Two Key Studies of Biological Explanations of Major Depressive Disorder (MDD) (SL IB Psychology)

Key study one - (Genetic Explanation): McGuffin et al. (1996)

Aim: To investigate concordance rates for MDD in MZ and DZ twins.

Participants: 

• 177 twin probands who were registered between 1948 and 1986 with the Maudsley Hospital in London as suffering from MDD
• The same-sex twin siblings of each proband were used as comparison participants 

Procedure: 

• The twins were assessed by clinicians via a series of tests and interviews (about their emotional state, their mood etc.) by blind researchers i.e. the researchers were unaware as to whether the twin was MZ or DZ 
• The blind researchers were also blind as to whether each twin had MDD 
• The data also consisted of the twins’ medical records

Results: 

• MZ twins showed a 46% concordance rate for MDD compared to 20% in DZ twins
• Shorter periods of depression in one MZ twin was matched by similar depression in their twin sibling
• MZ twins aged 65+ showed heightened levels of MDD compared to the general population

Conclusion: 

• MDD may be highly heritable rather than a product of environmental factors
• Short-term MDD  in one MZ twin appears to increase the probability of their MZ twin sibling also developing MDD, which reinforces the idea that depression is genetic

Evaluation of McGuffin et al. (1996)

Strengths

• This was a longitudinal study, conducted over decades which means that it is high in validity as the participants could be tracked over time to look for real differences in behaviour
• The use of a blind interviewer/researcher means that the study was free from bias which increases the validity of the findings

Weaknesses

• The concordance rate for MZ twins was 46%: if depression is entirely genetic then it should show a 100% concordance rate
• This research was conducted prior to DNA testing so it is possible that some of the MZ twins may actually have been DZ i.e. they may have looked identical but in fact have been biologically non-identical

High concordance rates for MZ twins support the argument that some behaviours are inherited.

Key study one - (Neurochemical Explanation): Caspi et al. (2003)

Aim: To investigate the link between the alleles of the 5-HTT serotonin transporter gene and MDD.

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 the length of each participant's alleles. 

Results: 

• The highest number of depressive episodes in response to stressful life events was reported by the participants who had two short 5-HTT alleles compared to the other two groups
• The participants with two long alleles reported fewer depressive symptoms overall  

Conclusion: 

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

Evaluation of Caspi et al. (2003)

Strengths

• The possible link between the 5-HTT serotonin transporter gene and propensity to depression means that genetic screening could be conducted to help protect those with short alleles and to provide them with treatment as a preventative measure against MDD
• 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