The diagnosis of depression is not straightforward and that people with depression can vary from one another in the type and severity of their symptoms.
There isn't a single explanation for depression that can be held accountable for all cases and can be assumed that more than likely different factors interact to produce depressive symptoms.
A depressive episode can often be triggered by stressful life events. People with depression often experience a greater number of such events during a month just before the start of the disorder than do other people (Kendler et al. 2004)
stress can only be one of the many contributory factors. It is also the case, that, whatever the underlying origin of the disorder, depressive symptoms express themselves though chaanges in the brain and the central nervous system .
Biological explanations of Depression
There are several factors that suggest that biological mechanisms may play a part in depression:
physical changes - The symptoms of depression include physical changes (e.g. sleep and appetite distubance, weight changes, fatigue and lack of energy).
similar symptoms - There is great similarity in the symptoms across cultures, races, gender and ages.
Families - Depression runs in families (although this could be due to a shared environment).
Drug treatments - Antidepressant medication can be successful in treating depression (although this does not in itself prove an underlying biological cause).
Genetic explanations of Depression
Depresion seems to run in families and a number of studies have shown an increased risk for depression in first-degree relatives of people with unipolar depression.
Biological closeness of the relationship and the likelihood of developing depression.
People with a relative diagnosed with bipolar disorder have three times the greater risk of developing MDD than people without relatives suffering witht he disorder. This suggests that the two disorders might share some genetic component even though the main symptoms are different.
Research into the influence of genetics has been carried out in the form of family, twin and adoption studies.