In psychology Psychology is the scientific study of human or other animal mental functions and behaviors. In this field, a professional practitioner or researcher is called a psychologist. Psychologists are classified as social or behavioral scientists. Psychological research can be considered either basic or applied. Psychologists attempt to understand the, as well as other social The social sciences are the fields of academic scholarship that explore aspects of human society. "Social science" is commonly used as an umbrella term to refer to a plurality of fields outside of the natural sciences. These include: anthropology, archaeology, economics, geography, history, linguistics, political science, international and behavioral sciences The term behavioural sciences encompasses all the disciplines that explore the activities of and interactions among organisms in the natural world. It involves the systematic analysis and investigation of human and animal behaviour through controlled and naturalistic experimental observations and rigorous formulations. (E. D. Klemke, R. Hollinger,, aggression (also called combativeness) refers to behavior between members of the same species In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are often used, such as based on similarity of DNA or that is intended to cause pain or harm. Predatory In ecology, predation describes a biological interaction where a predator feeds on its prey (the organism that is attacked). Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption. The other main behavior between members of one species towards another species is also described as "aggression." To exhibit aggression towards members of another species is common, such as in these examples: "Lions The lion is one of the four big cats in the genus Panthera, and a member of the family Felidae. With some males exceeding 250 kg (550 lb) in weight, it is the second-largest living cat after the tiger. Wild lions currently exist in Sub-Saharan Africa and in Asia with a critically endangered remnant population in Gir Forest National Park in India, are aggressive hunters of antelopes Antelope is a term referring to many even-toed ungulate species found all over the world in places such as Africa, Asia and North America. The term refers to a ‘miscellaneous’ group within the family encompassing the old-world species which are not cattle, sheep, buffalo, bison,or goats. A group of antelope is called a herd," and "Eagles Eagles are large birds of prey which are members of the bird family Accipitridae, and belong to several genera which are not necessarily closely related to each other. Most of the more than 60 species occur in Eurasia and Africa. Outside this area, just two species can be found in the United States and Canada, nine more in Central and South are aggressive hunters of small mammals Mammals are a class of vertebrate, air-breathing animals whose females are characterized by the possession of mammary glands while both males and females are characterized by hair and/or fur, three middle ear bones used in hearing, and a neocortex region in the brain. Some mammals have sweat glands, but most do not."

Aggression takes a variety of forms among human beings Humans commonly refers to the species Homo sapiens , the only extant member of the Homo genus of bipedal primates in Hominidae, the great ape family. However, in some cases the term is used to refer to any member of the genus Homo, and it can be physical, mental, or verbal. Aggression should not be confused with assertiveness Assertiveness is a behavioral skill taught by many personal development experts and behavior therapists as well as cogntive behavior therapists. It is linked to self-esteem and considered an important communication skill[according to whom?]. It was orginally explored by Joseph Wolpe in his book on treating neurosis. It is commonly employed as an, 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 aggression and instrumental, predatory, or goal-oriented aggression.^[1][2][3][4] Empirical research indicates that there is a critical difference between the two, both psychologically and physiologically. Some research indicates that people with tendencies toward "affective" aggression, defined in this study as being "impulsive, unplanned, overt, or uncontrolled" have lower IQs than those with tendencies toward "predatory" aggression, defined here as being "goal-oriented, planned, hidden, or controlled".^[1]

Across many different human cultures, men are more likely than women to express aggression by means of direct physical violence. Women are more likely to instead express aggression through a variety of indirect or nonphysical means.

Evolution

Like most behaviors, 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.^[5]

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, a new organism produced by one or more parents. Because aggression against a much larger enemy or group of enemies would lead to the death of an animal, animals have developed a good sense of when they are outnumbered.^[6] 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 selfless concern for the welfare of others. It is a traditional virtue in many cultures, and a core aspect of various religious traditions such as Judaism, Christianity, Islam, Hinduism, Jainism, Buddhism, Confucianism, Sikhism, and many others. Altruism is the opposite of selfishness. 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.^[7]

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.^[8]

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.^[9] In general, the more dominant animals will be more aggressive than their subordinates. ^{[10] [11]} The majority of conspecific aggression ceases about 24 hours after the introduction of the animals being tested. ^{[9] [12]}

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 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 . The time interval of a gestation plus 2 weeks is called gestation period, and the length of time plus 2 weeks that the offspring have spent developing in the uterus is called gestational age and lactation Lactation describes the secretion of milk from the mammary glands, the process of providing that milk to the young, and the period of time that a mother lactates to feed her young. The process occurs in all female mammals, and in humans it is commonly referred to as breastfeeding or nursing. In most species milk comes out of the mother's nipples; periods, 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 well being 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 human, 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 ^[13]. 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^[14].

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^[15]. A wide variety of studies have been done on these situations.

Culture

Culture is a distinctly human factor that plays a role in aggression. Kung Bushmen The ǃKung, or ǃXũ as it is also spelled in English, also called the Juǀʼhoansi, are a people living in the Kalahari Desert in Namibia, Botswana and in Angola. They speak the ǃKung language, noted for using click consonants, generally classified as part of the Khoisan language family. To pronounce "ǃKung" one must make a click 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 Bushmen is actually higher than that of most modern industrial societies (Keeley, 1996). Lawrence Keeley argues that the "peaceful savage" is a myth that is unsupported by the bulk of anthropological and archeological Archaeology, or archeology , is the study of past human societies, primarily through the recovery and analysis of the material culture and environmental data which they have left behind, which includes artifacts, architecture, biofacts and cultural landscapes. Due to the fact that archaeology employs a wide range of different procedures, it can be evidence. Hunter gatherer societies do not have possessions to fight over, but they may still come to conflict over status and mating opportunities.

Empirical Empirical research is research that derives its data by means of direct observation or experiment, such research is used to answer a question or test a hypothesis . The results are based upon actual evidence as opposed to theory or conjecture, as such they can be replicated in follow-up studies. Empirical research articles are published in peer- 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 The Southern United States—commonly referred to as the American South, Dixie, Down South, or simply the South—constitutes a large distinctive region in the southeastern and south-central United States. Because of the region's unique cultural and historic heritage, including Native Americans, early European settlements of English, Ulster Scots, 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). Changes in dominant behavior or in social status causes changes in testosterone levels. Reports of changes in testeosterone of young men during athletic events, which involve face-to-face competition with a winner and a loser, reveal that testosterone rises shortly before their matches, as if in anticipation of the competition. Also, one to two hours after the competitive match, the testosterone levels of the winners are high relative to those levels of the losers ^[16]. It is also important to take into account the type of conflict that is occurring when assessing aggression. Is the conflict between groups, within a group, within a family? The sex of those involved in the conflict is also critical. Male-male, male-female and female-female encounters should all be clearly distinguished from one another. Same sex encounters are more frequent than inter-sex encounters and this could affect the level of aggression present ^[17].

Media

Behaviors like aggression can be learned by watching and imitating the behavior of others. A considerable amount of evidence suggests that watching violence on television increases the likelihood of short-term aggression in children (Aronson, Wilson, & Akert, 2005), though for a dissenting viewpoint, see Freedman (2002). Individuals may differ in how they respond to violence. The greatest impact is on those who are already prone to violent behavior. Adults may be influenced by violence in media as well. A long-term study of over 700 families found "a significant association" between the amount of time spent watching violent television as a teenager and the likelihood of committing acts of aggression later in life. The results remained the same in spite of factors such as family income, parental education and neighborhood violence (Aronson, Wilson, & Akert, 2005).

Although exposure to violence in media is associated with likelihood of short-term increases in aggression, none of these studies provide evidence for a definitive causal mechanism. Instead, violence in media may be one of many factors, or it may play a maintenance role since violent media tend to be selected by people who are prone to violence.

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.

Pain and discomfort also increase aggression. Even the simple act of placing ones hands in warm 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^{[clarification needed]}).

Frustration is another major cause of aggression. The 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 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^{[clarification needed]}).

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 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 The Milgram experiment on obedience to authority figures was a series of 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 conflicted with their personal conscience. Milgram first. 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 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.

A new proposal links military experience to anger and aggression, thus creating serial killers. Castle and Hensley state, “The military provides the social context where servicemen learn aggression, violence, and murder”.^[18] Post Traumatic Stress Disorder Posttraumatic stress disorder is a severe anxiety disorder that can develop after exposure to any event which 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, overwhelming the individual's ability to cope. As an (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 flash backs and nightmares, causing severe stress. This can be contributed to serial killing, as well; however, these studies are still being further investigated.

Gender

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, 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 physical violence. Relational aggression is more common and more studied among girls than boys and 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. Furthermore, rejection can be either active, by bullying, teasing, or, 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 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, comprised primarily of raised coral limestone. Bellona Island is densely populated and its interior is lush and fertile. There, a culture based on male dominance and physical violence Violence is the expression of physical or verbal force against self or other, compelling action against one's will on pain of being hurt. Worldwide, violence is used as a tool of manipulation and also is an area of concern for law and culture which take attempts to suppress and stop it. The word violence covers a broad spectrum. It can vary from, 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 ^[17].

In children

The frequency of physical aggression in humans peaks at around 2–3 years of age. It then declines gradually on average.^[19] 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 ^[20]. 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.
• 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
• Family difficulties
• Learning, neurological, or conduct/behavior disorders
• Emotional trauma

Corporal punishment Corporal punishment is 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. The term usually refers to methodically striking the offender with an implement, whether in judicial, domestic, or educational settings such as spanking increases subsequent aggression in children.^[21]

The Bobo doll experiment Bandura hoped that the experiment would prove that aggression can be explained, at least in part, by social learning theory. The theory of social learning would state that that behaviour such as aggression is learned through observing and imitating of others. Social learning theory would state behaviours like aggression have a social and cognitive was conducted by Albert Bandura in 1961. In this work, Bandura found that children exposed to 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.^[22].

Summaries of best practice recommendations

1. American Academy of Pediatrics : '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.'^[23]
2. National Association of School Psychologists: '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.' ^[24]

In female youth

A web search for “girl fights” or “news about girl violence” yields thousands of links to videos of and news stories about girls fighting intensely with each other. Public perception is that these events are ever increasing, girls are getting out of control, and violence and aggression are no longer predominately the realm of boys.^{[citation needed]} Statistics of youth violence lend some support to these concerns. Although boys continue to commit the majority of violent offences, in Canada between 1994 and 1998 the rate of violent crime committed by female youth increased steadily. In 1994 4,882 females age 12-17 were charged with a violent crime, while this number rose to 5,652 in 1998 ^[25]. Similarly in the U.S., arrests of female youth for simple assault, aggravated assault, and offences included in the Violent Crime Index (homicide, forcible rape, robbery, aggravated assault) all increased from 1980 to 2000 ^[26]. More recent statistics are also concerning: in 2008 the U.S. National Survey on Drug Use and Health found that 18.6% of females age 12-17 got into a serious fight at school or work, 14.1% participated in a group-against-group fight, and 5.7% attacked another with the intent to seriously harm ^[27]. These statistics have increased research interest on this issue and on how best to prevent and intervene on girls’ use of aggression and violence.

Aggressive and violent behavior not only harms the victim but is also associated with negative outcomes and other risky behaviors for the aggressive individual. Adolescent females who are not enrolled in or attending school are more likely to engage in violent behavior than those who are in school^[27]. Moreover, there is an increase in violent behavior as school grades decrease, and violent adolescent females are also more likely to engage in binge drinking and illicit substance use than non-violent females^[27].

There is some suggestion that the causes and correlates of aggression in female youth are different from those in males. Ellickson, Saner, and McGuigan (1997)^[28] found that violent female high school students were more likely to report poorer mental health than violent males. Violent females report engaging in self-harm^[29], and unpopularity and depression may be more likely to be reported by aggressive female children than aggressive male children^[30]. Based on teacher reports, overtly aggressive females are perceived to be more maladjusted than overtly aggressive males^[31]. There is also evidence for differential gender effects of gender-neutral school-based violence prevention programs. A universal violence prevention program for seventh-graders resulted in stronger support of nonviolence and less acceptance of the use of violence in males but not in females ^[32]. Similarly a program aimed at improving social competency reduced physical fighting in boys but not in girls ^[33].

Aggressive and violent female youth is a growing issue and warrants attention. Despite this there are very few school-based violence prevention or intervention programs for females specifically. Although there are numerous school-based unisex programs for a variety of ages, only one school-based female-targeted social aggression prevention program has been developed^[34], and one group-counseling relational aggression intervention for female youth living in a residential home^[35]. Thus, there is a need for a school-based female-specific violence prevention programs aimed at improving self-confidence, self-efficacy, and social skills^[36]. An effective program should also incorporate gender-specific content such as discussions about what it means to be a girl, societal expectations of being a girl and the roles girls are expected to play, and how girls communicate and interact with each other^[35].

Biology

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 encoded by pheromones 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. There are alarm pheromones, food trail pheromones, sex pheromones, and many others that. In mice, Major urinary proteins (Mups) have been demonstrated to promote innate aggressive in behavior males.^{[37] [38]} Mups were demonstrated to activate olfactory sensory neurons in the 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 receptors 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. In response to stimuli the sensory receptor initiates sensory transduction by creating graded potentials or action potentials in the same cell or in an adjacent one, of mice ^[38] and rats.^[39]

The hypothalamus The hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions of the hypothalamus is to link the nervous system to the endocrine system via the pituitary gland and periaqueductal gray Periaqueductal gray is the gray matter located around the cerebral aqueduct within the midbrain. It plays a role in the descending modulation of pain and in defensive behaviour. The ascending pain and temperature fibers of the spinothalamic tract also send information to the PAG via the spinomesencephalic tract. The spinomesencephalic tract is so- 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^[40] the hypothalamus expresses receptors that help determine aggression levels based on their interactions with the neurotransmitters serotonin and vasopressin.^[41]

The amygdala The amygdalae 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,^[42][43] while lesions of an evolutionarily homologous In evolutionary biology, homology refers to any similarity between characteristics of organisms that is due to their shared ancestry. The word homologous derives from the ancient Greek ομολογειν, 'to agree'. There are examples in different branches of biology. Anatomical structures that perform the same function in different biological area in the lizard greatly reduce competitive drive and aggression^[44](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, c-Fos is a protein that in humans is encoded by the FOS gene 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 ^[45].

The 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 (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 ^[46].

Neurotransmitters and hormones

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. It is the principal male sex hormone and an anabolic steroid. In one source, it was noted that concentration of testosterone most clearly correlated with aggressive responses involving provocation. 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. 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. 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. However, no specific response of testosterone levels to competition was observed in female athletes, although a mood difference was noted^[47]. Testosterone has been shown to correlate with aggressive behavior in mice and in some humans,^[48] but in contrast to some long-standing theories, various experiments have failed to find a relationship between testosterone levels and aggression in humans.^[49][50][51] The possible correlation between testosterone and aggression could explain the "roid rage" that can result from anabolic steroid Anabolic steroids, officially known as anabolic-androgenic steroids , are drugs which mimic the effects of the male sex hormones testosterone and dihydrotestosterone. They increase protein synthesis within cells, which results in the buildup of cellular tissue (anabolism), especially in muscles. Anabolic steroids also have androgenic and use^[52][53], although an effect of abnormally high levels of steroids does not prove an effect at physiological levels.

Another 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 that is responsible for a key step in the biosynthesis of estrogens. Because estrogens also promote certain cancers and other diseases, aromatase inhibitors are frequently used to treat those diseases 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 A knockout mouse is a genetically engineered mouse in which one or more genes have been turned off through a gene knockout. Knockout mice are important animal models for studying the role of genes which have been sequenced, but have unknown functions. By causing a specific gene to be inactive in the mouse, and observing any differences from normal, 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 (also oestradiol) is a sex hormone. Estradiol is the predominant sex hormone present in females. It is also present in males, and at a higher level because it is being constantly produced. In females it is only produced 3 out of 30 days of the cycle. It represents the major estrogen in humans. Estradiol has not only a critical impact on is dependent on daylength: under long-days (16h of light) estradiol reduces aggression, and under short-days (8h of light) estradiol rapidly increases aggression ^[54].

Glucocorticoids Glucocorticoids are a class of steroid hormones that bind to the glucocorticoid receptor (GR), which is present in almost every vertebrate animal cell. The name glucocorticoid (glucose + cortex + steroid) derives from their role in the regulation of the metabolism of glucose, their synthesis in the adrenal cortex, and their steroidal structure ( 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 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 ^[54].

Dehydroepiandrosterone 5-Dehydroepiandrosterone is an endogenous steroid that has been implicated in a broad range of biological effects in humans and other mammals. DHEA is produced by the adrenal glands, gonads, and the brain. It acts on the androgen receptor both directly and through its metabolites, which include androstenediol and androstenedione, which can undergo (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 ^[54].

Another chemical messenger with implications for aggression is the neurotransmitter serotonin Serotonin or 5-Hydroxytryptamine (5-HT) is a monoamine neurotransmitter, biochemically derived from tryptophan, that is primarily found in the gastrointestinal (GI) tract, platelets, and central nervous system (CNS) of humans and animals. It is a well-known contributor to feelings of well-being. In various experiments, serotonin action was shown to be negatively correlated with aggression^[55](Delville et al. 1997). This correlation with aggression helps to explain the aggression-reducing effects of selective serotonin reuptake inhibitors such as fluoxetine EU EMA:link, US FDA:link (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 causes an increase in aggressive behavior when present in large amounts in the anterior hypothalamus (Delville et al. 1997). The effects of norepinephrine, cortisol, and other neurotransmitters are still being studied.

Genetics

In a nonmammilian example, the fruitless gene in Drosophila melanogaster 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^[45].

Alcohol

There has been some links between those who prone to violence and their alcohol use. Those who are prone to violence and use alcohol are more likely to carry out violent acts.^[56] For example, Ted Bundy, an inherently violent individual, became more violent with his murders after much alcohol abuse. ^[57]

See also

• Aggressionism
• Anger
• Conflict
• Genetics influencing aggression
• Passive aggressive behavior
• Relational aggressive behaviour
• Resource holding potential
• School violence
• Social defeat

Notes

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