Sample response

One biological theory of depression is genetic inheritance. Genetic inheritance is the theory that abnormal behaviour, in this case, depression, can be passed down through genes. Psychologists believe that if an individual has a specific combination of genes, they are more vulnerable to depression; life stressors – particularly aversive childhood experiences – can cause the genes to be expressed, leading to the disorder.  Originally, most research on the genetics of depression was done using family and twin studies; today, since the Human Genome Project, research has focused on specific genes.

A twin study was done by Kendler to investigate the rate at which both identical (MZ) twins and fraternal (DZ) twins inherit depression. Using the Swedish Twin Registry, Kendler looked at over 40.000 twins and found that the concordance rate for female MZ twins was 44% and for DZ only 16%.  In males, the rates were 30% and 10%.

When looking at the results of identical twins we can firstly see that the percentage is not 100%. This indicates that if depression is genetic, having the genes for depression is not enough to make someone depressed. Instead, only through interacting with the environment, resulting in gene expression, may depression occur.  The fact that the MZ twins may not both have depression may have less to do with genes and more to do with the stressors that they have personally experienced. Inheriting the genes does not mean that the person will automatically develop depression. The results for the DZ twins being lower than the MZ twins supports the theory of genetic inheritance because fraternal twins are much less likely to have the same gene make ups.

A limitation of this study is that twins are a very small part of our global population. Although this study seems to support the theory of genetic inheritance we cannot know for sure if these results can be generalized to everybody. There is also an issue with a self-fulfilling prophecy, as identical twins may think that they are more likely to develop depression they might start to exhibit more symptoms. This also causes question into issues of undo stress or harm both because the study may contribute to the onset of depression, but that the study could also leave participants in fear that they will too develop the illness.

Modern research does not only look at twin research but also at specific genes.  Caspi carried out a prospective longitudinal study of the effect of the mutation of the 5-HTT serotonin transport gene. Caspi argued that people with two short alleles (the mutation) of the 5-HTT gene would be more likely to develop depression.  

Participants were allocated to groups based on the length of the allele of their 5-HTT genes.. The first group consisted of those with one short allele and one long, the second had two short, and the third had two long.  Participants were evaluated for depression and asked to fill out a questionnaire detailing major life events.  Those with the mutation and with major life stressors were more likely to exhibit symptoms of depression, and suicidal ideation.  Caspi found that participants with the mutation who had three or more stressful life events were the most likely to show depressive symptoms. 

The study shows that genes are not destiny, but that a combination of a genetic predisposition and environmental stressors may be.  Modern genetics is a holistic approach – recognizing that a gene-environment interaction often leads to behaviour, and not the genes alone. There have been replications of the study, so the research has been shown to be reliable. However, when dealing with depression, there are complications with the construct. It is difficult to know if depression is the same across all cultures and if serotonin plays a role in the origins of the disorder.

Genetic research has helped us to understand research in the sociocultural approach. For example, in Brown & Harris’s study on women and depression, they found that women who had fewer protective factors were more likely to get depression. This study is quite old and today we can hypothesize that it was most likely because of gene-environment interaction that the women developed depression.

With modern technology we are able to isolate specific genes and carry out research to test the strength of the correlation with depression.  This avoids the problem of early research on MZ and DZ twins that made assumptions about their genes with no way to verify them.   However, genetic studies are still correlational in nature, meaning that it’s difficult to establish a cause and effect relationship.