Family Pedigree Studies

Researchers select “probands” with depression and examine whether their close relatives have a higher incidence of depression than the general population (or the relatives of non-depressed probands). This type of research has confirmed that as many as 20% of the relatives of depressed probands are depressed, compared with 5-10% of the general population (Harrington et al, 1993; Nurnberger & Gershon, 1992).

Twin Studies

Studies of monozygotic and diCognitive Development zygotic twins have also supported the genetic theory. One Danish study, for example, found that when a monozygotic twin has unipolar depression there is a 43% chance that the other twin will develop the disorder. If a dizygotic twin has the disorder the probability is only 20% (Bertelsen et al, 1977). Moreover, there is research evidence that suggests that being around a person that is depressed can lower a persons mood. Coyne (1976), for example, found that participants reported feeling worse than usual after a short telephone conversation with a depressed person. Interacting with a depressed person, may, therefore, contribute to the development of depression in that person’s relatives. Additionally, the twin studies demonstrate that factors other than genetics contribute to the disorder: in Bertelsen el al’s study 57% of the MZs of depressives did not develop depression, despite having identical genes.

Adoption Studies

A study of the families of adoptees who were hospitalised with severe depression found that their biological parents were more likely to have severe depression than the biological parents of a non depressed control group (Wender et al, 1986), but this was not the case with mild depression.

How could genes cause depression?

The mechanism of how genes lead to depression was investigated in a fascinating piece of research was by Kendler & Karkowski-Shuman (1997). They examined female MZ and DZ twins who were sufferers of major depression using the degree of genetic similarity as an index of genetic predisposition to depression. The number of stressful life events experienced by the participants was measured, these included: assault, serious marital problems, divorce/breakup, job loss, serious illness and major financial problems. They found that exposure to these stressful events was predicted by the closeness of the genetic relationships. Stressful life events that occurred during depressive episodes were not included in the analysis.

Kendler & Karkowski-Shuman concluded that, “In women, genetic risk factors for Major Depression increase the probability of experiencing Stressful Life Events in the interpersonal and occupational/financial domains. Genes can probably impact on the risk for psychiatric illness by causing individuals to select themselves into high risk environments.” While this may appear to be compelling evidence for a genetic basis of depression there are a number of issues to be considered:

• The research is based on the premise that genetic factors predispose a person to develop depression. I.e., twins of those who are depressed are more likely to develop depression because of genetic factors.
• The increased risk of developing the disorder in MZ twins of sufferers may be due to shared environmental factors.
• These shared environmental factors may be what leads them to “select themselves into high risk environments”.
• Alternatively, the shared environment itself may be high risk and, thus, account for the findings of the above study.