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Passamonti et al, 2012:
Healthy volunteers were given a diet that lacked  tryptophan, which is an important amino acid that helps build serotonin,   A lack of tryptophan in the diet will reduce levels of serotonin available in the brain.  The participants were put in fMRIs and their brain activity was measured while they were seeing images of happy, angry or neutral faces,  educed activity in the frontal lobe during the low serotonin conditions when the participant was viewing the angry face.

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Reduced serotonin levels have previously been implicated in aggression.

The chemical helps regulate behaviour in the brain as well as why some individuals may be more prone to aggression.

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Passamonti et al, 2012

Participants were given a diet that lacked tryptophan, which is an important amino acid that helps build serotonin. A lack of tryptophan in the diet will reduce levels of serotonin available in the brain.

The participants were put in fMRIs and their brain activity was measured while they were seeing images of happy, angry or neutral faces. The researchers could see the activation of different areas of the brain as the different faces were shown.

The results showed that there was reduced activity in the frontal lobe during the low serotonin conditions when the participant was viewing the angry face.

The reduced activity in their PFC as a result of the low serotonin may also affect their ability to inhibit impulsive reactions and think through their actions, so if someone has a tendency towards violence they may not be able to reduce an impulsive reaction towards an individual who is threatening them.

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Key Study: Raine et al. (1997)

Aim: To identify brain regions specific to offenders charged with murder or manslaughter, who had pleaded not guilty by reason of insanity.

Method: The participants were 41 murderers (2 female) who had been charged with murder or manslaughter and had pleaded not guilty by reason of insanity. The researchers used a (positron emission tomography) PET scanning method to highlight areas of brain activity and these results were compared to an age and gender matched control group.

Results: They found reduced activity for the offender group in areas such as the prefrontal cortex and corpus callosum (the nerve fibres responsible for swift communication between the hemispheres). Additionally, there were abnormalities in the activity of the limbic system, including the amygdala and thalamus.

Conclusion: There is indication that offenders (specifically violent offenders) have abnormal brain function when compared to normal controls. As there is largely reduced activity, it would suggest that the brains of offenders are slowed and perhaps unable to make the swift decisions to react appropriately in certain situations. For example, the frontal lobes are linked to planning behaviour; therefore, perhaps the decreased prefrontal cortex activity indicates that offenders are unable to consider the consequences of their actions and control their behaviour.

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Frontal Lobe Lesions - Grafman et al (1996):

57 normal controls and 279 veterans, matched for age, education, and time in Vietnam, who had suffered penetrating head injuries during their service in Vietnam, were studied.

Family observations and self-reports were collected using scales and questionnaires that assessed a range of aggressive and violent attitudes and behavior. Two Aggression/Violence Scale scores, based on observer ratings, were constructed.

The results indicated that patients with frontal ventromedial lesions consistently demonstrated Aggression/Violence Scale scores significantly higher than controls and patients with lesions in other brain areas.

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Serotonin
Crockett and Passamonti (2011) : http://www.cam.ac.uk/research/news/serotonin-levels-affect-the-brain%E2%80%99s-response-to-anger
For the study, healthy volunteers’ serotonin levels were altered by manipulating their diet.  On the serotonin depletion day, they were given a mixture of amino acids that lacked tryptophan, the building block for serotonin. On the placebo day, they were given the same mixture but with a normal amount of tryptophan.

The researchers then scanned the volunteers’ brains using functional magnetic resonance imaging (fMRI) as they viewed faces with angry, sad, and neutral expressions. Using the fMRI, they were able to measure how different brain regions reacted and communicated with one another when the volunteers viewed angry faces, as opposed to sad or neutral faces.

The research revealed that low brain serotonin made communications between specific brain regions of the emotional limbic system of the brain (a structure called the amygdala) and the frontal lobes weaker compared to those present under normal levels of serotonin. The findings suggest that when serotonin levels are low, it may be more difficult for the prefrontal cortex to control emotional responses to anger that are generated within the amygdala.

Using a personality questionnaire, they also determined which individuals have a natural tendency to behave aggressively. In these individuals, the communications between the amygdala and the prefrontal cortex was even weaker following serotonin depletion.  'Weak' communications means that it is more difficult for the prefrontal cortex to control the feelings of anger that are generated within the amygdala when the levels of serotonin are low.  As a result, those individuals who might be predisposed to aggression were the most sensitive to changes in serotonin depletion.

Liao et al (2004): Possible association between serotonin transporter promoter region polymorphism and extremely violent crime in Chinese males

In the present study, they investigated this 5-HTTLPR genetic polymorphism in a group of Chinese males who had been convicted for extremely violent crime (n = 135) and a normal control group (n = 111). The proportion of S-allele carriers was significantly higher in the criminal group than in the controls (p = 0.006). A significant association was not demonstrated for the relationship between the 5-HTTLPR polymorphism and antisocial personality disorder, substance abuse or alcohol abuse in the criminal group. Their findings demonstrate that carriage of the low-activity S allele is associated with extremely violent criminal behavior in Chinese males, and suggests that the 5-HTT may be implicated in the mechanisms underlying violent behaviors.

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Serotonin: Psychologists believe that low levels of serotonin leads to increased levels of aggression because serotonin is linked to emotional stimuli's which can lead to aggressive behavior. Bond (2005) conducted a study on aggression. Bond found that low levels of serotonin caused higher levels of aggression and increased levels of serotonin (given in the form of drugs) reduced aggression.

Frontal Lobe Lesions: Grafman et al (1996) studied the relationship between the frontal lobe and aggressive/violent behavior. Veterans with frontal lobe damage were studied and results showed that this caused increased levels of violence

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Grafman et al. (1996) examined the relationship between frontal lobe lesions and the presence of aggressive and violent behavior. They studied veterans who suffered penetrating head injuries during their service in the war.  The results indicated that patients with frontal lobe lesions consistently demonstrated violence scale scores significantly higher than the controls.

In addition to that Raine et el (1997) performed a PET study on a sample of murderers who had leaded “not guilty by reason of insanity”. The researchers predicted no abnormality in other areas of the brain. Generally, the murderers showed increased right-hemisphere activity.

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Grafman et. al (1996)

This study investigated the relationship between the frontal lobe lesions and the presence of aggressive and violent behavior.

They experimented on veterans who suffered penetrating head injuries during their service in the war and found that participants with frontal lobe lesions consistently demonstrated violence scale scores significantly higher than the control group's.

Raine et al. (1997)

Raine performed a PET study on a sample of murderers who had leaded “not guilty by reason of insanity”. The researchers predicted no other abnormality in the brain. Generally, the murderers showed increased right-hemisphere activity.