What Is the Definition of Mental Illness?

Mental illness refers to mental health conditions that have a negative effect on the way an individual thinks, feels, and behaves. Just like the phrase “physical illness” might describe a vast array of physical health conditions, mental illness encompasses many different types of mental health problems.


The National Institute of Mental Health estimates that one in five adults in the United States experience a mental illness in any given year. That means almost 20 percent of the population has a mental illness.

An estimated one in 25 adults in the United States experiences a serious mental illness in any given year that interferes with or limits one or more major life activity. Some mental illnesses, such as ADHD, begin during childhood. Other mental illnesses, like schizophrenia, are likely to become apparent during adolescence.

Mental illnesses, like depression or anxiety disorders, may begin at any age. They may be sparked by a stressful life experience or symptoms may appear for no apparent reason.

It’s also possible to have more than one mental illness at a time. For example, someone with generalized anxiety may also have ADHD. Or, someone with anorexia nervosa may also have depression.


There are over 200 mental illnesses. They are broken down into specific classes or types. The classes include:

  • Neurodevelopmental disorder
  • Schizophrenia spectrum and other psychotic disorders
  • Bipolar and related disorders
  • Depressive disorders
  • Anxiety disorders
  • Obsessive-compulsive and related disorders
  • Trauma and stressor-related disorders
  • Dissociative disorders
  • Somatic symptom and related disorders
  • Feeding and eating disorders
  • Elimination disorders
  • Sleep-wake disorders
  • Sexual dysfunction
  • Gender dysphoria
  • Disruptive, impulse-control and conduct disorders
  • Substance-related and addictive disorders
  • Neurocognitive disorders
  • Personality disorders
  • Paraphilic disorders
  • Other mental disorders

Signs and Symptoms

The symptoms of mental illnesses vary greatly depending on the condition. For example, someone with depression may experience decreased energy and trouble sleeping while someone with an eating disorder may binge and purge.

All mental illnesses have one thing in common—they interfere with a person’s ability to function.

In order to meet the criteria for mental illness, an individual’s symptoms must interfere with their social, occupational or educational functioning.

Everyone experiences peaks and valleys in their mental health. A stressful experience, such as the loss of a loved one, might temporarily diminish your psychological well-being. But that doesn’t mean you’re mentally ill. Most mental illnesses require that the symptoms last for a certain period of time, such as two weeks.

Some individuals have insight into their illness and recognize that they’re experiencing a problem. Someone with an anxiety disorder, for example, will likely recognize that their symptoms are affecting their everyday life.

However, someone who has obsessive-compulsive disorder may not realize that their thoughts are distorted. For example, someone who compulsively washes their hands due to a fear of germs may not realize that their fear is irrational.

In general terms, common symptoms of mental illness may include things such as:

  • loss of interest in leisure activities
  • changes in sleep
  • changes in appetite
  • withdrawal
  • unexplained physical symptoms, such as headaches and stomachaches
  • difficulty concentrating
  • changes in mood

Each mental illness has a different set of symptoms, but they tend to involve changes in thinking, mood, and behavior. If you suspect that you or a loved one has a mental illness it’s important to speak with a physician about your concerns.


The exact cause of most mental illnesses isn’t known. Instead, it’s thought that they stem from several different factors. The following are some factors that seem to influence whether someone develops a mental illness:

  • Genetics. Many mental illnesses seem to run in families. Individuals who have a relative with illnesses, like schizophrenia, may be at a higher risk of developing it, for example.
  • Biology. Brain chemicals play a major role in mental illnesses. Changes in neurotransmitters, the chemical messengers within the brain, play a role in depression, for example. 
  • Environmental exposures before birth. If your mother drank alcohol, used drugs, or was exposed to harmful chemicals or toxins when she was pregnant with you, may be at a higher risk of developing mental illness.
  • Life experiences. The stressful life events you’ve experienced may cause you to develop a mental illness. Enduring traumatic events might cause you to develop conditions, like PTSD. Changes in caregivers as a child may also cause you to develop an attachment disorder.


Mental illness can create a variety of complications in someone’s life. Common complications include:

  • Family conflict
  • Loss of interest in activities that were previously pleasurable
  • Sexual dysfunction
  • Increased absences at work or school
  • Decreased performance at school or work
  • Poverty
  • Homelessness
  • Legal issues
  • Drug or alcohol problems
  • Physical health problems
  • Increased risk of suicide
  • Behavioral issues


The Diagnostic and Statistical Manual (DSM-5) is the guidebook that is used by professionals to diagnose mental illness. It describes the criteria and symptoms for each mental illness. Mental illnesses may be diagnosed by a physician or a mental health professional, like a psychotherapist, psychologist, or psychiatrist.

To determine whether you have a mental illness, you will most likely be interviewed. A professional will want to understand the history of your illness, the symptoms you are experiencing, and the problems your illness is causing.

Family members may also be asked to participate in the interview so they can describe any symptoms they see.

Before a diagnosis is made, you may need to undergo a physical exam to rule out physical health issues. Thyroid issues, for example, may cause symptoms of depression or anxiety.

You may also be asked to complete questionnaires or to undergo psychological testing. Screening tools or psychological tests may assist a professional in pinpointing your exact diagnosis or help determine the severity of your illness.

For more information on CPTSD and other issues visit our YouTube Channel

If you need support or would like to connect with like-minded people join our Private and Closed online Facebook Group for Child Abuse Survivors and those with CPTSD. Click here to join

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