Two Key Studies of Genes & Behaviour: Caspi et al. (2003) & Brunner et al. (1993) (SL IB Psychology)

Aim: To investigate the violent, anti-social behaviour of specific male members of a large family in the Netherlands. The behaviour exhibited by the males in the family was borderline mental retardation (their average IQ was around 85), and violent behaviour. 

Participants: 5 males from a family in the Netherlands, all of whom had the same genetic condition, transmitted via the X chromosome on the MAOA gene. The family lived in a remote rural region of the Netherlands. Two carrier females and one non-carrier female were used as a control and compared with 3 clinically affected males. (Carrier means that some of the females carried the faulty gene in their genotype but it was not expressed in the phenotype i.e. their behaviour).

All of the affected males acted aggressively when angry, fearful, or frustrated. Examples of their violent, anti-social behaviour included attempted rape of one of the female members of the family, arson, attacking a mental institute warden with a pitchfork, voyeurism (spying on the females in the family at night), exhibitionism (appearing naked in public). Only one of the males in the family with the faulty gene finished primary education.

Procedure: A case study (close study of a small group of individuals from one family) and quasi-experiment. A quasi experiment is one in which the IV is naturally occurring i.e. it can’t be manipulated by the researcher – in this case the individuals involved either had the faulty gene or they didn’t have the faulty gene. Brunner conducted DNA analysis, obtained via urine samples. Observations of the males and interviews with the family provided qualitative data.

Results: None of the affected males had dysmorphic signs of the genetic mutation i.e. they didn’t ‘look abnormal’ or different physically to the unaffected males. Unaffected males in this family attended normal schools, and most had steady jobs. All the females (including several carriers) also functioned normally.

A base change in the DNA structure was identified in all 5 affected males. This in turn resulted in flawed monoamine metabolism, which is linked with a deficit of the enzyme monoamine oxidase A (MAOA) – an enzyme which (among other functions) regulates the supply of serotonin levels to the brain. The reason only males are affected is because it is specifically the single X chromosome which is responsible for the production of MAOA.

Conclusion: The dysfunctional MAOA gene may be linked to irregular serotonin metabolism which could in turn be responsible for the mental retardation and aggressive behaviour of the affected males. MAOA deficiency may account for an individual’s inability to regulate their aggression. This MAOA deficiency is now known as ‘Brunner syndrome’.

Evaluation of Brunner et al. (1993)

Strengths

• This is a case study of one family so the researchers were able to amass a good amount of data on which to build their theory
• By using one extended family the researchers were able to directly test their theory by using family members as control samples rather than an unrelated general population, thus validating the idea that the males’ behaviour was genetic rather than as a result of their environment

Weaknesses

• The use of only one family, however, does limit the generalisability of the findings
• The affected males may have encountered more adverse reactions from others e.g. hostility, aggression, confrontations due to their reduced IQ and lack of impulse-control which could have exacerbated their anti-social tendencies i.e. nurture may have influenced their behaviour as well as nature

Key terms:

• X Chromosome
• MAOA gene
• Serotonin

Aim: To investigate the link between the alleles of the 5-HTT gene and depression.

Participants: An opportunity sample of 847 participants aged 26 years.  The sample was split into three groups, depending on the length of the alleles on their 5-HTT transporter gene:

• Group 1 – two short alleles
• Group 2 – one short and one long allele
• Group 3 – two long alleles

Procedure: The participants were asked to report any stressful life events that had taken place between their 21st birthday up to their 26th birthday. The Diagnostic Interview Schedule was used to assess incidences of depression over the past year. 
The researchers carried out correlational analyses between the following co-variables:

• each participant’s stressful life events and incidences of depression;
• the length of each participant's alleles and incidences of depression;
• perceived stress and length of each participant's alleles. 

Results: More depression in response to stressful life events was reported from the participants who had two short 5-HTT alleles compared to the other two groups. The participants with two long alleles reported fewer depression symptoms overall.  

Conclusion: There may be a relationship between short 5-HTT alleles and depression - i.e. stressful life events are more likely to trigger depression in people with this genetic make-up. Long 5-HTT alleles may provide protection against stress-induced depression. The onset of depression appears to be an interaction between environment (stressful events) and genetic make-up. 

Evaluation of Caspi et al. (2003)

Strengths

• The researchers exerted control by restricting the measurement of stressful life events in their sample as falling between the ages of 21 and 26 which should ensure a degree of consistency across the measurement thus increasing reliability
• Conducting three separate correlational analyses means that each measure is checked by the findings of the other measures which should ensure internal validity.

Weaknesses

• The experience and aetiology of depression is complex and may be due to a number of factors, both biological and non-biological which makes this study somewhat reductionist as it provides an overly simplistic explanation
• Using a self-report to collect data is prone to bias (e.g. social desirability bias, response bias) which reduces the validity of the findings

Key terms:

• 5-HTT
• Allele
• Correlation