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?

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