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Saturday, September 12, 2009

Most People Experience "Mental Illness" By Age 32

Mental illness: how common is it? A popular answer is one in four - 25% of people will experience it at least once in their lives. In fact, most published research suggests that the lifetime rate is higher, around 30-50%, in Western nations.

That's a lot. But even this may be a serious underestimate, according to a new paper, How common are common mental disorders? The study compared the proportion of people reporting mental illness under two different research methods: retrospective and prospective.

Retrospective means asking people to think back and remember whether they ever have felt a certain way. A prospective study, however, recruits people and then follows them up for a certain length of time, asking them how they feel at regular intervals.

The obvious advantage of prospective studies is that there is less chance of forgetting. In a retrospective study, people are required to remember how they were feeling years, or even decades, ago. Human memory just isn't that good. A prospective study requires some remembering, as people are generally asked to report how they've felt over the last year, but this is clearly less problematic.

The prospective study in question here included 1,000 people from Dunedin, New Zealand. The volunteers were followed from birth to age 32, and were interviewed at ages 18, 21, 26 and 32. The results were compared to three large retrospective lifetime studies, two American and one from NZ. (1,2,3).

50% of the Dunedin prospective cohort reported at least one "anxiety disorder", 41% reported "depression", 32% confessed to "alcohol dependence" and 18% to "cannabis dependence". (Those were the only conditions studied.) For some reason, we're not told how much overlap there was, but even assuming there was a lot, well over half of all the cohort will have experienced at least one disorder. If the overlap was low, it could be almost all of them. And remember, this is just up to age 32. And there still may have been some forgetting...

Compared to the retrospective studies, these rates are all about twice as high. What does this mean for psychiatry?

First, it suggests that retrospective studies, which are by far the most common, are flawed. People just tend to forget a lot of "mental illness" when asked to remember across the lifetime. More evidence for this comes from the fact that the ratio of past-year to lifetime reported disorders was 38% in the prospective study compared to about 60% in the retrospective ones.

But there's a more profound implication. A growing number of critics have argued that the very high reported lifetime rates of mental disorders mean that the way most psychiatrists diagnose mental illness is flawed. The "Bible" of modern psychiatric diagnosis is the Diagnostic and Statistical Manual (DSM) of Mental Disorders of the American Psychiatric Association. DSM diagnostic criteria were used in the studies in question here.

These results suggest that DSM diagnoses are even more common than previously believed, which only strengthens the critics' case. According to DSM criteria, at least 40% of people experience "Major Depressive Disorder" by age 32.

In which case, what is it? A fairly usual part of human life. So, calling it a disease and treating it with drugs or therapy seems rather presumptuous. Especially since so many people who "suffer" from it manage to not only get over it, but actually forget it ever happened. (Of course, this shouldn't be taken to mean that real, serious clinical depression doesn't exist.)

The authors conclude - listen carefully -
This article is uninformative (and agnostic) about the validity of diagnoses as defined by DSM-IV ... [rather], objections voiced to surveys’ higher than expected lifetime prevalence of disorder are objections to prevalence that is only half what it could be in reality...

Researchers might begin to ask why so many people experience a DSM-defined disorder at least once during their lifetimes, and what this prevalence means for etiological theory, the construct validity of the DSM approach to defining disorder, service-delivery policy, the economic burden of disease, and public perceptions of the stigma of mental disorder.
That hammering sound you hear is another nail sealing the coffin of DSM's credibility. If many* DSM "disorders" are simply descriptions of normal parts of human life, we need to take a long, hard look at those "disorders", and rethink whether they need to labelled and treated as medical problems.

The newest edition of DSM, DSM-5, is currently in development. This would seem like a great opportunity to do just that. Unfortunately, the development process is rapidly degenerating into farce. If DSM-5 does not address the issues raised here, many people will be tempted to give up on DSM entirely.

* Not all: the great majority of people will never meet criteria for schizophrenia or bipolar disorder, for example.

ResearchBlogging.orgMoffitt, T., Caspi, A., Taylor, A., Kokaua, J., Milne, B., Polanczyk, G., & Poulton, R. (2009). How common are common mental disorders? Evidence that lifetime prevalence rates are doubled by prospective versus retrospective ascertainment Psychological Medicine DOI: 10.1017/S0033291709991036

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