that testosterone influences a specific brain area to control behavioral reactions. Studies in animal models indicate that aggression is affected by several interconnected cortical and subcortical structures within the so-called social behavior
Social behavior
In physics, physiology and sociology, social behavior is behavior directed towards society, or taking place between, members of the same species. Behavior such as predation which involves members of different species is not social...

 network. A study involved in lesion and electrical-chemical stimulation in rodents and cats revealed that such a neural network consists of the medial amygdala
The ' are almond-shaped groups of nuclei located deep within the medial temporal lobes of the brain in complex vertebrates, including humans. Shown in research to perform a primary role in the processing and memory of emotional reactions, the amygdalae are considered part of the limbic system.-...

, medial hypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...

 and periaqueductal grey (PAG), and it positively modulates reactive aggression. Moreover, a study done in human subjects showed that prefrontal-amygdala connectivity is modulated by endogenous testosterone during social emotional behavior . Lower endogenous testosterone levels were associated with greater activity in the Ventrolateral Prefrontal Cortex
Prefrontal cortex
The prefrontal cortex is the anterior part of the frontal lobes of the brain, lying in front of the motor and premotor areas.This brain region has been implicated in planning complex cognitive behaviors, personality expression, decision making and moderating correct social behavior...

 (VLPFC) and Frontal Pole (FP) during the affect-incongruent trials. In addition, fMRI scanning during the trials showed that testosterone influenced the coupling between left VLPFC/FP and right amygdala.
In human studies, extensive aggression research has focused on the role of the orbitofrontal cortex
Orbitofrontal cortex
The orbitofrontal cortex is a prefrontal cortex region in the frontal lobes in the brain which is involved in the cognitive processing of decision-making...

 (OFC). This brain area is strongly associated with impulse control and self-regulation systems that integrate emotion, motivation, and cognition to guide context-appropriate behavior. Patients with localized lesions to the OFC engage in heightened reactive aggression . Another study also supports the role of the OFC in aggressive behavior regulated by testosterone due to reduced medial OFC engagement following social provocation. When measuring participants’ salivary testosterone, higher levels can predict subsequent aggressive behavioral reactions to unfairness faced during the task. Moreover, brain scanning with fMRI during the task showed reduced activity in the medial OFC. Such findings may suggest that a specific brain region, the OFC, is a key factor in understanding reactive aggression.

Other neurotransmitters and hormones

Glucocorticoids also play an important role in regulating aggressive behavior. In adult rats, acute injections of corticosterone
Corticosterone is a 21-carbon steroid hormone of the corticosteroid type produced in the cortex of the adrenal glands.-Roles:In many species, including amphibians, reptiles, rodents and birds, corticosterone is a main glucocorticoid, involved in regulation of fuel, immune reactions, and stress...

 promote aggressive behavior and acute reduction of corticosterone decreases aggression; however, a chronic reduction of corticosterone levels can produce abnormally aggressive behavior. In addition, glucocorticoids affect development of aggression and establishment of social hierarchies. Adult mice with low baseline levels of corticosterone are more likely to become dominant than are mice with high baseline corticosterone levels.

5-Dehydroepiandrosterone is a 19-carbon endogenous steroid hormone. It is the major secretory steroidal product of the adrenal glands and is also produced by the gonads and the brain. DHEA is the most abundant circulating steroid in humans....

 (DHEA) is the most abundant circulating androgen and can be rapidly metabolized within target tissues into potent androgens and estrogens. Gonadal steroids generally regulate aggression during the breeding season, but non-gonadal steroids may regulate aggression during the non-breeding season. Castration of various species in the non-breeding season has no effect on territorial aggression. In several avian studies, circulating DHEA has been found to be elevated in birds during the non-breeding season. These data support the idea that non-breeding birds combine adrenal and/or gonadal DHEA synthesis with neural DHEA metabolism to maintain territorial behavior when gonadal testosterone secretion is low. Similar results have been found in studies involving different strains of rats, mice, and hamsters. DHEA levels also have been studied in humans and may play a role in human aggression. Circulating DHEAS (its sulfated ester) levels rise during adrenarche (~7 years of age) while plasma testosterone levels are relatively low. This implies that aggression in pre-pubertal children with aggressive conduct disorder might be correlated with plasma DHEAS rather than plasma testosterone, suggesting an important link between DHEAS and human aggressive behavior.

Another chemical messenger with implications for aggression is the neurotransmitter serotonin
Serotonin or 5-hydroxytryptamine is a monoamine neurotransmitter. Biochemically derived from tryptophan, serotonin is primarily found in the gastrointestinal tract, platelets, and in the central nervous system of animals including humans...

. In various experiments, serotonin action was shown to be negatively correlated with aggression (Delville et al. 1997). This correlation with aggression helps to explain the aggression-reducing effects of selective serotonin reuptake inhibitors such as fluoxetine
Fluoxetine is an antidepressant of the selective serotonin reuptake inhibitor class. It is manufactured and marketed by Eli Lilly and Company...

 (Delville et al. 1997), aka prozac.

While serotonin and testosterone have been the two most-researched chemical messengers with regards to aggression, other neurotransmitters and hormones have been shown to relate to aggressive behavior as well. The neurotransmitter vasopressin
Arginine vasopressin , also known as vasopressin, argipressin or antidiuretic hormone , is a neurohypophysial hormone found in most mammals, including humans. Vasopressin is a peptide hormone that controls the reabsorption of molecules in the tubules of the kidneys by affecting the tissue's...

 causes an increase in aggressive behavior when present in large amounts in the anterior hypothalamus (Delville et al. 1997). The effects of norepinephrine
Norepinephrine is the US name for noradrenaline , a catecholamine with multiple roles including as a hormone and a neurotransmitter...

, cortisol, and other neurotransmitters are still being studied.


In a nonmammilian example, the fruitless gene in Drosophila melanogaster
Drosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...

is a critical determinant for how fruit flies fight. Patterns of aggression can be switched, with males using female patterns of aggression or females using male patterns, by manipulating either the fruitless or transformer genes in the brain. Candidate genes for differentiating aggression between the sexes are the Sry (sex determining region Y) gene, located on the Y chromosome and the Sts (steroid sulfatase) gene. The Sts gene encodes the steroid sulfatase enzyme, which is pivotal in the regulation of neurosteroid biosynthesis. It is expressed in both sexes, is correlated with levels of aggression among male mice, and increases dramatically in females after parturition and during lactation, corresponding to the onset of maternal aggression.


There has been some links between those prone to violence and their alcohol use. Those who are prone to violence and use alcohol are more likely to carry out violent acts. For example, Ted Bundy
Ted Bundy
Theodore Robert "Ted" Bundy was an American serial killer, rapist, kidnapper, and necrophile who assaulted and murdered numerous young women during the 1970s, and possibly earlier...

, an inherently violent individual, became more violent with his murders after much alcohol abuse.

See also

  • Aggressionism
    Aggressionism is the philosophical theory that the only real cause of war is human aggressiveness. This theory has dominated much evolutionary thought about human nature. Nonetheless, many evolutionary biologists discount aggressionism as it promotes extinction....

  • Aggressive narcissism
  • Anger
    Anger is an automatic response to ill treatment. It is the way a person indicates he or she will not tolerate certain types of behaviour. It is a feedback mechanism in which an unpleasant stimulus is met with an unpleasant response....

  • Conflict (disambiguation)
    Conflict (disambiguation)
    - Movies and television :* Conflict , a boxing film starring John Wayne* Conflict , a suspense film starring Humphrey Bogart* Conflict , a 1956 American television series- Games :...

  • Frustration-Aggression Hypothesis
    Frustration-Aggression Hypothesis
    The frustration–aggression hypothesis, otherwise known as the frustration–aggression–displacement theory, attempts to explain why people scapegoat...

  • Genetics influencing aggression
    Genetics Influencing Aggression
    The field of psychology has been greatly influenced by the study of genetics. Decades of research has demonstrated that both genetic and environmental factors play a role in a variety of behaviors in humans and animals . The genetic basis of aggression, however, remains poorly understood...

  • Parental abuse by children
  • Passive aggressive behavior
  • Relational aggressive behaviour
    Relational aggression
    Relational aggression, also known as covert aggression or covert bullying, is a type of aggression in which harm is caused through damage to relationships or social status within a group rather than by means of actual or threatened physical violence...

  • Resource holding potential
    Resource holding potential
    In biology, resource holding potential is the ability of an animal to win an all-out fight if one were to take place.The term was coined by Geoff Parker to disambiguate physical fighting ability from the motivation to persevere in a fight . Originally the term used was 'Resource Holding Power',...

  • School violence
    School violence
    School violence is widely held to have become a serious problem in recent decades in many countries, especially where weapons such as guns or knives are involved...

  • Social defeat
    Social defeat
    Social defeat refers to losing a confrontation among conspecific animals, or any kind of hostile dispute among humans, in either a dyadic or in a group-individual context, generating very significant consequences in terms of control over resources, access to mates and social...

External links

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