What is a Mental Disorder? - Definition & Treatment

Instructor: Sarah Lavoie

Sarah has taught Psychology at the college level and has a master's degree in Counseling Psychology.

Not all painful wounds are visible on the outside. Just like cancer or other illnesses, mental disorders can affect a person's whole life. In this lesson we will discover how mental disorders are defined, diagnosed, caused and treated.


You may have heard people refer to mental illness as a single type of illness, but mental illnesses or disorders actually encompass a wide range of symptoms, causes and treatments.

A mental disorder is a broad term used to group physical and psychological symptoms that cause abnormal thoughts and behaviors. Mental disorders are more commonly referred to as mental illnesses. These illnesses cause abnormal behavior that is disruptive to a person's life. Mental illnesses may be associated with the brain, but they have more in common with other bodily illnesses than they do differences. In fact, as we learn about mental disorders, it is good to keep in mind how similar they are to physical illnesses.


The most common model used by psychologists to explain why mental disorder occurs is called the biopsychosocial model. If you break that word down to its parts it simply means that biological, psychological and social factors all contribute to mental disorders. Just like in any other disease, these factors all work together to create mental disorders. It is very often many of the same factors that create other diseases such as cancer or diabetes. Many psychologists believe that any model that does not contain all three is incomplete.

Biopsychosocial model of mental disorder

Biological components include brain chemistry and heredity. Often a socially driven factor, prescription or recreational drug use can play a role in changing the brain's chemistry. As nobody lives in a vacuum, the social environment is pretty much anything that happens in the person's environment. In fact, all three factors, biological, psychological and social are all connected and intertwined and all affect each other.

Example of the Biopsychosocial Model

Let's create an example of an imaginary boy to demonstrate the model. Johnny Exampleson grows up in a poor family with a mother who abuses him. He starts being quiet in class and behaving weird at home. Johnny grows up with serious anxiety and depression that follows him through life. When he becomes an adult, he begins to use alcohol to make himself feel better, as he saw his father do. Alcoholism causes further physical and emotional problems that make it difficult for Johnny to work. When he can work, Johnny is bullied for being quiet and angry and told he should see a 'shrink.' This makes Johnny more angry, since he feels that they are calling him crazy and gives up on his friends at work. Eventually, Johnny is so depressed and anxious and drinking so much that he loses his job.

Biological Factors:

The boy has had physical abuse trauma. His father's alcoholism may be genetic. His alcohol consumption creates poor nutrition.

Psychological Factors:

Abuse creates stress in young Johnny. He is afraid of being beaten. He is scared and quiet at school and doesn't want to bring possible friends to his house. He is lonely and doesn't want to be at school or at home. Later, alcoholism creates further sadness and depression. Losing his job is extremely depressing and makes him worry about paying for rent and food.

Social Factors:

Johnny's parents are both considered social factors because they are in his environment. Johnny has trouble at school, then at work. However, when we get to the end of his story, Johnny's relationship with his doctor, therapist and his group at AA meetings are also social factors that affect him, and luckily, they are very positive.

These are just some of the possible bio, psycho and social factors. Each one connects to another, and another. Can you think of other factors that could have affected Johnny?

Diagnosis and Classifications

When these three biopsychosocial factors of brain, biology and environment combine in ways that make it difficult or even impossible for a person to function, it can be diagnosed as a mental disorder. Mental illness can often be diagnosed by a physician, but can also be diagnosed by a psychiatrist and other licensed health professionals such as therapists or counselors.


Just as the biological, psychological and social factors can never be the same for two individuals, the experience of a mental disorder is never the same for two individuals. We may think of mental disorders as being all emotional in nature, but most disorders have both physical and emotional symptoms.

Diagnostic manual for mental disorders
DSM IV-TR cover

The same as with physical disorders, diagnosis can be tricky. However, doctors and psychologists have guidelines. The guidelines for diagnosis and classification of mental disorders are set by the American Psychiatric Association in a book called the DSM (Diagnostic & Statistical Manual of Mental Disorders).

Classifications and Symptoms of Mental Disorders

mental disorders umbrella

The disorders shown in the first umbrella image are only some of the common disorder classifications. There are many more disorders within each classification, but we are only going to cover the most common and important terms. With the classifications of animals, each of these categories contain subcategories. We will learn more about two of the most common, anxiety disorders and mood disorders.

Anxiety Disorders

Anxiety disorders are well known and commonly diagnosed. Anxiety disorders are characterized by excessive worrying, uneasiness, fear and sometimes uncontrollable panic. Just like with symptoms of depression, these symptoms can make it very hard for an affected person to function at all! Although anxiety disorders are found in both men and women, women are more likely to seek treatment.

anxiety disorders umbrella

Mood Disorders

Mood disorders have many sub-classifications as well, but we will focus on the most common one. Depression is a term that many people have heard about. Depression symptoms include many that we could guess from the name, such as feeling sad, empty, tearful and hopeless. Some more surprising symptoms of depression are weight loss, extreme fatigue or insomnia, and thoughts of death or suicide.

It is always important to call a doctor or emergency room if you or someone you know is showing symptoms of being suicidal!

In our example, Johnny suffered from both an anxiety and a mood disorder as a result of his mother's abuse.


Just like any other type of illness, mental illness is difficult to live with. Just like physical illnesses, mental illnesses can be short or long term and vary in pain and intensity. Sometimes the disorders can become so difficult to live with that the affected person has to spend time in the hospital.


Psychopharmacology is just a big word for prescription medications that work on the chemistry of the brain. Pharmacies have developed prescription medications for most mental illnesses. These can help the person to live more comfortably with their illness. Most often, some combination of medications and therapy with a licensed mental health professional is the most effective. However, many people can be treated with one or the other.

Talk Therapy

Talk therapy has been given a strange reputation, and conjures images of stuffy rooms and long couches. However, talk therapy is simply conversation with a mental health professional who is very knowledgeable about mental disorders. Just as podiatrists study the feet and dentists study teeth, therapists are mental health professionals who study theories of talk therapy.

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