THEORIES OF AGGRESSION
Aggression is a form of self-assertion (putting yourself forward or standing up for yourself). However, it is done through causing harm to yourself, to other people or to your environment. This can be physical harm involving breakages and bruises or mental harm, involving fear and anxiety. Physical aggression is violence and social aggression includes rumour spreading, insults and breaking off friendships. Threatening someone is aggressive because it causes them fear and anxiety, even if no violence occurs.
Aggression can take many forms but it is defined as any action that is aimed at causing either physical and/or psychological pain to oneself to others or to objects in the environment. The expression of aggression can occur in a number of ways, including verbally, mentally and physically - Charlotte Thomas (2012) There are two camps when it comes to the psychology of aggression: the nature camp and the nurture camp.
Nativists (nature) argue that aggression is innate - it comes from within us. We are born with aggressive urges which never entirely go away, although self-discipline and a good upbringing might help us to control or re-direct these urges.
Nurturists (nurture) argue that aggression comes from our environment and no one is born aggressive. Aggressive behaviour is learned or else produced by social pressures. Put anyone in the right situation and they will behave aggressively, but anybody's aggressive behaviour can be reduced or removed if they are put in better surroundings.
Do we teach boys sports like rugby so that they can get rid of their innate aggression safely and productively... or are we teaching boys to be aggressive who wouldn't be otherwise?
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THE NEUROPSYCHOLOGY OF AGGRESSION
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A famous example is Phineas Gage, an American railway worker who suffered a terrible accident in 1848. The railway workers used dynamite to make the ground flat for laying the rail tracks, but an unexpected explosion nearly killed Gage. It blasted a "tamping iron" (a metre-long iron nail) through Gage's skull; the iron entered through Gage's cheek, passed through his brain and shot out of the top of his head.
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Amazingly, Gage survived the accident, thanks to treatment from Dr John Martyn Harlow. However, his personality greatly changed. In his case study, Harlow reported that Gage became
fitful, irreverent, indulging at times in the grossest profanity (which was not previously his custom), manifesting but little deference for his fellows, impatient of restraint or advice when it conflicts with his desires - John Harlow (1868)
After he died, 12 years later, Gage's skull was preserved and studied. Modern computer-assisted design has reconstructed the damage to his brain: it was damage to the frontal lobe, which is responsible for decision-making and self-restraint.
However, the Phineas Gage case is not as clear-cut as it sounds. It is discussed in more detail on the page on Individual Differences (Personality)
Another case study is Charles Whitman, a Texan marine who, in 1966, murdered his family then shot a dozen strangers in a killing spree, before taking his own life. Whitman left a suicide note:
I don’t really understand myself these days. I am supposed to be an average reasonable and intelligent young man. However, lately (I can’t recall when it started) I have been a victim of many unusual and irrational thoughts. |
Before the shootings, Whitman had visited 5 doctors and a psychiatrist for help with his urges.
I talked with a Doctor once for about two hours and tried to convey to him my fears that I felt overwhelming violent impulses
The psychiatrist noted in his records that Whitman was "oozing with hostility". When an autopsy was carried out after the shootings, Whitman was found to have a brain tumour the size of a pecan nut pressing on his amygdala. A Commission that investigated the case concluded that this brain damage may have been responsible for Whitman's behaviour.
THE CEREBRAL CORTEX
As the Phineas Gage case suggests, the brain's frontal lobe plays an important part in decision-making and self-restraint. In particular, a region called the pre-frontal cortex seems to be particularly important. If it is damaged or malfunctions, aggression is a possible side-effect.
Another brain structure that plays a role in aggression is the corpus callosum that links the left and right hemispheres. The two hemispheres need to communicate over long-term planning and thinking through consequences. Damage to the corpus callosum might also lead to more reckless behaviour.
In the Classic Study by Raine et al. (1997), brain scans of a group of murderers revealed they had much less activity in the frontal lobe and corpus callosum compared to a control group of non-murderers.
Another brain structure that plays a role in aggression is the corpus callosum that links the left and right hemispheres. The two hemispheres need to communicate over long-term planning and thinking through consequences. Damage to the corpus callosum might also lead to more reckless behaviour.
In the Classic Study by Raine et al. (1997), brain scans of a group of murderers revealed they had much less activity in the frontal lobe and corpus callosum compared to a control group of non-murderers.
It's important to note that damage to the frontal lobe or corpus callosum doesn't create aggression itself. It just makes you less self-controlled and more inclined to act on the spur of the moment, especially in unfamiliar or confusing situations. This MAY turn out to mean aggressive behaviour, but it doesn't have to.
THE LIMBIC SYSTEM
The limbic system is a sub-cortical area - part of the "old brain" that we share with other animals. It is also the brain's emotion centre where our most basic urges and desires (appetite, sleep, sex drive, fear) are regulated. For example, the thalamus is the brain's "switchboard" which receives signals and sends messages out to all the other areas. The hypothalamus has an important role in producing hormones.
However, the most important part of the limbic system for understanding aggression is the amygdala. The amygdala takes information from the thalamus and interprets it as a threat or not; it produces fear or aggression, the famous "fight or flight" response. Of course, if the amygdala malfunctions, then things which are threatening will not produce a fear response - or else harmless events will be interpreted as a threat, producing aggression. The case of Charles Whitman might be an illustration of this.
However, the most important part of the limbic system for understanding aggression is the amygdala. The amygdala takes information from the thalamus and interprets it as a threat or not; it produces fear or aggression, the famous "fight or flight" response. Of course, if the amygdala malfunctions, then things which are threatening will not produce a fear response - or else harmless events will be interpreted as a threat, producing aggression. The case of Charles Whitman might be an illustration of this.
This 10-minute video looks at the link between the amygdala and the pre-frontal cortex. It also includes Adrian Raine talking about his famous study.
The relationship between the amygdala and the frontal lobe is very important. If the pre-frontal cortex is healthy, then willpower can resist the amygdala's urges of fear or aggression. In Adrian Raine's murderers, the amygdala behaved erratically and the pre-frontal cortex was under-active - a dangerous combination.
Animal studies also support the link between the limbic system and aggression. John Flynn carried out studies on cats in the 1960s, using electrodes to stimulate the amygdala directly. Egger and Flynn (1963) introduced a rat to the cat's cage and found that the two animals would ignore each other. However, when the amygdala was electrically stimulated, the cat immediately attacked and killed the rat. This is predatory aggression. Egger & Flynn found that stimulation of a different part of the amygdala caused the cat to ignore the rat and attack the experimenter! This is affective (emotional or fear-based) aggression.
AGGRESSION & HORMONES
Testosterone is the hormone linked to aggression. Testosterone is produced in spurts, so the testosterone levels can rise suddenly and have an effect within minutes. It also varies seasonally in some animals, which is why red deer become aggressive in the mating period in the Spring. Males produce more testosterone than females (although female ovaries do produce testosterone) - and this is an explanation for why males are are aggressive than females on average.
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Castration reduces testosterone levels in males. Wagner et al. (1979) castrated mice and observed that aggression levels dropped. When the castrated mice are injected with testosterone, their aggression levels (measured by biting attacks on other mice) rose back to pre-castration levels. This clearly suggests that testosterone is a cause of aggression in mice and may cause aggression in humans too.
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Over 19 sessions, you can see that mouse aggression varied a lot, but after castration it clearly drops. When the mice are injected with 150 micrograms of testosterone a day, their aggression creeps back up to the old levels.
A study of prisoners (James Dabbs et al., 1987, 1995) found testosterone levels were higher in those who had been convicted of a violent crime. Those with high testosterone levels were rated higher by other prisoners for being "tough".
Mazur & Booth (1998) review studies showing that men with higher levels of testosterone are more likely to:
Mazur & Booth report a study of 2,100 male Air Force veterans who received four medical examinations over a ten year period: testosterone levels fell and remained low with marriage, and rose with divorce. These results are consistent with Mazur & Booth's reciprocal model: testosterone varies with a person's dominance. Unattached males need to be socially dominant in order to attract mates so testosterone levels rise when a male is single. This explains why single/divorced males are more likely to be convicted of crimes, even when the statistics are adjusted for individuals with certain characteristics like calmness and empathy being more likely to marry in the first place (Ryan King, 2007).
- divorce, or remain single
- be arrested (for offenses other than traffic violations)
- to buy and sell stolen property
- incur bad debts
- use a weapon in fights
Mazur & Booth report a study of 2,100 male Air Force veterans who received four medical examinations over a ten year period: testosterone levels fell and remained low with marriage, and rose with divorce. These results are consistent with Mazur & Booth's reciprocal model: testosterone varies with a person's dominance. Unattached males need to be socially dominant in order to attract mates so testosterone levels rise when a male is single. This explains why single/divorced males are more likely to be convicted of crimes, even when the statistics are adjusted for individuals with certain characteristics like calmness and empathy being more likely to marry in the first place (Ryan King, 2007).
Another hormone linked to aggression is cortisol. Cortisol is a hormone produced in the adrenal glands. While it is responsible for "waking us up" in the morning, its main job is managing stress levels. Cortisol seems to inhibit aggression, the same way that testosterone increases it (Van Goozen et al., 2007). Virkkunen (1985) report low levels of cortisol in violent offenders and Tennes & Kreye (1985) report low levels of cortisol in aggressive school children.
People with lower levels of cortisol are more aggressive because it means their autonomic nervous system (ANS) is under-aroused; aggressive behaviour is an attempt to create stressful situations which provoke cortisol release, stimulating the ANS.
There's also evidence linking aggression to HIGH levels of cortisol (eg. Gerra et al., 1997). Clearly, cortisol is complicated and does a lot of different things in the brain.
Animal studies also support the link between hormones and aggression. Rachel Adelson (2004) used rats as test subjects and used electricity to stimulate the hypothalamus; this led to the release of a stress hormone called corticosterone, which is part of the aggressive response.
If the rats had their adrenal glands removed and couldn't produce their own hormones, their aggression faded. However, when they were then injected with corticosterone, the hypothalamus activated. This shows a "feedback loop" in aggression: the hypothalamus triggers the release of the hormone but the hormone also activates the hypothalamus. This might explain the phenomenon of rage, when aggression spirals out of control.
If the rats had their adrenal glands removed and couldn't produce their own hormones, their aggression faded. However, when they were then injected with corticosterone, the hypothalamus activated. This shows a "feedback loop" in aggression: the hypothalamus triggers the release of the hormone but the hormone also activates the hypothalamus. This might explain the phenomenon of rage, when aggression spirals out of control.
AGGRESSION & NEUROTRANSMITTERS
Serotonin is a neurotransmitter linked to mood and sadness. Treatment for depression often involves medicines that boost serotonin levels in the brain. However, low serotonin levels are also associated with increased aggression. This is because serotonin seems to inhibit aggression.
Dee Higley et al. (1996) studied rhesus monkeys living wild on an island. The researchers used behavioural observations of the monkeys fighting and leaping from trees. They also took samples of spinal fluid from the monkeys to measure 5-HIAA levels. 5-HIAA is a chemical that is produced by the re-uptake of serotonin, so high levels of 5-HIAA means high levels of serotonin. If serotonin inhibits aggression, you would expect aggressive monkeys to have low levels of serotonin, and therefore low levels of 5-HIAA (because there would not be need for much serotonin re-uptake).
The researchers found a negative correlation between 5-HIAA and aggression: aggressive monkeys had lower levels of 5-HIAA (and therefore of serotonin too); less aggressive monkeys had higher levels.
Low 5-HIAA/serotonin was associated with high risk-taking behaviour, such as aggression towards older, larger animals and taking long leaps from tree to tree. Many died as a result of this. |
The poor monkeys with low levels of serotonin got themselves killed jumping off trees and attacking older, bigger monkeys. Basically, being jerks.
There is also a link between dopamine and aggression. Dopamine is a neurotransmitter linked to attention and pleasure. Increased dopamine levels are associated with increased aggression and anti-psychotic drugs (which reduce dopamine levels in people suffering from schizophrenia) seem to reduce aggressive moods and behaviours.
Ferrari et al. (2003) studied the link between neurotransmitters and aggression in rats. They allowed a rat to fight every day for 10 days at approximately the same time. This was done by introducing an "intruder rat" into the test rat's cage. On the 11th day, no intruder rat was introduced. The researchers measured the levels of serotonin and dopamine in the test rat's brain. The test rat’s dopamine levels had increased and serotonin levels decreased, because it was anticipating a fight.
This study also links in with Classical Conditioning. The rat had been conditioned to fight at a certain time each day and its brain chemistry had altered to prepare it for aggressive action. It also illustrates brain plasticity, since the rat's brain chemistry changed, showing that the brain adapts to what we experience.
This study also links in with Classical Conditioning. The rat had been conditioned to fight at a certain time each day and its brain chemistry had altered to prepare it for aggressive action. It also illustrates brain plasticity, since the rat's brain chemistry changed, showing that the brain adapts to what we experience.
APPLYING THE BIOLOGY OF AGGRESSION TO REAL LIFE
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In Shakespeare's play "Othello", the conniving servant Iago drives his master Othello mad with jealousy by making Othello think his beautiful young wife has given her handkerchief to another man. It all ends BADLY.
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Sexual feelings also come from the limbic system. As part of its job in regulating emotions, the amygdala handles trust and intimacy. It is responsible for helping us recognise familiar faces and feel secure around people we are on intimate terms with. Sexual jealousy (when we feel insecure around people we are intimate with) is also produced by the amygdala.
Of course, some people just get unhappy when they are jealous; they don't get mad. Jealous rage seems to require hormones as well, such as testosterone in men. Testosterone is produced in the testes but the message to increase production comes from the pituitary gland in the brain, just beneath the limbic system.
Mazur & Booth (1998) showed how testosterone rises in men who need to show their dominance; this includes single males and males in failing relationships. If jealousy makes men feel insecure in a relationship, their body will start to produce more testosterone to prepare them to assert their dominance and this can make them aggressive.
One of the conclusions from all this is that the best way to reduce male aggression is to marry men off! This is the conservative idea that "men are civilised by marriage". A similar idea, popular with people of a different political outlook, is that the best way to reduce male aggression is to give more power to women.
Of course, some people just get unhappy when they are jealous; they don't get mad. Jealous rage seems to require hormones as well, such as testosterone in men. Testosterone is produced in the testes but the message to increase production comes from the pituitary gland in the brain, just beneath the limbic system.
Mazur & Booth (1998) showed how testosterone rises in men who need to show their dominance; this includes single males and males in failing relationships. If jealousy makes men feel insecure in a relationship, their body will start to produce more testosterone to prepare them to assert their dominance and this can make them aggressive.
One of the conclusions from all this is that the best way to reduce male aggression is to marry men off! This is the conservative idea that "men are civilised by marriage". A similar idea, popular with people of a different political outlook, is that the best way to reduce male aggression is to give more power to women.
Since violence is largely a male pastime, cultures that empower women tend to move away from the glorification of violence and are less likely to breed dangerous subcultures of rootless young men - Stephen Pinker (2011)
However, not everyone agrees that men need to be tamed. Schacht et al. (2014) carried out a meta-analysis of 20 studies into male violence in different countries. They found "that violence was equally likely to be associated with extra women as with extra men." Nine studies showed more violence in societies where men outnumbered women, and nine showed the opposite. Two studies were not conclusive.
Social Aggression
Not all aggression is physical aggression. There is also teasing, name-calling, rumour-spreading socially excluding people. Social aggression has two components:
Statistics on bullying show that 30% of children may be the target of it at some point in time (Analitis et al., 2009). A study by Atlas & Pepler (1998) revealed that other children were present in 85% of all bullying episodes on a school playground, yet these bystanders intervened to stop the bullying just 15% of the time.
Gordon Ingram (2014) shows that young children show more physical aggression than social aggression, but, as they grow into adolescence, this reverses and social aggression (gossiping, rumour-spreading) dominates.
Brendgen et al. (2015) considers this as part of the Biological Contemporary Study: young children don’t have the verbal or social skills to practise social aggression, but they acquire these once they start school. The Brendgen study goes on to show a genetic connection in twins that links to social aggression as well as physical aggression.
Not all aggression is physical aggression. There is also teasing, name-calling, rumour-spreading socially excluding people. Social aggression has two components:
- Indirect aggression, which is covert (hidden), such as spreading malicious gossip
- Relational aggression, which is overt (in the open) but non-physical, such as breaking off a friendship, pulling faces or “bitchiness”
Statistics on bullying show that 30% of children may be the target of it at some point in time (Analitis et al., 2009). A study by Atlas & Pepler (1998) revealed that other children were present in 85% of all bullying episodes on a school playground, yet these bystanders intervened to stop the bullying just 15% of the time.
Gordon Ingram (2014) shows that young children show more physical aggression than social aggression, but, as they grow into adolescence, this reverses and social aggression (gossiping, rumour-spreading) dominates.
Brendgen et al. (2015) considers this as part of the Biological Contemporary Study: young children don’t have the verbal or social skills to practise social aggression, but they acquire these once they start school. The Brendgen study goes on to show a genetic connection in twins that links to social aggression as well as physical aggression.
EVALUATING THE BIOLOGY OF AGGRESSION
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EXEMPLAR ESSAY
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