How common are common mental disorders? By Sofie Hampton

The Dunedin Study has had a considerable amount of media coverage in the last year, including a television series titled “Why am I ?”. This study is famous because it tracks the lives of just over 1,000 people born in Dunedin during 1972-1973, and it is still continuing 45 years later. I am particularly interested in mental health, so I decided to a search for interesting research in this area that has come from the Dunedin Study. 

I found a paper from 2010 that compared the incidence of the four most common mental health conditions in New Zealand: anxiety, depression, alcohol dependence and cannabis dependence using retrospective studies (NZ Mental Health Survey) and prospective studies (Dunedin Study). The results showed that the retrospective study underreported on all four conditions, and amazingly, it underreported by almost one half. Indeed, by age 32, 49.5% of the study population had suffered episodes of anxiety, 41.4% from depression, 31.8% from alcohol dependency and 18% from cannabis dependency. This indicates that the number of people struggling with mental health problems may be a lot more common in the population than previously thought, and is much higher than the number of people seeking help. 

The problem with retrospective studies is they ask respondents to try and remember past experiences of mental illness. The research showed that they didn’t accurately report the severity of the effect of the mental health condition on their lives. An earlier study had shown that half of respondents with a documented prior episode of depression didn’t recall it when interviewed years later. The rate of recall failure for depression has been estimated to be 30% in men and 40% in women. If people couldn’t remember these episodes you might predict that they were relatively minor. However, some of those individuals had been hospitalized for depression and suffered major debilitating effects.  

This study has helped me with my nursing training by firstly teaching me the value of directly reading scientific research rather than the sensationalist stuff that appears in the media. More importantly, however, it has made me question the statistics of mental health conditions in New Zealand. There are obviously many people that are struggling that our system doesn’t even know about. We need to consider how we can engage all sufferers to access healthcare that is relevant to their condition. We should be encouraging people to reach out and focus on a positive recovery. In the long run, waiting for them to come to us when things get worse puts bigger costs and pressure on the system because the conditions become harder to treat. 



How common are common mental disorders? Evidence that lifetime prevalence rates are doubled by prospective versus retrospective ascertainment. Moffitt et al. Psychol Med 40:899-909 (2010).