Genetic Explanation of Aggression

• Genes alone don't cause aggression.

  --> They affect the production of hormones and neurotransmitters which in turn impact/create aggression
• What do genetic explanations of aggression assume

  Aggressive behaviour is the result of inherited traits/characteristics that have adapted across generations because of their evolutionary usefulness
• What are the 3 types of studies used in genetic studies.

  1. T win studies
  2. A nimal studies
  3. Ad option studies
• Animal Study: Turner (2007)

  Turner studied aggression in pigs (pigs are highly intelligent and become aggressive when they are bored)

  PROCEDURE:

  He selectivley bred the animals and found that he could breed less aggressive pigs by breeding from the least aggressive male and female.

  AO3 POINT:

  REAL WORLD + ECONOMIC IMPLICATIONS

  • This has allowed vets to significant reduce their bills as well as increase the stocks of pork to sell
• What is the correlation coefficient

  It measures the correlation (a statistical measure of the strength of relationship between 2 variables)

  -> 60% concordance rate = 0.6 correlation coefficient
• Why do genetic studies usually involve twins

  Twins share DNA, especially MZ twins who share 100% of DNA

  • This allows researchers to determine the extent to which genetic factors contribute to certain behaviours.
• What are monozygotic (MZ) twins

  MZ twins develop from a single fertilized egg that splits into 2

  • They share 100% of DNA
• What are dizygotic (DZ) twins

  DZ twins develop from 2 separately fertilized eggs.

  • They share 50% of DNA (the same as siblings)
• What do researchers do with MZ and DZ twin study results

  They compare how similar the behaviour is of MZ twins to that of DZ twins - This would display if behaviour is genetic or has environmental influences
• Twin Studies: Dinic et al (2020): MZ and DZ twin studies

  478 adult male twins (316 MZ, 172 DZ) + non twin control group

  PROCEDURE

  Using competitive reaction time, the loser was subjected to a large blast of noise given by the winning twin

  FINDINGS

  • There was no significant difference in aggression between the control group and the experimental twin group
  • Concluded that in this case, aggression was not genetically linked and was subject to may extraneous variables
• COUNTERPOINT (AO3 LIMITATIONS) of Dinic et al (2020) Twin studies

  1: Low mundane realism (artificial task)

  2: Potential of extraneous variables -> for example twins may have had an argument before the test

  • This could be in support of the frustration aggression hypothesis??
• Twin Studies: Coccaro et al (1997) MZ/DZ twins and aggression

  Studied male adult MZ and DZ twins

  Direct physical aggression: concordance rates for MZ = 50%, DZ = 19%

  Verbal aggression = 28%, DZ = 7%

  This demonstrates that aggression is likely due to genetic factors because for both variations, MZ twins had higher concordence rates that DZ twins.

  • This displays that aggression is genetic as it is expected that these sets of twins were raised in the same environment, meaning the correlation is likely due to genes/inheritance rather than environmental learning
• AO3 Limitation: Twin Studies

  How do we operationalise aggression (kicking, spitting etc)

  • This method of data collection may vary across studies

  This may result in differences between studies, meaning there is low replicability as well as low comparability which makes it difficult to draw overarching conclusions due to the un standardised methods possibly being the reason for variations in results. This reduces reliability of findings/conclusions
• What do adoption studies suggest about aggression

  similarities in aggressive behaviours between an adopted child and adopted parents suggest the similarity is due to the environment + the role of social learning rather than genetic inheritance
• Adoption Studies: Rhee and Waldman (2002): meta analysis.

  Studies adoption studies and found that genetic influences accounted for 41% of the variance in aggression between adopted children and adopted parents

  (i.e. adopted children were aggressive even when the adopted parents weren't because of the role of genetic influences)

  THIS SHOWS THE IMPORTANCE OF GENES AND THE ROLE THEY HAVE IN CREATING AGGRESSIVE BEHAVIOURS
• What is the MAOA gene

  The MAOA gene codes for the production of the MAOA enzyme
• What does the MAOA enzyme do

  This enzyme is involved in the break down of serotonin in the synapse
• The break down of serotonin is a part of neurotransmission and the MAOA enzyme efffectivley sweeps up any serotonin left in the synaptic gap after neurotransmission is complete
• What is the role of serotonin in modulating aggression + the role of serotonin in the amygdala

  serotonin has been linked to mood, specifically in balancing mood.

  • low/disrupted levels of serotonin may be a factor in mood affective disorders such as depression, as well as aggression

  ++ serotonin typically inhibits the firing of the amygdala, meaning it calms the brain and has a role in controlling aggressive impulses

  • This is because a hyperactive amygdala has shown to be linked to aggression
• What effect does serotonin have on the brain

  Serotonin typically has a calming effect on the brain, which is linkeed to lower levels of aggression
• What happens if the MAOA enzyme (caused by the mutation in the MAOA gene) isn't working

  It would cause a build up of serotonin in the synapse which normally has a calming effect on the brain.

  • This means that synaptic transmission of serotonin is partially inhibited and could lead to aggression
• What are the 2 types of MAOA genes + which leads to aggression

  1: n ormal variant

  2: l ow variant (MAOA - L)

  The low variant is the one that has a predisposition towards aggression due to the build up of serotonin in the synapse
• AO1/AO3: Maori research on the MAOA gene

  MAOA-L variant was possessed by 56% of New Zealand Maori males

  • These men were notably aggressive, meaning this research supports claims on the function of the MAOA gene in relation to aggression
• Brunner et al: Dutch family + MAOA - L variant

  Studied 28 male family members who were repeatedly convicted for violent crimes. (e.g. rape)

  FINDINGS

  All the aggressive males in the family shared the same dysfunctional genes which led to low levels of the MAOA enzyme, leading to unregulated levels of serotonin in the synapse - this is linked to aggression.
• What did Brunner et al's Dutch study on the MAOA gene suggest about male and female aggression?

  It suggested that aggression differences may be linked to a genetic defect on the X chromosome.
• How was the trait related to aggression passed according to the study?

  The trait was passed from mother to son.
• How do women differ in their vulnerability to X chromosome defects compared to men?

  Women have two X chromosomes, so a normal second one can mask the effects of a defect on the first.
• Why are men more vulnerable to defects on the X chromosome?

  Because men have only one X chromosome.
• What does it mean for women to be carriers of the MAOA gene defect?

  It means they can carry the gene without showing aggressive behavior themselves.
• What was noted about some women in the study concerning the MAOA gene?

  Some women who were not aggressive also had the MAOA gene marker, indicating they were carriers.
• AO3 Strength: Biological Evidence

  The genetic explanation provides empirical evidence (such as saliva, DNA analysis etc)

  • This means we can compare structures between individuals and identify repeating patterns which could lead us to draw accurately informed conclusions about which specific genes influence aggression
• AO3 Strength: Twin/Adoption studies use large sample sizes

  These studies tend to use large samples (e.g. Dinic - 478 twins, Rhee & Waldman meta analysis = a collation of multiple studies)

  • This means that conclusions drawn are more likely to be valid due to the high population validity achieved through large sample sizes, making it more informed and accurately representative
• AO3: weakness: Inconsistent concordance rates

  If genetic factors were soley responsible for aggression, the concordence rates for MZ twins would be 100%, which is not the case - Dinic et al MZ twins = 50% and 28%.

  This is because they share 100% of DNA, so if aggression was purely genetic and one twin was aggressive, both twins would have the same gene expressed and would display the same rates of aggression.

  • This reduces the validity of the genetic explanation when used in isolation and indicates that there must be environmental influences upon behaviours
• Frazetto et al: Diathesis Stress Model - AO3 Weakness

  Found an association between antisocial aggression + the MAOA-L variant, HOWEVER only in those who experienced significant trauma during the first 15 years of life.

  • Those with no trauma weren't significantly aggressive as adults even if they possessed the MAOA-L variant.

  This provides strong evidence for an interactionist model in influencing aggressive behaviour and highlights the dual importance of the environment and genetics.
• AO3 : Strength: Mertins et al (2011): High variant

  Found people with a high variant MAOA gene were more coperative and less aggressive in a money distribution game than those with normal/low.

  • These findings support the relationship between MAOA gene activity/ it's impacts on the levels of MAOA enzyme alongside it's subsequent effect on aggression
  • This demonstrates that genes can impact our biology in terms of neurotransmitters which in turn regulates our aggression