Why, for example, does it exist at all? Has it some adaptive evolutionary function that enables the depressed to survive better than others? Or is it a normal function that has gone wrong?
It is helpful to liken depression to cancer, which is the result of a highly adaptive process in which cell growth and multiplication become abnormal and thus pathological. Cells multiplication is under strict control, but this control can fail, allowing cells to divide too often and to acquire abnormal properties that make them malignant and even lethal.
By analogy, I believe that depression is sadness that has become pathological. Sadness is the emotion that most closely resembles depression, and Freud pointed out the similarities between depression and grief caused by bereavement.
Depression is thus a disorder of a basic emotion. But what is emotion? Among the universal emotions are such states as sadness and fear; these different states may represent responses to signals that may help a person gain reward or avoid harm.
But how do sadness and grief fit into this scheme? Sadness, like depression, is an unsatisfactory term. It covers a range of feelings and gives no sense of the intensity, for instance, of sadness associated with grief.
Why should the loss of a loved one be so devastating? The answer seems to to lie in attachment. Attachment - the need for a child to maintain a close bond with its mother, and for the mother to be attached to her child - is adaptive, helping to ensure the survival of the child and therefore of the mother's genes. For attachment to be effective, the loss or removal of the mother from the child must necessarily cause it distress. Attachment also promotes survival in reproductive couples.
Sadness, anxiety and fear can appropriately be described as "malignant", since they can, just like malignant cancer cells, affect, even invade, other mental processes. They can even invade each other, causing a reciprocal influence: a common characteristic of depressed individuals is that they are negative about everything to an unrealistic degree.
Malignant sadness, it seems, distorts their thinking - because they are overwhelmingly sad, they rationalise the feeling by seeing everything in a negative light. This in turn makes them feel even sadder and so a vicious loop of sadness and negativity is set up, each reinforcing the other.
The origin of depressive illness is complex, but vulnerability to depression is known to have a strong genetic component. Abuse or severe neglect in childhood can accentuate this vulnerability.
A small structure in the brain, the amygdala, is overactive in depressives. It is intimately involved in processing emotion, and its overactivity may initiate, and be sustained by, negative thinking.
Therapy can break this vicious cycle, either by reducing this brain activity or controlling negative thoughts. Antidepressant drugs work well at reducing the relevant brain activity, and cognitive therapy also does well at changing negative thinking. A combination can be particularly effective.
I am sad, but not depressed, that I am losing my column - this is my last. Thanks for reading.
Lewis Wolpert's 'Malignant Sadness - The Anatomy of Depression' is published by Faber&Faber at pounds 9.99Reuse content