How many people have been depressed?

It sounds like a simple question to answer; in reality it's anything but. 

Depression isn't just about feeling unhappy or fed up. 'Clinical depression' is treated as an illness by the NHS, but there's surprisingly little clarity on how widespread the condition actually is.

Going by what the Daily Mail said this week, half of UK adults say they've been depressed at some point in their lives, "twice as many people as official statistics show".

The 'half' being referred to is actually 45% - the proportion of adults who answered yes to a recent ICM poll question asking, along with some background information about the condition: "Have you ever experienced depression?".

But polls like this aren't necessarily all that revealing. Just two reasons are:

• people answering the poll are likely to have different views about what counts as depression (and report varying degrees of severity);
• not everyone will perfectly recall how they've felt over the course of a lifetime.

Taking it to the doctors

If the problem is not having a single, reliable definition of depression, one solution is to find out how many people are being diagnosed by doctors based on clinical symptoms. Here there's a more concrete number: 11.7% of adults in England (a narrower geography) had a diagnosis of depression in 2011/12.

But this too isn't all that revealing:

• not everyone with symptoms of depression is necessarily diagnosed with the condition;
• diagnoses only tell us how many people are currently suffering from a condition, they don't show the lifetime prevalence.

'Official' statistics?

The Mail refers to 'official statistics' suggesting that only about one in four of us have ever experienced depression. As far as we can tell, no such official statistic exists. 

Instead, the Mental Health Foundation, the Royal College of Psychiatrists, the British Medical Journal and the Department of Health all cite an identical figure - one in four - but this actually refers to all mental illnesses rather than just depression alone. They all cite the same source: the 2007 Adult Psychiatric Morbidity Survey.

The Survey used a series of interviews based on matching symptoms people showed or reported to standard clinical definitions of depression and other mental disorders, so it potentially gives us a fuller picture than a poll asking about whether or not people had ever 'felt depressed'.

When it comes to depression, however, the study only looked at how many people presented with the condition in the week prior to the interviews. Less than 3% of adults aged 16-74 had a 'depressive episode' in the previous week.

So not much help there either.

As many as two thirds?

The BBC did a feature about depression last year. Curiously, they cited a much higher figure: "about two in three adults have depression at some time in their life", sourcing this to Patient.co.uk.

The medical information site didn't cite a working source, but we tracked a similar claim back as far as a 1992 research paper which stated: "approximately 60-70% of the adult population will at some time experience depression or worry of sufficient severity to influence their daily lives". No further source was given.

So not only is the claim - as far as we can tell - at least two decades old, no one making use of it actually seems to know where it comes from. Until we know more, this doesn't inspire much confidence.