Study - Caspi et al. (2003): 5-HTT Gene & Depression

What were the aims of the study?

1. Investigate why stressful life events led to depression in some but not others

2. Whether stressful life events more likely led to depression in people with variation of 5-HTT gene

Who were the participants in this study?

• 847 Caucasian non-Maori participants from New Zealand

• Applied opportunity sampling

→ They were members of Dunedin Multidisciplinary Health & Development Study

What were the different variants of the 5-HTT gene?

• Two copies of short allele - 17%

• One copy of short & one copy of long allele - 51%

• Two copies of long allele - 31%

What ages were the participants studied at?

3-21, and virtually again at age 26

What was the procedure of the study?

Longitudinal study

1. 847 members were split into 3 groups based on variant of 5-HTT gene:

2. Participants were asked to complete a questionnaire about stressful life events between 21-26 years old

3. Symptoms of depression one year before 26th birthday measured with Diagnostic Interview Schedule & an informant was asked via questionnaire

Questionnaire 1 - What were the results on stressful life events in the study?

• No stressful life events: 30%

• One stressful life event: 25%

• Two stressful life events: 20%

• Three stressful life events: 11%

• Four or more stressful life events: 15%

Questionnaire 2 - What were the results on depressive symptoms in the study?

• DSM-IV depressive episode experienced in past year: 17%

• Past-year suicide attempt: 3%

What were the findings of the study?

1) There was no difference in number of stressful life events experienced across the three groups

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2) Individuals w/ a short allele had a significantly stronger interaction between life events & self-reported depression at age 26.

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3) Individuals w/ an short allele whose life events occurred after their 21st birthday experienced increases in depressive symptoms from 21-26.

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4) Stressful life events predicted suicidal ideation among carriers of an short allele.

What were the conclusions of the study?

1) The 5-HTT gene interacts with life events to predict depressive symptoms & suicidality [Nature & Nurture]

2) Those w/ a long allele were less likely to develop depression or suicidality

What are strengths of the study?

1) INCREASING GENERALISABILITY

A large sample was used (847 participants), → more accurately reflect results of the target population

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2) PRACTICAL APPLICATIONS

Findings can be useful for psychologists/doctors

→ should consider different causes of depression (stressful events, short allele of 5-HTT gene)

What are weaknesses of the study?

1) SUBJECTIVITY

Questionnaires use self-reported data

→ some may exaggerate, whilst others hide information = unreliable

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2) LOWERS INTERNAL VALIDITY

• Gene & life events may not directly cause depression

• Instead researchers thought short allele of 5-HTT made people prone to putting themselves in stressful situations → depression