New data suggests one in two of us experience mental illness in our life-times

Mental health charities and campaigners typically claim that one in four of us will experience a mental illness at some point in our life-times. This prompts disbelief in some quarters. The rates can't possibly be that high, so the argument goes, there must be something wrong with the figures. A new study led by Terrie Moffitt confirms that 'Yes', there is something wrong with the one in four figure - it should be one in two!

Previous estimates for the prevalence of mental illness are largely based on retrospective surveys. People are typically asked whether they've experienced any mental illness in the last year or at any point in their lives. The problem with this approach is that people forget or prefer not to mention a bout of illness that happened a long time ago.

Moffitt's new study avoided this problem by using a longitudinal, prospective design. Over 1000 people born in Dunedin in New Zealand between 1972 and 1973 were assessed four times between the ages of 18 and 32 by clinically trained interviewers using standard (DSM; Diagnostic and Statistical Manual) psychiatric criteria.

At each assessment point, any instances of depression, anxiety, or alcohol/cannabis dependence over the previous year were noted (comparable to the procedure for standard retrospective surveys). The key difference from past research is the data from each successive assessment was on record, thus allowing a cumulative life-time prevalence total to be generated for each participant (up to age 32), rather than relying on participants' recall.

Moffitt's team first checked the average 'past year' prevalence rates for mental illness and found they were similar in the new study and the old retrospective studies. This is as you'd expect and provides a validation of the new prospective data.

The big differences emerged when looking at the life-time prevalence data. By the age of 32, 49.5 per cent of the new sample had experienced at least one bout of anxiety disorder; 41.4 per cent had experienced depression; 31.8 per cent alcohol dependence; and 18 per cent cannabis dependence. These rates are approximately double those found in the classic retrospective survey data. For example, life-time prevalence of depression up to age 32 is 19 per cent according to the New Zealand Mental Health Survey and for anxiety it is 25.4 per cent according to the US National Comorbidity Survey.

So how can we account for this massive discrepancy? A clue comes from the fact that half of the participants in the new prospective study who'd had an incidence of mental illness before age 32 had had this diagnosis just once. It's highly likely then that it is people who experience short-lived, single episodes of mental illness who are missed by the classic retrospective surveys (unless that one episode has occurred in the previous year).

Of course a limitation of the data is that it's based exclusively on a New Zealand sample. However, the similarity of the classic retrospective data from New Zealand and the USA suggests there's nothing especially weird about New Zealand participants.

Another caveat is that the new data is based on diagnoses made according to the standard DSM psychiatric criteria. This study is not about the validity of those criteria - it's just saying that based on those criteria, the life-time prevalence rates of mental illness are much higher than has traditionally been claimed. In fact, the new findings suggest approximately one in two of us can expect to experience a bout of anxiety or depression in our lives as defined by the DSM. What's more, if anything, these new figures are underestimates because the new study ignored childhood problems, ended at age 32, and there were inevitable gaps in the four 12-month assessment windows used to generate the life-time prevalence data.

'...[T]he findings can be taken as evidence that existing and oft-cited retrospective prevalence rates undercount not trivially, but substantially,' Moffitt's team concluded. 'Researchers might begin to ask why so many people experience a DSM-defined disorder at least once during their life-times, 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.'
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Moffitt, T., Caspi, A., Taylor, A., Kokaua, J., Milne, B., Polanczyk, G., & Poulton, R. (2010). How common are common mental disorders? Evidence that lifetime prevalence rates are doubled by prospective versus retrospective ascertainment. Psychological Medicine, 40 (06), 899-909 DOI: 10.1017/S0033291709991036

Link to Guardian blog post on the one in four figure.
Link to related Mind Hacks blog post.

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