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Very much linked to the nature vs. nurture debate, which we will consider in more detail in the next section, the research by Caspi et al. lies very much on the ‘nature’ side of the argument because their research concerned genetic influence in whether or not an individual was susceptible to the development of depression because of their innate reactions to stressful environmental events.
Their initial argument was that individuals who are genetically predisposed to depression are more likely to develop it after experiencing a stressful life event, such as divorce or bereavement. This is because their 5-HTT gene, which transports serotonin around the body, is different or ‘adapted’.
Aim
Caspi et al. aimed to investigate whether a functional change in the 5-HTT gene is linked to a higher or lower risk of depression in an individual and why some people are able to successfully deal with stressful events when others are not.
Method
A total of 847 subjects, all of whom were 26 years of age, were used for the research. Each had their 5-HTT gene identified to see if its accompanying allele was ‘long’ or ‘short’. An allele is a different version of the same gene, so, for example, in the genes which determine eye colour, there may be one allele that codes for brown eyes and one allele that codes for blue eyes.
In the general population approximately 57% of people are known to have a ‘long’ allele and 43% are known to have a ‘short’ allele. The ‘long’ allele is thought to lead to the production of a greater number of serotonin transporters, giving greater control over the levels of serotonin in the synapse.
Once the type of allele had been determined, the participants were split into three groups:
- Group one: two short alleles (s/s)
- Group two: one short and one long allele (s/l)
- Group three: two long alleles (l/l).
As well as separating the participants according to their alleles, the researchers also identified if participants had experienced one or more stressful events since their 21st birthday. This was done by using a life-history calendar and it was found that:
- 30% of participants had experienced no stressful life events
- 25% had experienced one
- 20% had experienced two
- 11% had experienced three
- 15% had experienced four or more.
Additionally, it was determined if participants had developed depression in the last year and it was found that 17% of participants met the criteria as having experienced a major depressive episode. This 17% sample was made up of 58% female and 42% male participants.
A correlation was then tested for between stressful life events and depression, between the length of the alleles and depression and an interaction between perceived stress and the length of the alleles.
Results
- Individuals having two short forms (s/s) of the gene were most likely to be depressed after stressful events
- Individuals with one long and one short (s/l) form were moderately likely to be depressed
- Those with two long alleles (l/l) typically responded as if no stressful events had occurred.
Conclusion
It was determined that while there is no direct relation between short alleles on the 5-HTT gene and depression, there is a relationship between these and incidences of stress and subsequent depression.
The long alleles seem to protect against experiencing depression as a result of stress.
Strengths of the study
- The study does appear to have produced clear evidence of a genetic factor in the development of depression
- The sample size was very large
- The independent variable (i.e. the length of the alleles) is naturally occurring, which suggests that the results are more reliable.
Weakness of the study
- The function of genes is extremely complex and it is impossible to determine if the 5-HTT gene alone is responsible for the findings
- Whether or not an individual finds an event to be ‘stressful’ is very subjective and therefore individuals may have reported different reactions to the same ‘stressful’ event – self-report is unreliable.
