Difficulty: Intermediate | Prerequisites: Introductory psychology, basic neuroscience terminology (neurotransmitters, brain anatomy).
Schizophrenia is one of the most studied disorders in abnormal psychology, and for good reason: it touches nearly every system the course covers, from neurotransmitter pathways to gene–environment interactions. This set of notes covers the three symptom categories (positive, negative, cognitive), the genetic evidence, and the neurobiological hypotheses that attempt to explain why these symptoms occur. If you have not yet reviewed basic brain anatomy and neurotransmitter function, do that first. Everything here builds on it.
Schizophrenia produces three broad categories of symptoms: positive (additions like hallucinations and delusions), negative (losses like flat affect and social withdrawal), and cognitive (deficits in attention, memory, and problem-solving). The disorder has a strong genetic component, with twin studies showing roughly 50% concordance in identical twins. Two major neurochemical hypotheses, the dopamine hypothesis and the glutamate hypothesis, explain different symptom clusters, and structural brain findings like hypofrontality tie the neurobiology to the clinical picture.
Positive symptoms
Symptoms that represent an addition of abnormal experiences or behaviours, such as hallucinations, delusions, and disorganised thinking. In simple terms, these are things that appear that should not be there.
Negative symptoms
Symptoms that represent a loss or reduction of normal functioning, such as flat affect, poverty of speech, and social withdrawal. Think of it as something being taken away from the person's baseline.
Cognitive symptoms
Deficits in mental processes including attention, memory, psychomotor speed, and abstract reasoning. In simple terms, these are disruptions to how well someone can think, learn, and process information.
Hallucinations
Perceptions of stimuli that are not present. Auditory hallucinations (hearing voices) are the most common type in schizophrenia.
Delusions
Strongly held false beliefs that persist despite contradictory evidence. Common subtypes include persecution ("someone is out to get me"), grandeur ("I have special powers"), and control ("an outside force is directing my actions").
Thought disorder
A disruption in the logical organisation of thinking, making it difficult to arrange ideas coherently or reach plausible conclusions. In simple terms, the person's train of thought goes off the rails.
Anhedonia
The inability to experience pleasure. Think of it as the reward system going quiet: activities that once felt enjoyable no longer register.
Flat affect / flattened emotional response
A marked reduction in emotional expressiveness, including facial expression, vocal tone, and gestures. The person may still feel emotions internally but shows very little outward sign.
Poverty of speech (alogia)
A significant reduction in the amount of speech produced. The person gives very brief or empty replies, not because they are choosing to be quiet but because the drive to speak is diminished.
Hypofrontality
Decreased activity in the frontal lobes, particularly the dorsolateral prefrontal cortex (dlPFC). This is consistently observed in schizophrenia and is linked to negative and cognitive symptoms.
Dopamine hypothesis
The theory that positive symptoms of schizophrenia result from overactivity at dopamine synapses, especially in the mesolimbic pathway.
Glutamate hypothesis
The theory that impaired glutamate activity, particularly in the frontal lobes and involving NMDA receptors, contributes to the negative and cognitive symptoms of schizophrenia.
DISC1 gene
A gene involved in neurodevelopment and neuronal function. Mutations in DISC1 are associated with a markedly increased risk of developing schizophrenia.
Concordance rate
The probability that both members of a twin pair will share a trait. For schizophrenia in monozygotic twins, this sits at roughly 50%.
Mesolimbic pathway
A dopamine pathway running from the ventral tegmental area to the limbic system. Overactivity here is thought to drive positive symptoms.
Dorsolateral prefrontal cortex (dlPFC)
A region of the frontal lobe involved in executive function, working memory, and planning. Reduced activity here is a hallmark of schizophrenia's cognitive and negative symptom profile.
Positive symptoms add something abnormal to a person's experience. They are the most recognisable features of schizophrenia and the primary target of antipsychotic medication.
Hallucinations are the most common positive symptom. Auditory hallucinations (typically hearing voices) predominate, though visual, tactile, and olfactory hallucinations can also occur.
Delusions are fixed false beliefs. Persecution delusions ("they are watching me"), grandiose delusions ("I am chosen"), and control delusions ("they are putting thoughts in my head") are classic exam examples.
Thought disorder refers to disorganised thinking visible in speech patterns: tangential answers, loose associations, and word salad in severe cases.
Negative symptoms subtract from normal functioning. They tend to be harder to treat than positive symptoms and often cause more long-term disability.
Flat affect: reduced facial expression, monotone voice, diminished gestures.
Poverty of speech (alogia): short, content-poor responses.
Anhedonia: loss of interest or pleasure in previously enjoyed activities.
Social withdrawal: the person pulls away from relationships and social engagement.
Cognitive symptoms are disruptions to thinking processes that interfere with daily functioning.
Attention deficits: difficulty sustaining focus on tasks.
Low psychomotor speed: slower reaction and processing times on cognitive tasks.
Learning and memory deficits: impaired ability to encode new information and retrieve it later.
Poor abstract thinking and problem-solving: difficulty grasping non-concrete concepts or working through multi-step problems.
Schizophrenia has a substantial genetic component. Close biological relatives of affected individuals carry a higher risk than the general population.
Monozygotic (identical) twin concordance is approximately 50%. This is strong evidence for genetic involvement, but the fact that it is not 100% tells you that genes alone do not determine the outcome; environment matters too.
Many genes have been implicated. The DISC1 gene is a frequently cited example: it plays a role in neurodevelopment and neuronal function, and mutations in it significantly elevate risk.
Older paternal age at conception is associated with a higher incidence of schizophrenia in offspring.
The proposed mechanism is the accumulation of de novo mutations in sperm cells over a father's lifetime. Sperm cells continue to divide, and each division carries a small chance of copying errors.
Neuroimaging studies consistently show reduced activity in the frontal lobes of people with schizophrenia, particularly in the dorsolateral prefrontal cortex (dlPFC).
This pattern, called hypofrontality, maps onto the negative and cognitive symptoms: the dlPFC is critical for working memory, planning, and executive control.
Hypofrontality may arise from developmental disruption, degenerative processes, or both.
The dopamine hypothesis is the oldest and most established neurochemical model of schizophrenia.
Core idea: positive symptoms stem from excessive dopamine activity in the mesolimbic pathway.
Supporting evidence:
Antipsychotic drugs that block D2 dopamine receptors reduce positive symptoms.
Dopamine agonists (e.g., amphetamines) can trigger or worsen psychotic symptoms.
Limitation: the dopamine hypothesis does a poor job of explaining negative and cognitive symptoms, which is where the glutamate hypothesis picks up.
The glutamate hypothesis addresses the symptoms the dopamine model does not cover well: the negative and cognitive clusters.
Core idea: impaired glutamate neurotransmission, especially at NMDA receptors in the frontal lobes, contributes to these symptom categories.
Supporting evidence:
NMDA receptor antagonists like PCP (phencyclidine) and ketamine produce symptoms in healthy people that closely resemble schizophrenia's negative and cognitive profile.
Post-mortem studies have found reduced glutamate concentrations in the frontal cortex of individuals with schizophrenia.
The dopamine hypothesis is the direct basis for every antipsychotic medication on the market: these drugs work by blocking dopamine receptors, particularly D2. Understanding hypofrontality also explains why cognitive rehabilitation programmes for schizophrenia focus on tasks that engage the prefrontal cortex, such as working memory training.
"Positive symptoms" does not mean good symptoms. "Positive" here means an addition, not a value judgement. Students mix this up regularly on exams.
A 50% twin concordance rate does not mean schizophrenia is "50% genetic." It means that if one identical twin has it, there is a roughly 50% chance the other will too. Heritability estimates for schizophrenia are actually higher (around 80%), because concordance and heritability are different statistics.
The dopamine hypothesis does not explain all of schizophrenia. It accounts mainly for positive symptoms. Students who write as if dopamine overactivity is the whole story will lose marks.
Negative symptoms are not the same as depression. There is overlap (anhedonia, withdrawal), but flat affect and poverty of speech are distinct features that do not map neatly onto a depressive episode.
⚠️ Be able to sort any given symptom into the correct category (positive, negative, or cognitive). This is a very common exam question format.
⚠️ Know the difference between the dopamine hypothesis and the glutamate hypothesis, and which symptom cluster each one addresses.
⚠️ The DISC1 gene and the 50% MZ concordance figure are frequently tested recall items.
⚠️ Hypofrontality and dlPFC are linked to negative and cognitive symptoms, not positive symptoms. Do not mix this up.
True or false: Auditory hallucinations are classified as a negative symptom.
Fill in the blank: The concordance rate for schizophrenia in monozygotic twins is approximately ___%.
True or false: The dopamine hypothesis best explains the cognitive symptoms of schizophrenia.
Fill in the blank: Decreased activity in the frontal lobes, especially the dlPFC, is called ___________.
True or false: Older paternal age is associated with an increased risk of schizophrenia in offspring.
Answers: 1. False (positive symptom). 2. 50%. 3. False (it explains positive symptoms). 4. Hypofrontality. 5. True.
Q: Name the three broad categories of schizophrenia symptoms and give one example of each.
A: Positive (e.g., hallucinations), negative (e.g., flat affect), and cognitive (e.g., attention deficits).
Q: What does the dopamine hypothesis propose, and which symptom cluster does it primarily explain?
A: It proposes that positive symptoms result from overactivity of dopamine synapses in the mesolimbic pathway. It primarily explains positive symptoms.
Q: How does the glutamate hypothesis differ from the dopamine hypothesis in its explanatory scope?
A: The glutamate hypothesis focuses on impaired NMDA receptor activity in the frontal lobes and is used to explain negative and cognitive symptoms, which the dopamine hypothesis does not account for well.
Q: What evidence supports a genetic component to schizophrenia?
A: Monozygotic twin concordance of approximately 50%, higher risk in close biological relatives, and identification of specific risk genes such as DISC1.
Q: Why is the 50% concordance rate in identical twins important for understanding the aetiology of schizophrenia?
A: It confirms a strong genetic contribution but also demonstrates that genes alone are insufficient, because if schizophrenia were purely genetic the concordance would be 100%. Environmental factors must also play a role.
Q: What is hypofrontality and which symptoms is it associated with?
A: Hypofrontality is reduced activity in the frontal lobes, particularly the dorsolateral prefrontal cortex. It is associated with negative and cognitive symptoms.
This material connects directly to neurotransmitter systems covered earlier in the course. The dopamine and glutamate hypotheses build on your understanding of synaptic transmission and receptor pharmacology. The genetic findings connect to the broader discussion of diathesis-stress models: genes create vulnerability, and environmental stressors may trigger onset. The next set of notes covers those environmental and developmental factors, along with treatment approaches.
schizophrenia, positive symptoms, negative symptoms, cognitive symptoms, hallucinations, delusions, thought disorder, flat affect, anhedonia, alogia, poverty of speech, social withdrawal, attention deficits, psychomotor speed, dopamine hypothesis, mesolimbic pathway, glutamate hypothesis, NMDA receptor, hypofrontality, dlPFC, dorsolateral prefrontal cortex, DISC1, concordance rate, monozygotic twins, heritability, paternal age, abnormal psychology, psychosis, antipsychotic mechanism