What the world’s longest study of mental disorders reveals about mental health during a person’s lifetime

Mental health is a topic many people are increasingly aware of and seeking treatment for nowadays, especially during the COVID-19 pandemic. But according to the world’s longest running study of mental disorders, a far larger portion of the population is significantly impacted by mental disorders over the course of their lifetimes than was previously believed.

Each year, the Marjorie Pay Hinckley Endowed Chair in Social Work and the Social Sciences sponsors a prominent social sciences scholar to visit Brigham Young University and speak on a topic of interest. For the 18th annual lecture on Feb. 3, Terrie E. Moffitt, professor of social development at King’s College in London and the Nannerl O. Keohane University professor of psychology at Duke University, spoke about her work in following 1,000 New Zealanders for several decades and what that revealed about mental health over a person’s lifetime.

“Most people that I talk to assume that mental disorder is very rare and because it’s rare, it can safely be ignored by policy makers and the rest of us,” Moffitt said during her lecture. “But virtually nobody realizes that it’s very common, especially in young people. And it’s a major source of inequality, it’s a major burden of cost to the public purse, and most important, mental health is an under-appreciated and fantastic prevention opportunity to improve our old age.”

Mental health clinicians and university researchers typically only see patients at one point in their lives, either when they seek treatment or participate in a study.

“We focus on making the diagnosis because our thinking is that the patient’s diagnosis adequately represents the patient and it tells us what treatment we want to choose for them and what the long term prognosis for their illness will be,” Moffitt said.

However, after joining the Dunedin Multidisciplinary Health and Development Study in 1985, Moffitt’s research caused her to “rethink what I thought I knew about mental disorder.” 

The study “began in 1972 as a birth cohort of all the babies born in one city. Ethnically, they’re of white European background,” Moffitt explained. The individuals came into the study’s clinic for a full day of assessment and data collection on 13 occasions since they were babies. It was the first to use a standardized diagnostic assessment to measure mental disorder in children, surviving four editions of the American Psychiatric Association’s diagnostic manual.

Terrie E. Moffitt, professor of social development at King's College and Nannerl O. Keohane University professor of psychology at Duke University, gives the Marjorie Pay Hinckley Lecture at the Hinckley Center at BYU on Feb. 3, 2022.
Terrie E. Moffitt, professor of social development at King’s College and Nannerl O. Keohane University professor of psychology at Duke University, gives the Marjorie Pay Hinckley Lecture at the Hinckley Center at BYU on Feb. 3, 2022. Credit: Ellie Alder, BYU

They assessed 16 mental disorders in three types of categories: externalizing (ADHD; conduct disorder; alcohol, cannabis, tobacco and other drug dependence), internalizing (depression, general anxiety, social phobia, simple phobia, panic, PTSD, eating disorders), and thought (schizophrenia, mania, OCD). Diagnosis for the disorders required impairment to the patients’ lives and corroboration by an informant.

They discovered that 85% of the cohort experienced a mental disorder during the course of the study between ages 11 and 45, with the highest rate of diagnosis happening when the participants were 15-18. Rates of mental disorders in the cohort went down as the participants aged, but part of the reason for this is that “people with mental disorders tend to die young from suicide, from overdoses, from accidents, and also from physical diseases as well,” Moffitt said.

The Dunedin cohort are not the only ones to show this high prevalence of mental disorders. Moffitt and her team found similar rates of mental disorders in the Danish national treatment register, reports done by the World Health Organization, and other longitudinal studies done in North Carolina; Oregon; Zurich, Switzerland; and Christchurch, New Zealand. She noted that mental illness is often undertreated, so the numbers from some of these sources should be higher.

As Moffitt likes to tell her students, “If you stay mentally well your whole life, you’re not normal.”

One surprising thing Moffitt and her team learned was that “the same person moves in and out of different mental disorders across their life,” she said, noting that the diagnosed disorders also moved between externalizing, internalizing and thought disorders without a noticeable pattern. 

Additionally, the earlier a participant was diagnosed with a mental disorder in their life, the more frequently they were later diagnosed with a mental disorder and the greater variety of diagnosed disorders they received.

Moffitt said: “The important point here to take away from this is that some people will have a mental health history that starts young, lasts a long time, and includes many different disorders, whereas other people will have a mental health experience that starts later in life, that comprises only a brief episode, and maybe just one diagnosis. And most of us will have a pattern that falls somewhere in between.”

Those who are diagnosed with mental disorders earlier in life also see physical effects in their later years. “People with mental disorders will age faster and age younger, they’ll have less ability to manage their health and finances as they need to prepare for old age, they’ll have more chronic physical diseases as they age, they’re going to use more health care and incur more healthcare costs for physical diseases, and they’re more likely to develop dementia as well,” Moffitt said.

Mental disorders are just as important for early mortality as physical diseases are, she said. “But mental disorders occur much younger and this gives us an advance opportunity for preventive treatment.”

Recognizing that most people will most likely have a brush with mental health problems will help reduce the stigma against mental illness, she said. “And if there’s less stigma than that ought to promote more people to feel safe to get treatment and get it sooner.”

Watch the livestream of the Marjorie Pay Hinckley lecture at hinckleychair.byu.edu/2022-hinckley-lecture.