Risk factors of schizophrenia

Genetics

Schizophrenia appears to have a genetic component, but carrying the associated genes does not guarantee the illness will develop. It is considered a polygenic condition, meaning multiple genes interact with environmental factors to create risk. Studies of twins show that if one identical twin has schizophrenia, the other has about a 50% chance of developing it. Family research confirms that the more closely related someone is to a person with schizophrenia, the higher their own risk becomes.

A father's age at conception may play a role, as sperm cells undergo many divisions over a lifetime, increasing the chance of DNA copying errors. In contrast, a woman's eggs divide only twenty-three times before birth, so fewer mutations accumulate from this process. These copying errors can lead to mutations that raise schizophrenia risk. Concordance rates are higher in identical twins than in fraternal twins, and evidence suggests that conditions before and during birth may also influence these rates in identical twins.

Despite strong evidence from twin and family studies that schizophrenia is highly heritable, the precise genetic causes are still not fully understood. Large-scale research has started to clarify the genetic basis, distinguishing between two main types of risk factors: common variants that carry lower risk, often identified through genome-wide association studies, and rare variants that carry higher risk, which can arise from new mutations or copy-number variations.

Environment

A range of environmental factors can contribute to the risk of developing schizophrenia, many of which are tied to the period before birth. These include stress during pregnancy, complications during pregnancy or childbirth, and negative experiences in childhood. Later in life, substance use disorders also play a role. While pregnancy complications are a known risk factor, their effect is very small and not specific to schizophrenia.

The only direct experimental evidence that schizophrenia's brain pathology stems from multiple causes in human tissue comes from studying prenatal neural cells grown from patient stem cells, highlighting the prenatal period as a critical but still poorly understood window. It is important to note that about 25 to 30 percent of people experience obstetric complications, yet the vast majority never develop schizophrenia, and most people with the disorder have no clear history of such events.

Still, the link is consistently observed, and these complications may influence how genetic or other environmental risks play out. Researchers are still working to identify which specific complications matter most and how they work. Exposure to toxins like lead may also raise the risk. Another factor is advanced paternal age, which is thought to increase risk due to new mutations in sperm that can affect early brain development.

A small but notable pattern, called the seasonality effect, shows that people with schizophrenia are more often born in winter or spring in the Northern Hemisphere. Possible explanations include higher rates of viral infections or lower vitamin D levels during those seasons. Interestingly, a similar birth-season pattern has been observed in healthy groups, such as chess players.

Life experiences

A higher number of adverse social factors during childhood, such as poverty or social isolation, is linked to an increased risk of developing schizophrenia. Stressful life events often occur shortly before the illness begins. Having personally migrated or having a close family member who recently migrated is a significant risk factor, connected to challenges like social disadvantage, feeling like an outsider, racial discrimination, family instability, joblessness, and substandard housing.

Among British African Caribbean communities, higher rates of schizophrenia compared to native African Caribbean populations are tied to unemployment and early parental separation. This highlights how social hardship, alongside genetic predisposition, contributes to the disorder. Data from the Dutch national register, examining immigrants from Turkey, Morocco, the Netherlands Antilles, and Surinam, showed that schizophrenia rates in these groups living in the Netherlands were higher than in the native-born population.

Experiencing abuse or trauma in childhood raises the likelihood of a schizophrenia diagnosis later on. Large population studies indicate that the risk grows with each additional instance of maltreatment, though some critical reviews note that further research is needed due to conceptual and methodological concerns. There is evidence that such adversities can lead to cognitive biases and changes in dopamine signaling, a process sometimes called sensitization. Childhood trauma, as well as bereavement or family separation, are recognized risk factors for schizophrenia and psychosis.

Certain social experiences have been tied to specific psychological processes and psychotic symptoms in schizophrenia. Additionally, brain imaging studies of people who survived sexual abuse or other trauma have occasionally shown patterns similar to those seen in some patients with psychosis, including a thinner corpus callosum, reduced volume in the anterior cingulate cortex, and a smaller hippocampus.

Other views

Schizophrenia is classified as a mental illness because its origins remain unclear. Psychiatrists such as R. D. Laing, Silvano Arieti, and Theodore Lidz proposed that what we call mental illness symptoms are actually understandable responses to overwhelming pressures from society and, especially, family life. These doctors stood out for treating psychotic experiences as meaningful material worth interpreting, rather than dismissing them as mere side effects of psychological or neurological problems.

Laing examined eleven cases of people diagnosed with schizophrenia and concluded that their words and actions made sense when viewed within their family and life situations. In 1956, Gregory Bateson, along with Paul Watzlawick, Donald Jackson, and Jay Haley, introduced a related theory: schizophrenia arises from double bind situations, where a person receives conflicting messages. They saw madness as an expression of this distress, something valuable as a cathartic and transformative experience.

In their books Schizophrenia and the Family and The Origin and Treatment of Schizophrenic Disorders, Lidz and his team argued that parental behavior often leads to mental illness in children. Arieti’s Interpretation of Schizophrenia received the 1975 National Book Award in science in the United States.

Psychologist Julian Jaynes expanded on the idea that schizophrenia is linked to civilization. In his 1976 book The Origin of Consciousness in the Breakdown of the Bicameral Mind, he suggested that before historical times, a condition similar to schizophrenia was the normal human state.

He called this the bicameral mind: a low-affect state for daily routines, interrupted during crises by mysterious voices giving commands, which early people interpreted as divine guidance. Psychohistorians accept psychiatric diagnoses but, unlike the current medical model, they propose that poor parenting in tribal societies creates the schizoid personalities seen in shamans. Commentators like Paul Kurtz have supported the view that major religious figures experienced psychosis, hearing voices and showing delusions of grandeur.

Did You Know?

• A meta-analysis found that oxidative DNA damage, measured by 8-hydroxy-2'-8-deoxyguanosine or 8-oxo-7,8-dihydro-2'-deoxyguanosine, was significantly increased in schizophrenia.
• It has been argued that a small endowment of schizotypy-increasing genes may increase reproductive success by increasing traits like creativity, verbal ability, and emotional sensitivity.