Most people regard themselves in more positive terms than other people regard them, and have an unduly optimistic outlook on their circumstances and possibilities. Such positive illusions, as they are called, are of three broad kinds, an inflated sense of one’s qualities and abilities, an unrealistic optimism about the future, and an illusion of control over external occurrences. For instance, most people think that they are a better than average driver, citizen, or parent, collectively implying that the average driver, citizen, or parent is in fact not at all average. A couple on the verge of getting married is likely to over-estimate the odds of having a gifted child but underestimate the odds of having a miscarriage, falling ill, or getting divorced. Positive illusions may confer certain advantages such as an ability to take risks, persevere with major undertakings, and cope with traumatic events. In the longer term, however, the loss of perspective and poor judgement that come from undue self-regard and false hope are likely to set us up for failure and disappointment, not to mention the emotional and behavioural problems (such as anxiety and anger) that are associated with a defended position.

It is interesting to note that positive illusions are particularly prevalent in Occidental and Occidentalised cultures; in East Asian cultures, for example, people do not tend to be self-enhancing and may even be self-effacing. Positive illusions are also more marked in unskilled than in highly skilled people, who tend to assume, falsely, that those around them enjoy a similar level of competence. This so-called Dunning-Kruger effect is encapsulated in a short line from Darwin’s Descent of Man: ‘Ignorance more frequently begets confidence than does knowledge.’

In contrast to most, people who might be diagnosed with depression are prone to a number of cognitive biases or distortions that might be thought of as ‘negative illusions’. Three examples of cognitive biases that occur in depression are ‘selective abstraction’, ‘dichotomous thinking’, and ‘catastrophic thinking’. Selective abstraction involves focusing on a single negative event to the exclusion of other, more positive ones. For instance, a depressed student may focus on a test that he has failed at the expense of all his previous academic achievements and all the other good things in his life, his strong physical health, confiding friendships, supportive family, and so on. Dichotomous thinking is a form of thinking in which an outcome is either all good or all bad, with no room at all for shades of grey. A common example of dichotomous thinking in hospital in-patients with depression is, ‘If he doesn’t come to see me today then he doesn’t love me.’ Another, more subtle, one is, ‘If I am not out of hospital by Jack’s birthday, he’ll know that I do not love him.’ Catastrophic thinking involves exaggerating the possible consequences of an event or situation, as in, ‘The pain in my knee is getting worse. When I’m in a wheelchair, I won’t be able to go to work and pay the mortgage. So I’ll end up losing my house and living in the street.’

People who might be diagnosed with depression may be prone to a number of such cognitive biases, but the scientific literature suggests that they can also display more accurate judgement about the outcome of so-called contingent events (events which may or may not occur) and a more realistic perception of their qualities, abilities, and limitations. This phenomenon of so-called depressive realism is most pronounced at the milder end of the depressive spectrum, and it may be that the most accurate perception of self and reality belongs not to the tranquil or joyful but to the mildly discontented.