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How are mental health illnesses like or unlike physical ones?

By Nicolas Scrutton Alvarado

For over 2000 years, doctors and philosophers alike have been struggling to understand the causes of mental illnesses. Opinions have swung back and forth, ranging from the possession of demons in the Middle Ages, to moral disturbances, to a physical illness no different than the common cold.

The idea that mental illnesses are different from physical ones only really began in the late 1700s, with the development of public “madhouses”. The separation between asylums and traditional hospitals emphasized a distinction in how we approached the two types of illness.

A painting of a madhouse by Franscico Goya. A group of people are painted In a room, in various states of movement.
Franscico Goya’s famous “Casa de Locos” (1812-1819)

The establishment of the York Retreat in 1794, by the Quaker William Tuke in the United Kingdom, famously seeded the idea that mental illnesses were not a physical ailment, but a moral one. Shocked by the recommended treatments in hospitals at the time, which often included restraint and isolation, Tuke went on to establish a radically different method, one focused on compassionate moral treatment. The success of such treatments, both in the UK and other similarly-minded institutions, left an impression on the public. It was around this time that the term “mental illness” began being widely used, separating them from physical illnesses. This separation was only strengthened as autopsies of the time were unable to observe any obvious physical changes accompanying mental illnesses.

We now understand that the separation between physical and mental illnesses is not a biological one. The mind and body are highly interconnected. Many mental illnesses present physical symptoms which are clearly observed with current techniques, and many physical illnesses present general mental symptoms. Pain, for example, is the primary symptom of many physical injuries, like a broken bone or digestive issues. However, pain is a purely psychological phenomenon. Likewise, depression is often associated with a lack of energy, a physical symptom. While the biological causes of many mental illnesses remain an active area of study, it is now understood that they are firmly grounded in the body – the same as physical illnesses. As a result, modern medicine now treats mental illnesses as another type of disease (it’s currently listed as Category 6 in the International Classification of Diseases manual).

That being said, mental disorders do have identifying characteristics. Mental disorders are primarily characterized by significant changes in how someone thinks, feels, or behaves. While there are physical symptoms, such as tiredness or digestive issues, they are treated as secondary. Many mental illnesses, such as Parkinson’s Disease, show clear changes in the brain which can now be observed through modern techniques. The research into the biological causes and impacts of mental illnesses is still ongoing, and we don’t fully understand the mechanism by which our most common treatments help, nor why they help some but not others. For example, sertraline (also known as Zoloft) is a common antidepressant used to treat a range of disorders, including major depression and social anxiety. Sertraline belongs to a class of molecules known as SSRI’s, or selective serotonin reuptake inhibitors. In short, they cause an increased concentration of serotonin, a neural messaging molecule, in the brain. Setraline has been shown to significantly reduce social anxiety symptoms after 6 weeks, and modestly decrease major depressive symptoms after 12 weeks. However, recent evidence has put the role of serotonin in major depression into question. How does a medicine that affects serotonin levels in the brain reduce depression symptoms if serotonin isn’t a major player in depression? We don’t know the answers to these questions yet, and they’re not easy answers to find. They require rigorous (and confusing) research which can’t often be conducted in humans.

Additionally, the identification of mental illnesses, while much improved relative to 50 years ago, is still difficult. Misdiagnosis rates for several of the most common illnesses, such as depression and general anxiety, are over 60% — a number much higher than the average 10% misdiagnosis rate in general medicine. Similarly, stigma against mental illnesses remains a huge issue, and occurs much more commonly than stigma against general illnesses.

It’s tempting to separate mental illnesses into their own world. You wouldn’t be entirely wrong to, given how mental illnesses are and have been commonly treated in our society. But it’s important to understand that mental illnesses are just another kind of illness, and need to be treated no differently than more obvious physical ones.

If you’re interested in reading on some of the things mentioned in this article, feel free to check out the resources used while researching the answer!

  • An overview on the history and philosophy behind separating physical and mental illnesses

  • A history on the foundation of the York Retreat

  • A study on physical symptoms in depression

  • A recent study exploring the relationship between gut and mental health

  • A report on diagnostic error in medicine

  • An analysis of diagnosis rates of common mental illnesses

  • An investigation on the role of serotonin in major depressive disorder

  • A clinical trial investigating the effectiveness of sertraline in treating social anxiety disorder and major depressive disorder

  • A fact sheet on sertraline

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