Genetics: Violence as a biological
predisposition
2 constants across cultures:
1.
Men are most likely to commit violent acts.
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Sex difference is a universal.
Average man is more aggressive then women even in infancy prior to
sex role socialization by adults.
In USA 85% of arrests for violent crimes are men.
2. Young persons are more likely to be violent than older persons
Adoption studies
• 1,000 boys adopted in Denmark between 1927-1947.
• Groups
– 1. Children of criminal Biological Parents adopted to non-criminal parents
– 2. non-criminal biological parents adopted by criminal parents
• Group 1: were the most likely to be criminals, plus the
more extensive criminal history of biological parents the
higher risk the child is a criminal. Risk factor was
unrelated to whether adopted parents knew of biological
parents history or not.
Phineas Gage
• Railroad Accident – Sept. 1848
• Leveling land with Dynamite
• 3 foot inch thick tamping rod was projected in to his
brain
• Entered via cheek, left Eye and into the frontal lobes
• Driven by other workers in a ox-cart to Doctor’s office
What happened?
• Lost conscious and had convulsion immediately, but awoke
quickly and was talking and walking soon afterwards
• Never showed any impairment of movement or speech
• Memory was intact, and was capable of learning new things
• However, within months his personality had changed
dramatically
– He became extravagant and anti-social, a foul mouth liar with bad
manners, frequently got into fights and assaults.
– could no longer hold a job or plan his future
– According to friends “Gage was no longer Gage”
• Died after an accident 13 years later
– A penniless, epileptic
• Harlow (physician who treated him)
discussed the link between the frontal
damage and behavior change.
• 130 years later Antonio Damasio used
computer modeling and imaging to determine
the path and damage of the tamping rod
• The rod damaged the ventromedial frontal
lobes. This region is implicated in personality
changes and aggression/violence.
Implicated Brain structures
• Rat lesion studies suggest that different types of
aggression may be controlled by different
subsets of brain structures.
– Limbic sites: amygdala, septum and hypothalamus
• Lesion in the Septum of rats
– increase predatory aggression but reduce social aggression)
• Neural circuit in frontal cortex: medial, frontal and orbitofrontal
cortex.
Neurobiologist definitions of aggression
• Reactive:
– This aggression involves unplanned, enraged attacks on the
object perceived to be the source of the threat or frustration.
• Instrumental:
– This aggression is purposeful, goal directed and can be
described as cold-blooded (involves executive function)
Frontal Cortex
• Damage to medial frontal and orbitofrontal cortex is associated with
increase risk of reactive aggression in humans.
• Neuroimaging studies show evidence of frontal dysfunction in
aggressive individuals.
• Violent individuals showed significantly less CBF in frontal cortex
than comparison individuals.
• Neural integrity of prefrontal cortex (proton magnetic resonance
spectroscopy (H-MRS) in violent patients showed reductions in Nacetylasparate (NAA-a marker of neural density). (Chritchley, et.al. 2000)
•
CBF (cerebral blood flow) measured by PET during performance of
a continuous performance task in murderers pleading not guilty by
reason of insanity and matched comparison individuals was studied.
They found that a subset of murders (the reactive murderers vs the
instrumental) had lower prefrontal cortex. (Raines, 1997)
Amygdala
• Possible to increase aggression by modulation of
the amygdala.
– Electrical stimulation of amgydala increases all types of
aggression
– Charles Whitman (Austin, Texas University tower mass murder)
left a note begging for his brain to be studied. His autopsy
revealed he had a tumor pression into his amygdala
Neurotransmitters:
• Serotonin
– 5HT exerts inhibitory control over impulsive aggression
– 5HT binding sites in frontal area of alcoholic impulsive violent offenders
was lower than in healthy control subjects or non-violent alcoholics.
– 5-H IAA reduction in some impulsive offenders with personality disorders
– Gene for TPH (tryptophan hydroxalyse) associated with aggressive
behavior in males but not females with personality disorders.
– 5-ht promotor genes are associated with early onset alcoholism and
violent behavior.
– 5-HT receptor activation decreased aggression, while those that decrease
receptor activation have been found to increase aggression .
– Tryptophan depletion increase aggression in both men and women.
– Aggressive effect of tryptophan depletion is mediated via the 5-HT1
receptor.
-Amiobutyric Acid
•Alcohol and benzodiazepines have consistently shown
to increase aggression.
•Specifically, GABAA receptor complex (receptor
agonists benzodiazepines, barbiturates and alcohol.)
Increase Cl-flux in ion channel.
• Alcohol and Benzo’s heighten aggression effects.
• Administration of Benzo’s is associated with increased behavioral
aggression following provocation in anxious patients.
• Behavioral aggression occurred after one dose of benzodiazepines in
normal healthy subjects.
• Both alcohol and diazepam affects the lateral orbitofrontal cortex.
• Both alcohol and benzodiazpines impair recognition of angry
expression in others.
Summary of 5-HT and GABA
• Both 5-HT and GABA appear to modulate
aggression.
• 5-HT leads to a suppression of aggression
• GABA leads to a increase in the probability of
aggression.
• However, this is only developed for reactive
aggression.
• These transmitters have little effect (GABA) and
no effect on instrumental aggression.
Instrumental aggression which is goal directed
behavior requires executive control to implement it.
The only transmitters that might modulated the
systems crucial for socialization are likely be be
related to instrumental aggression.
• Norepinephrine
– The amygdala is implicated in socialization. NE in amygdala
have been implicated in modulation of emotional memory
– Some studies show relationship between aggression and
increased NE activity.
– Beta-adrenergic blocking agents have been used clinically to
suppress violent behavior in psychiatric patients
– Propanolol in a high dose increase recognition of sadness which
is impaired in psychopathic individuals. (they cannot easily
process sad expressions).
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Monoamine Oxadase
• Low levels of MAO-A subtype related to increase in
violence and aggression
• Intergenerational Dutch Family Violence
– Men spanning 4 generations
– Excessive levels of violence: rapes, assaults, homicides, arson, etc…
– 1993: sero tests indicated men were missing MAO-A enzyme (breaks
down NE, 5HT, and DA)
– Men had excessively high levels of 5-HT
Testosterone
• In animals, increased levels of testosterone is related
to social aggression
– reducing testosterone in the alpha male eleminates his dominant
social status, and restoring testosterone (through injection) causes
him to regain his social status.
– However, giving testosterone to non-alpha dominant males does
not make them dominant or alpha
• So, for animals testosterone does not increase
violence or aggresion, but does increase social
aggression in alpha males.
Humans and testosterone
• Abnormally high levels of testosterone in
humans is related to increased social aggression,
but there is no evidence they are more violent.
• Thus, there is no evidence that testosterone
levels have any predictive value in identifying
violent behavior, nor does it increase violent
behavior.
Genetics and violent behavior
• 1960: researchers looked at men born with an extra Y
chromosome.
• However, further studies showed while more men in
prison had the extra Y then men in the population, they
were not necessarily violent. Many were incarcerated for
non-violent crime.
• Furthermore, XYY males are extremely rare, thus this
syndrome could not explain all the violent behavior
Genetic mutations studies
• Tg-8 knockouts
– Blind mice with really lousy tempers
– Normally mice with plenty of room can share same
environment
– Tg-8 mice (who lack MAO-A enzyme) attack any other mice
or animal in cage
Summary
•No specific genes for aggression,
perhaps some for impulsivity
•Testosterone: no empirical evidence
•XYY prisoner study flawed
•Amgydala: site of rage
•Electrode in bulls
•Pre-frontal lobotomies