Difficulty: Intermediate | Prerequisites: Introductory psychology, basic understanding of genetics (alleles, concordance rates, heritability)
Schizophrenia is one of the most studied psychiatric disorders in abnormal psychology, and its causes are far from simple. This set of notes covers the genetic and environmental evidence for how schizophrenia develops. You will need this material to understand the diathesis-stress model (genetic vulnerability + environmental trigger) that underpins most modern thinking about the disorder. If you have not yet reviewed basic Mendelian and polygenic inheritance, do that first. This topic connects directly to neurobiology and treatment (covered in Part 2).
Schizophrenia runs in families, with twin and adoption studies confirming a strong genetic component, but genes alone are not sufficient. Environmental factors during pregnancy and early development (infections, obstetric complications, paternal age) interact with genetic risk to influence whether the disorder emerges.
Heritability
The proportion of variation in a trait within a population that can be attributed to genetic differences. In simple terms, it tells you how much of the risk for schizophrenia comes from genes versus environment at a population level. It does not mean a single person's risk is that percentage genetic.
Concordance rate
The probability that both members of a pair (e.g. twins) will have a given trait if one already has it. Think of it as a matching rate: if one identical twin has schizophrenia, the concordance rate tells you how likely the other twin is to develop it too.
Monozygotic (MZ) twins
Identical twins who develop from a single fertilised egg and share virtually 100% of their DNA. In simple terms, they are the closest genetic match two people can be.
Dizygotic (DZ) twins
Fraternal twins who develop from two separate eggs and share roughly 50% of their DNA, the same as any pair of siblings.
DISC1 gene (Disrupted in Schizophrenia 1)
A gene involved in neural development, cell migration, and synapse formation. Mutations in DISC1 have been linked to a sharply increased risk of schizophrenia, bipolar disorder, and major depressive disorder. Think of it as a gene that helps wire the brain during development; when it goes wrong, several psychiatric conditions become more likely.
5-HTT gene (serotonin transporter gene)
The gene encoding the serotonin transporter protein. Variations in its promoter region (short vs. long alleles) affect serotonin reuptake efficiency. In simple terms, the short version of this gene makes a person more vulnerable to depression, which frequently co-occurs with schizophrenia.
Epigenetics
Changes in gene expression that do not involve alterations to the DNA sequence itself, often triggered by environmental factors. Think of it as the environment turning genes "on" or "off" without rewriting the genetic code.
Seasonality effect
The observation that people born in late winter or early spring have a slightly higher incidence of schizophrenia, possibly due to maternal viral infections during critical periods of fetal brain development.
Rh incompatibility
A condition where the mother's and fetus's blood types are mismatched at the Rh factor, potentially leading to immune-mediated damage to the developing fetus. This is one of several obstetric complications associated with increased schizophrenia risk.
Twin studies provide the strongest evidence for a genetic component
MZ (identical) twins show a significantly higher concordance rate for schizophrenia than DZ (fraternal) twins
If schizophrenia were purely genetic, MZ concordance would be 100%. It is not, which confirms that environment matters too
Adoption studies separate genetic influence from shared environment
Biological relatives of individuals with schizophrenia have a higher incidence of the disorder than adoptive relatives
This holds even when the adopted child was raised in a household with no family history of schizophrenia
DISC1 gene
Implicated in neural developmental processes including cell migration and synapse formation
Mutations can drastically increase the risk of schizophrenia, bipolar disorder, and major depressive disorder
Animal models (mice with disrupted DISC1) show schizophrenia-like symptoms in adulthood, supporting a neurodevelopmental pathway
5-HT transporter polymorphisms
The promoter region of the 5-HTT gene has short and long allele variants
One or two short alleles increase susceptibility to depression
Relevant to schizophrenia because depression frequently co-occurs with the disorder, and these polymorphisms may influence treatment response to antidepressants
Obstetric complications
Maternal diabetes, Rh incompatibility, and fetal oxygen deprivation during birth all raise the likelihood of schizophrenia
These complications often produce reduced brain weight and enlarged cerebral ventricles, mirroring the structural brain anomalies seen in schizophrenia patients
Seasonality and viral epidemics
Higher incidence of schizophrenia among people born in late winter or early spring
The leading hypothesis is maternal exposure to viral infections (e.g. influenza) during critical windows of fetal brain development
Paternal age
Advanced paternal age is associated with higher schizophrenia risk in offspring
The mechanism is thought to be accumulated mutations in spermatocytes over time (older fathers have more cell divisions in sperm-producing cells, and each division carries a small risk of copying errors)
Preventative factors (emerging research)
Avoiding exposure to environmental toxins during pregnancy
Ensuring adequate nutrition during pregnancy
Maintaining supportive social environments
These may reduce risk in genetically predisposed individuals, though the evidence base is still developing
The diathesis-stress framework matters beyond the exam. Prenatal screening programmes and public health campaigns (e.g. encouraging flu vaccination during pregnancy) are informed by this research. Genetic counselling for families with a history of schizophrenia also draws on concordance and heritability data to provide realistic risk estimates, rather than deterministic predictions.
"Schizophrenia is entirely genetic." MZ twin concordance is well below 100%, so genes create vulnerability, not certainty. Environment plays a substantial role.
"If no one in your family has schizophrenia, you cannot develop it." Sporadic cases occur, sometimes linked to de novo mutations or environmental insults during development.
"Heritability of 80% means an individual has an 80% chance of getting schizophrenia." Heritability is a population-level statistic about variance, not an individual risk percentage.
"The seasonality effect proves that viruses cause schizophrenia." The correlation is real, but the causal mechanism (maternal viral exposure disrupting fetal brain development) remains a hypothesis, not a confirmed pathway.
⚠️ Be ready to explain why MZ concordance rates below 100% are evidence that schizophrenia is not purely genetic.
⚠️ Know the difference between heritability as a population statistic and individual genetic risk.
⚠️ DISC1 is a high-yield term. Know what it does (neural development) and what disorders it links to (schizophrenia, bipolar, MDD).
⚠️ Obstetric complications and the seasonality effect are commonly tested as examples of environmental risk factors.
⚠️ Paternal age as a risk factor often appears in multiple-choice items. Remember the mechanism: accumulated spermatocyte mutations.
True or False: Monozygotic twins have a 100% concordance rate for schizophrenia.
Fill in the blank: The _______ gene is implicated in neural development and, when mutated, increases risk for schizophrenia, bipolar disorder, and major depressive disorder.
True or False: Adoption studies show that adoptive relatives of people with schizophrenia have the same incidence as biological relatives.
Fill in the blank: The seasonality effect refers to higher schizophrenia incidence among people born in late _______ or early _______.
True or False: Heritability tells you an individual person's probability of developing a disorder.
Answers: 1. False (it is elevated but well below 100%). 2. DISC1. 3. False (biological relatives show higher incidence). 4. Winter, spring. 5. False (it is a population-level statistic about variance).
Q: Explain how twin and adoption studies together support a genetic contribution to schizophrenia.
A: Twin studies show that MZ twins (who share ~100% DNA) have a higher concordance rate than DZ twins (who share ~50%), pointing to genetic influence. Adoption studies confirm this by showing that biological relatives of people with schizophrenia have a higher incidence than adoptive relatives, even when the environment differs. Together, these designs separate genetic from environmental effects.
Q: What is the DISC1 gene, and why is it relevant to schizophrenia?
A: DISC1 (Disrupted in Schizophrenia 1) is involved in neural developmental processes such as cell migration and synapse formation. Mutations in DISC1 significantly increase the risk of schizophrenia, as well as bipolar disorder and major depressive disorder. Mouse studies confirm that disrupting DISC1 during fetal development produces schizophrenia-like symptoms in adulthood.
Q: Describe two environmental risk factors for schizophrenia and the mechanisms proposed for each.
A: (1) Obstetric complications such as fetal oxygen deprivation or Rh incompatibility can lead to reduced brain weight and enlarged ventricles, mirroring the brain anomalies found in schizophrenia patients. (2) The seasonality effect, where late winter/early spring births carry higher risk, is thought to result from maternal exposure to viral infections during critical periods of fetal brain development.
Q: Why does advanced paternal age increase schizophrenia risk?
A: Spermatocytes accumulate mutations with each cell division over time. Older fathers have undergone more of these divisions, raising the probability of de novo genetic errors that may contribute to schizophrenia risk in offspring.
Q: A student says, "Schizophrenia is 80% heritable, so there is an 80% chance I will get it if my parent has it." What is wrong with this statement?
A: Heritability is a population-level statistic describing how much of the variation in schizophrenia risk across a population is attributable to genetic differences. It does not translate to an individual's probability of developing the disorder. Individual risk depends on which specific genetic variants a person carries and on environmental factors.
This material connects directly to the neurobiology of schizophrenia (brain structure abnormalities, dopamine pathways), which is covered in Part 2 of these notes. The gene-environment interaction framework here also applies broadly across abnormal psychology, from depression (5-HTT and life stress) to anxiety disorders. Understanding heritability and concordance studies will recur in nearly every disorder you study this term.
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