A normal brain produces all of the symptoms that define most psychiatric disorders. In other words, no underlying brain disorder is needed to produce what we call a psychiatric disorder. Instead, certain clusters of otherwise normal emotion and behavior that last for defined lengths of time at a certain intensity level are defined as psychiatric disorders.

Why do different "normal" people have different emotions and behaviors? I believe it's because of their childhood experience. The How Personality Develops page on this website explains exactly how I think this happens. It is easy to see how the mechanisms described on that page could produce a constellation of emotions and behaviors in an otherwise normal child that is later defined as a psychiatric disorder.

People who are diagnosed with a psychiatric disorder often have symptoms that are more intense and last longer than people without psychiatric disorders. That's because the childhood experience that caused those symptoms was more intense and prevalent in those people than in people without psychiatric disorders. 

I'll use bipolar disorder as an example. Here are the symptoms and my comments (I'm only looking at the actual symptom, not the intensity or duration - see last paragraph):

Manic symptoms:

1.  Inflated self-esteem or grandiosity - If a child is praised enough, they will exhibit these. It is common for someone who is particularly talented to have these traits, probably because their talent was evident when they were young and they got a lot of attention and praise for it.

2. Decreased need for sleep - Many people experience this when they are very excited about something.

3. More talkative than usual or pressure to keep talking - excitement about something causes many people to talk more than they might otherwise, and if excited enough sometimes it's hard to get them to stop talking about something.

4. Flight of ideas or subjective experience that thoughts are racing - This is similar to #3. An example might be when some people fall in love and jump from one thing to another saying how wonderful their life will be.

5. Distractibility - Everyone is distracted at times.

6. Increase in goal-directed activity - This happens frequently, as when someone has a deadline to learn something for a job, for example.

7. (Excessive) involvement in activities that have a high potential for painful consequences (for example, engaging in unrestrained, buying sprees, sexual indiscretions, or foolish business investments) - Lots of people engage what might be called excessive sexual indiscretions. Other people make foolish business investments. 

Depressive symptoms:

1. Depressed mood (most of the day, nearly every day) - Many people experience depressed mood at times.

2. Markedly diminished interest or pleasure in all, or almost all, activities (most of the day, nearly every day) - Might happen while grieving.

3. Significant weight loss when not dieting, or weight gain - Significant stress can cause both of these.

4. Insomnia or hypersomnia (nearly every day) - Significant stress can cause both of these.

5. Psychomotor agitation or retardation (nearly every day) - Significant stress can cause both of these.

6. Fatigue or loss of energy (nearly every day) - Significant stress can cause both of these.

7. Feelings of worthlessness or excessive or inappropriate guilt (nearly every day) - Many people have felt worthless and guilty for no obviously good reason.

8. Diminished ability to think or concentrate, or indecisiveness, (nearly every day) - Significant stress can cause both of these.

9. Recurrent thoughts of death, recurrent suicidal ideation without a specific plan, or a suicide attempt or a specific plan for committing suicide - This might happen to someone incarcerated and tortured in a rat infested prison. A person's childhood experience can make the person experience the world is a horrible place and that they are a horrible person who doesn't deserve to live. 

My point here is that all of these symptoms can be experienced in a person who is considered normal (without any psychiatric diagnosis). I believe that childhood experience causes all of the above symptoms in otherwise normal people. The difference between them and people who have psychiatric disorders is the duration and intensity of the childhood experiences that caused the symptoms.

Note: Auditory hallucinations, commonly described as "hearing voices in my head," are identical to what happens when a non-psychotic person imagines hearing something such as a voice or song. For example, most people can imagine the voice of their mother talking to them, and many people have experienced hearing a song in their head that  "I can't get out of my head!". That is identical to what happens in many/most people who are said to have auditory hallucinations. They hear a voice that repeats continually that they can't get out of their head. In some people, those "voices" may tell them to do certain things, make derogatory comments, and so on, all of which are easily explained by some particular childhood experiences