Causes and Risk Factors of Schizophrenia

Schizophrenia is an illness of the brain that causes certain characteristics, abnormal experiences, and behaviors. For different people, schizophrenia can involve different clusters of symptoms. It is possible that slightly different disease processes are involved in these different clusters. However, many researchers believe that schizophrenia is a single disease which can have different effects depending on which brain regions are most affected. In the latest edition of the DSM, the subtypes of schizophrenia were removed.

Researchers do not yet know exactly what causes some people to develop schizophrenia. There is a very strong genetic component to schizophrenia. However, genes alone do not completely explain the illness.

Most scientists believe that genes don’t cause schizophrenia directly, but do make a person vulnerable to developing the disease. Scientists are studying many possible factors that might cause a person with a genetic predisposition to develop schizophrenia.

Schizophrenia causes and risk factors
Verywell / Cindy Chung

Family History and Genetics

The evidence of a genetic predisposition to schizophrenia is overwhelming. The frequency of schizophrenia in the general population is slightly less than 1 percent. Being related to someone with schizophrenia, though, greatly increases your risk of developing schizophrenia.

Family Connections

Just as with other medical and mental health conditions, genetic predisposition plays a role in whether someone will develop symptoms of schizophrenia. The numbers to support this are strong, for example:

  • If your brother or sister or one parent has the illness, your chance of having schizophrenia is around 10 percent.
  • If one parent has the condition, it raises your chances of developing schizophrenia by about 13 percent.
  • If your identical twin has the illness, you have a roughly 50 percent chance of developing schizophrenia.
  • If both of your parents have schizophrenia, you have a 40 percent likelihood of developing the illness.

We know these family risks are due to genetics rather than family environment, as these rates are the same whether a person is raised in the birth family or not. The children of people with schizophrenia are more frequently given up for adoption because their parents are too ill to care for them.

However, genes alone don’t cause schizophrenia. If they did, then identical twins, who share virtually the same genetic code, would have closer to a 100 percent likelihood of sharing the illness, rather than 50 percent.

Environment

As with many health conditions, certain environmental factors may contribute to the development of schizophrenia.

Viral Infections

Exposure to viruses are found to be a contributing factor in the development of schizophrenia. There are a variety of traits of viruses that might make this possible. For example, viruses can:

  • Attack certain brain regions and leave others intact
  • Alter certain processes within a brain cell without killing the cell
  • Infect someone and then lay dormant for many years before causing illness
  • Cause minor physical abnormalities, birth complications, and altered fingerprint patterns sometimes found in people with schizophrenia
  • Affect neurotransmitters

It is also important to note that some antipsychotic agents are also antiviral agents, which can also support the idea that viral infections can play a role in the development of schizophrenic symptoms, particularly psychotic symptoms.

Herpes Viruses

People who have recently developed schizophrenia very often have antibodies to two herpes viruses in their blood, HSV (herpes simplex virus) and CMV (cytomegalovirus). Studies have shown that when these herpes viruses infect someone with a particular set of genes, that person is much more likely to develop schizophrenia.

Other Infectious Influences

People with schizophrenia also are more likely to show antibodies to toxoplasmosis gondii, a parasite carried by cats that can also infect humans. Being raised around cats slightly raises a person’s likelihood of developing schizophrenia, and the illness is more common in countries and states where many people have cats as pets.

Infectious disease theories of schizophrenia are very exciting and promising. It is too early to know if researching these theories will reveal the cause of schizophrenia, but further study will go a long way toward informing future diagnosis and treatment of the condition.

Exposure to Toxins

Being exposed to harmful toxins are suggested to play a role in the development of schizophrenia, even as early as during fetal development. Toxins that have been researched to show possible influence include alcohol and lead.

Lead

Exposure to lead during fetal develop has been shown to significantly influence the likelihood of the development of schizophrenia, particularly psychotic features. Lead can be found in things such as:

  • Gasoline
  • Paint
  • Tap water
  • Children's toys

Although there are measures that have been taken in more recent years to remove lead from the things we come in contact with everyday, it is important to be aware that lead can still be in our environment. Even a home that has been painted with lead paint 25 years ago or more can release toxins.

Societal Factors

Because the causes of schizophrenia have been so difficult to define, much research has been devoted to investigating societal issues that could potentially contribute to the onset of the disease in some people.

Living in a Populated Area

It has been suggested that living in a densely populated area can be a risk factor in the development of schizophrenia. Research has shown that people who grew up in metropolitan areas were more likely to be diagnosed with schizophrenia than those who lived in the country or rural areas.

Prenatal Exposure to Hunger

The children of women experiencing famine during the first three months of their pregnancy have been shown to be more likely to develop schizophrenia.

Family Environment

Being raised in a family with schizophrenia greatly increases the stress and likelihood of abuse and trauma, and children from these homes are more likely to develop the illness themselves. However, the genetic contribution, rather than the psychological stress, explains most of the rate of schizophrenia in children from these families.

It is certainly possible to look in the history of many people with schizophrenia and find past trauma, but many more people with schizophrenia came from loving, supporting homes. One of the many tragedies of schizophrenia is the blame that well-meaning people often assign blame to parents already heartbroken by the illness of their beloved child.

It is important to realize that these risk factors do not, in themselves, indicate a cause of schizophrenia. Instead, scientists are looking for underlying differences among these groups of people that can explain why they experience different levels of risk.

Brain and Body Risk Factors

Developmental theories of schizophrenia suggest that something goes wrong when the brain is developing. Brain development, from the earliest stage of fetal development through the early years of life, is an extremely complicated process. Millions of neurons are formed, migrate to different regions of the forming brain, and specialize to perform different functions.

The “something” that goes wrong might be a viral infection, a hormonal imbalance, an error in genetic encoding, a nutritional stress, or something else. The common element in all developmental theories is that the causal event occurs during the brain’s development.

Even though these potential causes may be rooted in very early development, symptoms of schizophrenia typically emerge in late adolescence or early adulthood.

Early Disruptions

Developmental theories suggest an early disruption causes the brain structure to be disorganized. The start of puberty brings a number of neurological events, including the programmed death of many brain cells, and at that time the abnormalities become critical.

Developmental theories suggest there are a number of risk factors for schizophrenia related to critical periods in fetal development, such as:

  • Schizophrenia is more common in winter and spring births.
  • Children whose mothers experienced famine during the first trimester are more likely to develop schizophrenia.
  • Pregnancy and birth complications increase the risk of developing schizophrenia.

However, there is not yet enough evidence that the brains of adults with schizophrenia are disorganized in the ways that developmental theories predict. Also, these theories address the when of schizophrenia’s origin, but not the cause itself.

Neurochemicals

Schizophrenia clearly involves irregularities in the chemicals of the brain (neurochemicals) that allow brain cells to communicate with each other. We know this because blocking certain neurotransmitters with drugs (like amphetamine or PCP) can cause schizophrenia-like symptoms. Also, antipsychotic medications that block the action of the neurotransmitter dopamine can effectively reduce symptoms.

In fact, dopamine imbalance was once thought to cause schizophrenia. However, more recent antipsychotics work without blocking dopamine. Current research indicates that the neurotransmitters GABA and glutamate are involved in the cause of schizophrenia.

The difficulty of neurochemical theories is that most brain processes can affect neurotransmitter levels, and neurotransmitters (of which there are at least 100) all interact with one another.

When we say that one particular neurotransmitter or another is causing schizophrenia, we are basing that claim on a single frame of a very long and complicated motion picture, without being able to see the frames that led up to the change we’re observing.

The medical treatment of schizophrenia today relies almost entirely upon regulating levels of neurotransmitters, and so research in this area is vital to developing more effective treatments.

Lifestyle Factors

While certain lifestyle factors may seem to be associated with schizophrenia, the connections are more coincidental than causal.

Schizophrenia Discussion Guide

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Stress

Psychological stress has physiological effects and is implicated in causing or contributing to psychiatric disorders including post-traumatic stress disorder. Psychological stress also aggravates diseases like high blood pressure and heart disease.

Certain types of psychological stress, namely traumas in the aftermath of war, natural disaster, or concentration camp imprisonment, have not been shown to cause schizophrenia. This statement doesn’t make sense to many people familiar with schizophrenia. How can it be true? For one thing, schizophrenia does not become more common after these so-called single life incidents. But some research does show an increased risk of developing schizophrenia for individuals who experience traumatic stress disorders, especially if triggering incidents occurred early on in life and/or repeatedly as in cases of sexual abuse.

Stress also plays a significant role in the control of the illness. People with schizophrenia become very sensitive to stress and change. Psychological stress alone can be enough to trigger an episode. Developing and maintaining a routine is one of the most important aspects of avoiding relapse.

Life-Changing Events

People’s lives are often filled with loss during the time leading up to a first psychotic episode. However, those losses (like relationships, jobs, school, accidents, etc) are often the result of early-onset symptoms including suspicion, memory disturbance, withdrawal, and loss of motivation. Essentially, previously undiagnosed schizophrenia can be the cause of many life-changing events, and not the other way around.

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Additional Reading

  • Schizophrenia: A Detailed Booklet That Describes Symptoms, Causes, and Treatments, With Information on Getting Help and Coping.National Institutes of Mental Health. (2006)

  • What Causes Schizophrenia? (2007) National Institutes of Mental Health.

  • Torrey, E.F. (2006) Surviving Schizophrenia: A Manual for Families, Patients and Providers, 5th Edition. New York: HarperCollins Publishers.