While researchers often talk about ‘finding the cause’ of some disease or disorder this often obscures the fact that only part of the story is known. Some causes are pretty straightforward. We know that a broken leg is usually the result of some kind of pressure or strain being applied. Moreover, if you have a broken leg you typically know when it happened (leg was fine yesterday, today it is broken) and how it happened (this morning you went skiing).
Things are not so simple with depression. We have good ideas about what some of the ‘pressures or strains’ that result i n depression are – but they are not all agreed upon and there might be others.
For any one person there could be many ‘pressures’ in their life. It is often unclear when the depression started – much of the time it gradually has an effect.
We can see another complication by going back to the broken leg example. Some people suffer from osteoporosis which makes their bones more fragile (more vulnerable). If you only had a minor accident when you went skiing, your osteoporosis was probably as much the cause of your broken leg, since it made your leg more vulnerable to the effects of pressure. If you have a major accident, however, the leg will probably break, osteoporosis or not.
In other words, the causes of depression are some mixture of ‘pressure’ (mild to severe) combined with a vulnerability to depression (as a sort of ‘psychological osteoporosis’) which too can range from mild to severe.
As noted earlier, for each ‘type’ of depression, differing ‘mixtures of causes’ have differential relevance. Thus, for psychotic or melancholic depression physical and biological factors are generally more relevant. By contrast, for non-melancholic depression, the role of personality (osteoporosis) and life-event stressors (accident) are generally far more relevant.
Source: Black Dog Institute