Aim

To estimate the contribution of genes and shared family environment on 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.

Sample

177 twins (probands) suffering from major depression between 1948 and 1986 and their same‐sex co‐twins. The sample was taken from the Maudsley hospital (London) register.

Method

Diagnostic assessments and detailed interviews were carried out interviewers were blind as to whether the twin was one of identical (MZ) or non‐identical (DZ) twins and also had no information on the mental health of the other member of the twin pair. One twin was usually interviewed by a different researcher than was the other. The probands and their co‐twins were asked about their shared environment: whether they were still in regular contact; for how long they had lived together, and whether they had shared a bedroom and, if so, for how long. They were also asked the degree to which, as children, friends and parents treated them alike.Doctors' reports from the hospital and from family doctors were also investigated. Correlations in the presence or absence of depression in the co‐twin were calculated.

Results

The concordance rate of major depressive disorder for MZ twins was 46% and for non‐identical (DZ) twins was 20%. 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 aged 65 years with the expected incidence of depression in the general population at that age, they found that the heritability factor rose to 70%. There was no difference between MZ and DZ twins in the length of time they had lived together or shared a bedroom. However, monozygotic twins more often than DZ twins reported dressing alike as children, being treated alike by parents, having joint friends, or being in the same class at school. All of these differences were significant at p<.05.

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 of a major depressive episode in one twin increased the likelihood of the other twin being a sufferer, suggesting that this was one criterion for a higher genetic component.

Strengths

• 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

National figures may 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%.• The research took place nearly ten years after the last patient was added to the hospital register. This means that many of the interviews were retrospective where people were asked to remember back as many as ten or fifteen years (or sometimes even more) and give details of depressive episodes. 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