McGuffin et al. (1996)

A Hospital-Based Twin Registry of the Heritability of DSM-IV Unipolar Depression

Background information: There is consistent evidence from many studies that mental disorders run in families. However, this does not necessarily mean that they are genetic. Researchers have not always been able to determine if it is family environment and upbringing that results in mental disorders in some families, or if there is truly a genetic component.

Aim: To estimate the contribution of genes and shared family environment to the tendency to having major depressive disorder, and to examine the influence of characteristics of one twin suffering from the disorder on the likelihood of the other suffering from it.

Method: This was a study of 177 twins (“probands”) suffering from major depressionfrom the Maudsley HospitalRegister (London, England), between 1948 and 1986 and their same-sex co-twins. Diagnostic assessments and more detailed interviews were carried out on 69 of the original twins on the register and 75 co-twins. The interviewers were blind as to whether the twin was one of identical (MZ) or non-identical (DZ) twins and to information on the mental health of the other member of the twin pair. One of a twin pair was usually interviewed by a different researcher than was the other. Doctors’ reports from the hospital and from family doctors were also investigated. Correlations in liability were calculated.

Results: The concordance rate of major depressive disorder for MZ twins was 46% and for non-identical (DZ) twins was 20%. This shows that there is a heritability factor for major depression. Durations of depressive episodes that were less than 13 months were correlated more strongly with depression in the other twin. When the researchers compared the incidence of depression in the identical twins at age 65 years with the expected incidence of depression in the general population at that age, they found that the heritability factor rose to 70%.

Conclusion: There is a strong genetic component to major depressive disorder, and no evidence for it being caused by a shared family environment. A shorter duration in one twin increased the likelihood of the other twin being a sufferer, suggesting that this was a marker for a higher genetic component.

Evaluation

Strengths: Inter-rater reliability needed to be ensured, with different interviewers for each of a twin pair. Many of the interviews were retrospective where people were asked to remember back as many as ten or fifteen years or sometimes even more. This has to bring into question the reliability of such memories.

The sample used was of people who had been depressed enough to have sought medical treatment at a hospital and may not be representative of the general population of people suffering depression.

The strength of this study is its methodology. The researchers were careful to conduct a blind experiment with the researchers not knowing in advance whether they were interviewing the original twin, who had already been diagnosed with depression, or the co-twin, who may not have been. The study used data from a hospital register, details from family doctors and interviews with the people themselves. This triangulation of approach should raise the reliability of the data.

Limitations: When comparing the likelihood of having had major depressive disorder by the age of 65 between the participants in this study and the general population, figures for the general population were gathered from the surrounding London area in which the hospital was based. Yet people came from all over the UK to attend that particular hospital (the Maudsley), which was famous for its treatment of depression. National figures may therefore have been more representative. The researchers themselves admit that their general population estimates of the likelihood of major depressive disorder (8.4% for women and 3.6% for men) were much lower than the estimates of a similar study conducted in Virginia, USA, which gave the lifetime figure for women as over 30%.

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