The Big 5

The world's most common mental illnesses

Read on to learn more about the five most common mental illnesses: depression, bipolar disorder, obsessive compulsive disorder, schizophrenia, and addiction.

1. Depression

Depression is a mood disorder. Although some people may experience mild depression and think they have it under control, they are at risk of developing severe depression if they don't get treatment. About 14.8 million American adults (6.9% of the population) live with major depression.

Individuals with depression are:

  • Twice as likely to develop coronary artery disease
  • Twice as likely to have a stroke
  • More than four times as likely to die within six months from a heart attack

Signs and symptoms

  • Depressed mood nearly daily for more than two weeks
  • Emotionally numb
  • Trouble falling asleep, staying asleep, or sleeping too much
  • Substantial change in appetite
  • Low sex drive
  • Fatigue or loss of energy on a daily basis
  • Feeling slowed down or agitated
  • Loss of interest or pleasure in activities
  • Diminished ability to think or concentrate
  • Feelings of worthlessness, guilt
  • Thoughts of death or suicide


  • Severe loss, such as the death of a loved one, a break up, or a divorce
  • Financial loss
  • Job loss

Dr. Crosby explains the signs and symptoms of depression.

2. Bipolar disorder

Bipolar is a mood disorder. Though it has the same symptoms as depression, manic symptoms distinguish bipolar disorder from depression. Symptoms of mania can range from mild to severe and are often the opposite of depression symptoms. Severe mania is a medical emergency and requires hospitalization. About 6.1 million American adults (2.6% of the population) live with bipolar disorder.

Signs and symptoms

  • Elevated, irritable, or expansive mood
  • Racing thoughts
  • Increased energy
  • Decreased need for sleep
  • Impulsive behaviors that are uncharacteristic of the person, such as sexual indiscretions and extravagant spending
  • Psychotic symptoms, including delusions and hallucinations


  • Sleep deprivation
  • Transcontinental and transoceanic travel

Dr. McElroy explains the signs and symptoms of bipolar disorder.

3. Obsessive compulsive disorder (OCD)

Obsessive compulsive disorder (OCD) is different from obsessive compulsive personality. Some people are just highly organized individuals that can seem obsessive to those that aren't.

Signs and symptoms

  • Intrusive thoughts, experienced as unpleasant or uncomfortable
  • Anxious feelings
  • Feeling the need to do something over and over again or something bad will happen
  • Development of compulsive behaviors that takes a lot of time and gets in the way of important daily activities


  • Mostly unknown
  • Some cases of OCD have been triggered by streptococcal infections
  • Inheritable

Dr. Brady explains the signs and symptoms of obsessive-compulsive disorder.

4. Schizophrenia

About 1 in 100 (2.4 million) American adults will experience schizophrenia. It is one of the most severe and difficult to treat mental illnesses. Hospitalization is usually necessary.

Signs and symptoms

  • Disturbances in perception of reality
  • Delusions and beliefs not based in reality
  • Auditory and visual hallucinations
  • Inability to recognize that the signs and symptoms are abnormal


  • Brain chemistry and structure
  • Substance abuse
  • Early childhood trauma
  • Genetics (specific genes have been shown to influence the risk of developing schizophrenia]

Dr. Keck explains the signs and symptoms of schizophrenia.

5. Addiction

One in every 10 people struggle with addiction, but only 10% receive treatment. Addiction has three unique factors: quantity (the amount you engage in an activity or substance impairs your ability to function), frequency (an activity or substance you need repeatedly and can't resist), and disability (the activity or substance has the potential to damage your brain or physical health).

With addiction, choice is out of your control. Individuals that struggle with addiction cannot successfully resist the things that are bad for them, which could include alcohol, drugs, gambling, or another possibly harmful activity or substance.

Signs and symptoms

  • Participates in activities that impair the ability to function
  • Participates in negative activities that cannot be resisted
  • Uses substances that are medically damaging on a consistence basis


  • Stress
  • Family history
  • Another mental illness. Research shows 85% of individuals with addiction have another mental illness. This is called co-occurrence. These individuals are not usually not receiving treatment for another mental illness and develop an addiction from lack of treatment

Dr. Tuell explains the signs and symptoms of addiction.