Genetic Factors in Aggression

Question 1

what does the genetic approach to aggression suggest?

2

Answer 1

the propensity for aggressive behaviour lies in an individual’s genetic make up

aggressive tendencies may be inherited through genes

Question 2

what does this mean researchers must do?

2

Answer 2

researchers must try to establish whether genetically related individuals are more similar in their aggressive tendencies than non-related individuals

if they are, this would suggest that there are genetic factors involved in aggression

Question 3

what implications does this have?

1

Answer 3

this has important implications for understanding the origins of violent crime as research suggests that aggressive tendencies may partly be inherited through genes

Question 4

what are genetic factors?

2

Answer 4

the likelihood of behaving in a particular way is determined by a person’s genetic make up

in other words, behaviour is inherited from parents

Question 5

outline genetic factors in aggression

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Answer 5

trying to determine the role of genetic factors in aggression is essentially a question of nature and nurture

to work out the relative contributions of genetic inheritance (nature) and environmental influences (nurture) in aggression, researchers have used a variety of methodological techniques — including twin and adoption studies and studies of violent populations

genetic factors in aggression include...
• twin studies 
• adoption studies
• research on genetic factors in aggression
• the role of MAOA
• MAOA-H and MAOA-L
• the warrior gene

Question 6

twin studies

4

Answer 6

monozygotic, identical twins share all of their genes while dizygotic, non-identical twins share only 50% of their genes

in twin studies, researchers compare the degree of similarity for a particular trait such as aggression between sets of monozygotic (MZ) twins and compare this to the similarity between sets of dizygotic (DZ) twins

both types of twins share the same environment as each other but MZ twins are more genetically alike, so if the MZ twins are more alike in terms of their aggressive behaviour then this should be due to genes rather than the environment

most twin studies have focused on criminal behaviour — Coccaro et al (1997) used adult twin pairs and found that nearly 50% of the variance in direct aggressive behaviour (aggression towards others) could be attributed to genetic factors

Question 7

adoption studies

5

Answer 7

adoption studies can help work out the relative contributions of environment and heredity in aggression

if a positive correlation is found between aggressive behaviour in adopted children and aggressive behaviour in their biological parents, a genetic influence is implied

but if a positive correlation is found between the adoptees aggressive behaviour and the rearing family, then an environmental influence is implied

a study of over 14,000 adoptions in Denmark found that a significant number of adopted boys with criminal convictions had biological parents, particularly fathers, with convictions for criminal violence (Hutchings et al, 1975)

this provides evidence for a genetic influence in aggression

Question 8

research on genetic factors in aggression

3

Answer 8

Miles and Carey (1997) carried out a meta analysis of 24 twin and adoption studies that demonstrated the genetic basis of aggression

the results suggested a strong genetic influence that could account for as much as 50% of the variance in aggression

age differences were notably important, with both genes and family environment being influential in determining aggression in youth, but at later ages the influence of environment decreased and the influence of genes increased

Question 9

what is MAOA?

2

Answer 9

monoamine oxidase A (MAOA) is an enzyme that regulates the metabolism of serotonin in the brain

sometimes referred to as ‘the gene for aggression’

Question 10

the role of MAOA

4

Answer 10

although no individual gene for aggression has been identified in humans, a gene responsible for producing an enzyme called monoamine oxidase A (MAOA) has been associated with aggressive behaviour

MAOA regulates the metabolism of serotonin in the brain and low levels of serotonin are associated with impulsive and aggressive behaviour

in the 1980s, a study of a Dutch family found that many of its male members behaved in a particularly violent and aggressive manner and a large proportion had been involved in serious violent crimes, including arson and rape

these men were found to have abnormally low levels of MAOA in their bodies and a defect in this gene was later identified

Question 11

MAOA-H and MAOA-L

4

Answer 11

a study by Caspi et al (2002) involving 500 male children found a link between MAOA and aggressive behaviour

researchers discovered a variant of the gene associated with high levels of MAOA called MAOA-H and variant associated with low levels called MAOA-L

those with the MAOA-L variant were significantly more likely to grow up to exhibit antisocial behaviour, but only if they had been maltreated as children

children with the MAOA-H variant who were maltreated + those with the MAOA-L variant who were not maltreated did not display antisocial behaviour

Question 12

the warrior gene

5

Answer 12

according to research, people with a low activity version of the MAOA gene are more likely to respond aggressively when provoked

MAOA-L is much more frequent in populations with a history of warfare, with about 2/3 of people in these populations having this version of the gene

in contrast, only around 1/3 of people in western populations have the MAOA-L variation of the gene

this has led to it being referred to as the ‘warrior gene’

McDermott et al (2009) found that MAOA-L participants displayed higher levels of aggression when provoked than MAOA-H subjects did

Question 13

x5 evaluation points

Answer 13

problems with sampling in studies of aggression

problems with assessing aggression

evidence for the influence of the MAOA gene

difficulties determining the role of genetic factors

the MAOA gene can help explain gender differences in aggressive behaviour

Question 14

EVALUATION
problems with sampling in studies of aggression

8

Answer 14

many studies concerning genetic factors in aggression have focused exclusively on individuals convicted of violent crime

2 particular difficulties arise when trying to draw meaningful conclusions from these studies

the first problem lies with the participants themselves — convictions for violent crime are relatively few compared to the vast number of violent attacks by individuals that never result in a conviction

they therefore represent just a small minority of those regularly involved in aggressive behaviour

secondly, offenders designated as ‘violent’ on the basis of a court conviction or not necessarily the most serious, persistent offenders

for example, a convicted murderer could be designated as violent for one offence despite having otherwise had a lifetime free from crime, meaning that such individuals may not be appropriate to use for samples

this might explain why so many studies have found little or no evidence of heritability for violence and aggression

reduces the validity of findings as the sample may not be representative of typical aggressive behaviours, meaning that it may be hard to generalise the findings from such small populations of people to wider society

Question 15

EVALUATION
problems with assessing aggression

7

Answer 15

many of the aggression studies have relied on either parental or self-reports of aggressive behaviour, whereas other studies have made use of observational techniques

in the meta analysis conducted by Miles and Carey, mode of assessment was found to be a significant moderator of aggressive behaviour in the 24 studies that they analysed

they found that genetic factors explained a large proportion of the variance in aggressive behaviour in studies that used parental or self reports

however, those that had made use of observational ratings showed significantly less genetic contribution and a greater influence of environmental factors

for example, Reggie and Ronnie Kray were once considered the most dangerous men in Britain, their violent tendencies may have had less to do with the genetic similarity and more to do with similarities in upbringing, but it is difficult to assess this

these inconsistencies in findings make it difficult to accurately assess the relative contributions of genetic and environmental factors in aggression

it also demonstrates that the method used to assess aggression can significantly affect the findings of a study

Question 16

EVALUATION
evidence for the influence of the MAOA gene

5

Answer 16

in many countries, the majority of all violent crime is committed by a small group of persistent offenders

a recent study in Finland has added research support that the MAOA gene is implicated in severe violent behaviours such as murder

Tiihonen et al (2015) studied Finnish prisoners, revealing that the MAOA low activity genotype (MAOA-L) in combination with another gene called the CDH13 gene, was associated with extremely violent behaviour

there was no substantial evidence for either of these genes among non-violent offenders, indicating that this combination of genes was specific for violent offending only

however, critics argue that although these genes may make it harder for some people to control violent urges, they do not predetermine violent behaviour

Question 17

EVALUATION
difficulties determining the role of genetic factors

6

Answer 17

the connection between genetic factors and aggression is far from straightforward because of problems determining what is and what is not a product of genetic inheritance

it is difficult to establish genetic contributions to aggressive behaviour for the following reasons….

• more than one gene usually contributes to a given behaviour
• as well as genetic factors there are many non-genetic, environmental influences on the manifestation of aggressive behaviour
• these influences may interact with each other; genetic factors may affect which environmental factors have an influence and vice versa (gene-environment interaction)

in particular, the idea that genetic and environmental influences interact with each other is demonstrated in the study by Caspi et al (2002)

Question 18

EVALUATION
the MAOA gene can help explain gender differences in aggressive behaviour

6

Answer 18

an advantage of MAOA gene research is that offers an explanation for the uneven rates of violence for males and females

Niehoff (2014) suggests that the gender differences in aggressive behaviour may be due to the differential genetic vulnerability that males and females have to MAOA gene

the MAOA gene is linked to the X-chromosome, women have two X chromosomes whereas men only have one

when men inherit an X-linked gene from their mothers, they are more likely to be affected by it

whereas women inheriting the same gene are generally unaffected as they also have a second X chromosome with a normal gene for MAOA that prevents expression of the abnormal version of the MAOA gene

this can explain why males typically show more aggressive behaviour than females and suggests that genetic explanations of aggression can be very useful