Welcome to the third and final post examining the idea that one in four of us suffer mental illness at some point in our lives.
As I explained in parts 1 and 2, "one in four" has no basis in the scientific literature, although given how dubious the literature is, this is not necessarily a bad thing. It's not clear where the one in four meme originally came from, although most of the recent uses probably trace back to a 2001 WHO report which quoted it.
But why has one in four proven so popular? The simple answer is that it's high, but not too high. Were someone to say that one in every two people suffer from mental illness, most of us just wouldn't believe it. That's actually what most published studies have found, but it fails the laugh test. One in four is low enough to be believable but high enough to be striking, attention-grabbing, and memorable.
Why are so many people quoting the highest estimate they can get away with? After all, if we've (unconsciously) decided that one in two is "too high", as we seem to have done, we could equally well decide that one in four is too high. We could redefine "mental illness" such that the prevalence of it was, say, one in ten, or one in fifty. Those would be no more and no less valid than one in four. But we haven't, so why not? There are some obvious, but wrong, explanations -
Most obvious of all is Big Pharma. It's a well known fact that pharmaceutical companies are pure evil and that Satan sits on the board of directors of most them. And clearly, pharma do have a financial interest here. The more people who are deemed mentally ill, the more who might buy their pills. However, to think that pharma are primarily responsible for the spread of one in four is simplistic. Cynics that we are, no-one takes what pharma says seriously. At the least, they would need some accomplices to help convince people of the idea.
People sometimes accuse governments of talking up prevalence estimates. The Szaszian phrase "the therapeutic state" still crops up, with the implication that the government wants to use mental illness as an excuse to implement authoritarian policies. Well. This conspiracy theory doesn't seem all that realistic, given that the present British government, at least, couldn't conspire to get drunk in a brewery. Indeed if anything, modern governments generally treat the mentally ill as a financial burden. They require often-expensive treatment, and maybe also welfare payments. At least in Britain, the government currently trying to minimize the numbers officially considered mentally ill, to save money.
Finally, "Psychiatry" is said to be expanding its power by defining everyone as mentally ill. Anyone who has been to a psychiatry conference will find it hard to take the idea of such a grand plot seriously, but more fundamentally, this assumes that psychiatrists like treating people just for the sake of it. Why would they? In some countries, true, they do have a financial incentive to treat as many people as possible, although no more than any other medical professional. But in Britain and other countries with nationalized health-care, psychiatrists are paid a salary and every additional patient is just more work. And more chance of getting called up in the middle of a romantic dinner and having to talk down some suicidal person for three hours.
The true explanation, I think, is rather more boring than any of the above. Simply, one in four persists because everyone with an interest in talking about the prevalence of mental illness has an interest in talking it up.
Mental health charities and other advocates for the mentally ill like one in four because it's a great way of fighting the stigma attached to mental illness. One in four represents hard, scientific proof that mental illness is not rare, weird, and freakish - i.e. that the mentally ill are "just like everyone else" and have the same rights. Which is a perfectly good message, and remains one despite the fact that one in four is rubbish.
Academics like one in four - or rather, like high estimates of the prevalence of mental illness - because it gives their work an air of importance. Almost every research paper about depression, for example, starts with a paragraph of formulaic boilerplate to the effect that "Depression is really common". Here's the first paragraph of the first depression paper I plucked from PubMed at random (honestly) -
Major depression is a substantial public health problem, ranking first among the causes of worldwide disability. According to a study by the World Health Organization, depression was estimated as fourth leading cause of disability-adjusted life years (DALYs), a measure of disease burden, in all age groups and the second leading cause in people aged 15–44 years in 2001 (World Health Report, 2001). Lifetime incidence and prevalence estimates are not available for the UK or Scotland (Paykel et al., 2005). The one year prevalence for major depressive disorder (MDD) in Europe has been estimated at 3.9%, 5.0% in women and 2.6% in men (...). A large proportion of these patients remain untreated despite a number of campaigns aiming at increased awareness of depression.
This doesn't quote one in four as such (academics rarely do), but you can see why these authors and everyone else are not exactly lining up to debunk it. I'm not accusing the authors of this paper of being deliberately disingenuous; I'm sure they believe what they wrote. But they wrote it and made it their first paragraph for a reason. It's a running joke in modern science that when you're applying for grant money or trying to get work published, you have to talk up the "practical implications" of your research. In medicine a crucial aspect of this is talking up the seriousness and importance of the disease you're studying.
So, one in four persists because it's in no-one's interest not to say it. There's no conspiracy, just the collective action of various groups all of whom benefit from the idea that mental illness is extremely common. Fascinatingly, the very same mental health charities (and pop psychologists) who are traditionally opposed to academic psychiatry and Big Pharma have ended up promoting statistics which perfectly serve the interests of those groups. Not on purpose, of course. No-one is trying to be deceptive, everyone is just doing what they think is the right thing - but the end result is that this profoundly dubious statistic has become almost universally accepted.