Psychology  9-1 GCSE - 

3.2.1 - Caspi et al. (2003) Influence of Life Stress on Depression: Moderation by a Polymorphism in the 5-HTT Gene:

Aims - 

To investigate whether functional change in the 5HTT gene is linked to a higher or lower risk of depression in an individual.

Method - 

The researchers used an opportunity sample from a cohort of participants who were part of another longitudinal study. There were 847 participants of 26 years old and they were split into 3 groups, depending on the length of their alleles on their 5HTT transporter gene. Group 1 - Two short alleles, group 2 - one short and one long allele, group 3 - two long alleles.
1. Stressful life events occurring after the 21st birthday and before the 26th birthday were assessed using a life history calendar. 
2. Post-year depression was assessed using the diagnostic interview schedule.
3. A correlation was tested between stressful life events and depression. Between the length of the alleles and depression and on interaction between perceived stress and the length of the alleles.
4. A further test was done to see if life events could predict an increase in depression over time among individuals with one or two short alleles.

Results - 

The participants with two short alleles in the 5HTT gene reported more depression symptoms in response to stressful life events than either of the other groups. Those participants with the two long alleles reported fewer depression symptoms. Moreover, childhood maltreatment was predictive of depression in adulthood, only with 1 or two short alleles.

Conclusions - 

While there is no direct relation between short alleles on the 5HTT gene and depression, there is a relationship between these and incidences of stress and subsequent depression.

Strengths - 

  • Large cohort of females and males.
  • The age was controlled to isolate the variable of stressful life events.
  • It was a natural experiment, with the naturally occurring IV (independent variable) being the length of the alleles.

Weaknesses - 

  • Gene action is highly complex and actions of the other genes could not be controlled.
  • Even if events are of the same nature, some are more stressful than others, this effects the results.
  • Symptoms of depression were self reported, self reporting can be unreliable.

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