For Glass-Half-Empty types, this is the week in the year when the glass is liable to seem bone dry. Calls to the Samaritans peak on New Year's Day, the first of the cruellest month: January has the highest suicide rate. Could there be a better time for me to embark on a season of columns about the causes and treatment of depression?
For decades, psychiatrists have claimed depression is largely genetic. Yet the depression gene (along with those for almost any other mental illness) remains undiscovered. In fact, studies of identical twins prove that most people's depression is not genetic.
Suppose 100 people suffer from the mind-numbing, zombie-like agony of what is known as major depression. Further suppose that they have 100 duplicates of themselves in the form of an identical twin with exactly the same genes. On average, about half of the duplicates will also have the illness.
Oddly enough, this proves that genes cannot have been the cause of half the cases in even this extreme mental illness. In the cases where only one of the twins is afflicted, their environment - and only their environment - could have caused the depression: it could not be genes because they have the same ones.
The great majority of depression is not major but what is known as minor - depression-lite, you might think from the name, except that it is a hellish state, too. Whilst there may not be delusions or total withdrawal from the world, few of us have been spared some experience of this sad, angry negativity.
Whereas only four per cent of people at any one time have major depression, around one third suffer symptoms of the minor variety. In these cases, twin studies show little or no genetic origin, ergo, most people's depression is not caused by genes.
Rather, for the big picture, look to social class and to gender. Women are twice as likely to suffer as men. Poor mothers with small children are eight times as likely as rich men.
There are also large variations across time. Depending on which study you believe, a 25-year-old today is between three and 10 times more likely to suffer a major depression than one in 1950. And a recent study showed that for people in their twenties, rates of minor depression doubled between 1981 and 1996.
For the fine tuning, that explains why one sibling suffers but not the other, in most cases look not to genes but to differences in early parental care.
Among 800 British women, half of those who had been severely sexually abused as girls were depressed, so were 41 per cent of those who had been physically abused and one third of those who had been emotionally neglected.
By contrast, only eight per cent of women who had suffered none of these problems were depressed.
A recent British study showed that five-year-old children whose mothers had trouble coping and who lived in disharmonious homes were four times more likely to suffer depression at the age of 33 than children from undisturbed homes. Another study showed that five-year-olds whose parents had been rejecting and over-controlling were more likely to be severely self critical at the age of 31.
Depression takes two main forms. One revolves around a sense of the lack of others in your life, with feelings of emptiness and loneliness, similar to mourning. This has its roots in early childhood.
Unmet needs as a result of unempathic care in infancy leave the baby despairing that it can affect the world. Eventually, it develops an emotional default position in which it assumes that it is worthless and hopeless.
This can also be caused by unresponsive or erratic care as a toddler. If the main carer is depressed and does not react to the toddler, it feels unvalued and unimportant. Equally, if the carers keep changing, it is the same.
The other type of depression entails a sense of failure - the person is trapped in a web of hypercriticism from parents or of trying to meet impossibly high standards. This type is next week's cheery subject, the ideal preparation for the most depressing month of the year.