Caspi 2003: Life Stress and Depression

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

Background of the Study

Research evidence has found that stressful life events cause depression, but not everyone who experiences stressful life events becomes depressed.

This study looks at whether a gene linked to the neurotransmitter serotonin, a brain chemical known to be involved in depression, makes some people more likely to become depressed after stressful life events than others.

A group of antidepressant drugs called SSRIs (selective serotonin reuptake inhibitors) help to increase the amount of serotonin available in the brain and reduce symptoms of depression. 

If low levels of serotonin cause depression, then maybe the genes that cause someone to have lower levels of serotonin than the rest of the population could explain why some people become depressed after stressful life events.

Aims

Caspi et al aimed to investigate why stressful life events seem to lead to depression in some people and not others. 

They then investigated whether stressful life events were more likely to lead to depression in people with a certain variation of the 5-HTT gene (a gene linked to the amount of serotonin available in the brain).        

Procedure

Participants were members of the Dunedin Multidisciplinary Health and Developmental Study – a group of children who have been studied since birth. 

In total, 847 children took part.  They were divided into three groups based on the version of the 5-HTT gene they had:

Table: groups of participants and their version of the 5HTT gene

This was a longitudinal study (over years), with each participant completing a questionnaire that measured the life events that occurred between their 21^st and 25^th birthdays. 

The events included problems with money, health or relationships. 

Another questionnaire assessed whether they had any symptoms of depression in the year before their 26^th birthday, which gave the researchers quantitative data in the form of a depression score.

Results                           

1. Participants who were found to have at least one short version of the 5-HTT gene and who had been through stressful life events between the ages of 21 and 26 years old showed a significant increase in symptoms of depression after the event and up until they were 26 years old. They were more likely to be diagnosed with depression than participants with two long versions of the gene.

• Those with a short version of the gene were more likely to develop suicidal thoughts after stressful life events than those with two long versions of the gene.

• Those with two short versions of the 5-HTT gene were most likely to report severe depressive symptoms if they had suffered a number of stressful life events, [including a significantly higher report of suicidal thoughts or attempts if they had faced more than three stressful life events.]

Conclusions

The evidence seems to suggest that there is an interaction between life events and genetic influences in causing depression.  This suggests that both nature and nurture work together to increase the likelihood of a person developing depression. 

A person seems to need to have both a specific gene and stressful life events in order to become depressed.

Evaluation

• One strength of the study is the size of the sample.  For example, there were 847 participants, which is a large sample. This is a strength because it means that generalising the results from the study should be easier that if the sample had been very small.

• Another strength is that the information from this study could be useful for doctors as it tells them that someone with depression could have different causes for their symptoms. For example, doctors should look at a variety of factors in their patients, including whether they have been through stressful events and whether they have the short version of the 5-HTT gene, to understand why they have become depressed.  This is a strength because it might also help them to understand why some people become depressed after a life event, while others don’t.  Realistically, however, doctors find it very difficult to assess all possible causes of a person’s depression due to time and cost restraints.

• One weakness of the study is that the researchers thought there was a chance that some people were naturally more likely to put themselves in situation where stressful life events might happen.  For example, the short version of the 5-HTT gene might be a natural factor that makes people prone to situations that are stressful.  This is a weakness because the gene and life events may not be causing depression as such, but instead the person is in more stressful situation s that could cause depression.

• Another weakness is that the evidence from the questionnaires is self-report data.  For example, some people might exaggerate their symptoms of depression while others might not want to make a big deal of them.  This is a weakness because their answers may not be true (lack of validity) and can't be considered reliable (consistent every time).

Summary

• Caspi et al 2003 found that patients who became depressed after stressful life events were more likely to have at least one short version of the gene that controls the level of serotonin available in the brain. 

• This suggests that there is an interaction between nature and nurture when causing depressive symptoms. 

• This study provides some value for society as it suggests that rather than simply looking for one cause of depression, we must consider that depression may have both biological and social causes. 

• This also has implications for treatment. 

• Some people may naturally be more prone to depression due to their genetic make-up, meaning that it needs to be considered how to treat patients who may be likely to suffer depression again in the future when faced with another stressful life event.