IB Psychology HLTopic 1 — Mental Health DisordersPaper 1 & 2Biological approach~9 min read
How Genes Contribute to Depression
Depression runs in families. That much is easy to show. The hard part is that families share far more than DNA — they share homes, money worries, habits and stress. This page shows you how psychologists pull those two things apart, and what the famous twin numbers actually prove.
📘 What you need to know
MDD (major depressive disorder) is diagnosed from a cluster of symptoms: low mood, loss of interest in things you used to enjoy, sleep and appetite changes, low motivation, and thoughts about death.
The genetic explanation sits on the nature side of the nature–nurture debate: behaviour comes from inherited traits.
Heritability is studied using twin studies and adoption studies.
A concordance rate is the chance that, if one twin has MDD, the other twin has it too.
MZ (identical) twins share all their DNA. DZ (non-identical) twins share about half, like any brother or sister.
McGuffin et al. (1996) found roughly 46% concordance in MZ pairs and 20% in DZ pairs.
46% is a long way from 100%, so genes cannot be the whole story. That gap is where most of your evaluation marks live.
What we are trying to explain
Before you explain a disorder you have to know what the disorder is. Clinicians look for a set of symptoms that have been there most of the day, most days, for at least two weeks:
persistent low mood, hopelessness or unusual irritability
losing pleasure in activities that used to be enjoyable
eating much more or much less, with weight change
sleeping badly, or sleeping far too much
finding decisions hard, and feeling flat and unmotivated
thoughts about death or about ending one’s life
Notice that none of these is a brain scan or a blood test. MDD is defined by behaviour and self-report, which will matter later when you evaluate the research.
Learn the symptom list properly. Examiners give credit for a short, accurate description of MDD at the start of an essay, and it costs you about three sentences.
Why psychologists use twins
You cannot run the experiment you would really want. You cannot take a baby, copy their genes, and raise the copies in different homes to see who becomes depressed. Twins are the closest nature gets to that experiment.
The logic works like this:
🧩 The twin-study argument, step by step
MZ twins share 100% of their DNA. DZ twins share about 50%.
Both kinds of twin pair are usually raised in the same home at the same time, so their environment is roughly matched.
So if MZ pairs agree on MDD much more often than DZ pairs, the extra agreement is most likely down to the extra shared genes.
The size of the gap between the two concordance rates is your evidence for a genetic influence.
Read both halves of this chart. The gap between the bars supports a genetic influence; the empty space above the blue bar is why nobody claims MDD is purely inherited.
Research support: McGuffin et al. (1996)
Aim: to compare concordance rates for MDD between MZ and DZ twin pairs.
Participants: 177 twins who had been registered at a London psychiatric hospital with depression between 1948 and 1986. Each one’s same-sex twin was used for comparison.
Procedure: clinicians assessed the twins’ mood using interviews and standard tests, and also looked at medical records. The assessors were blind — they did not know whether a pair was MZ or DZ, or whether the co-twin had a diagnosis.
Results: about 46% of MZ pairs both had MDD, compared with about 20% of DZ pairs. Where one MZ twin had a short depressive episode, the co-twin often had a similar one.
Conclusion: genes make a real contribution to the risk of developing MDD.
Why blind assessment matters. If the clinician knew a pair was identical, they might unconsciously look harder for symptoms in the second twin and inflate the MZ figure. Removing that knowledge protects the study from researcher bias — an easy, specific strength to write about.
The missing 54% — the point most students skip
If depression were written into the genes and nothing else mattered, MZ concordance would be 100%. Identical DNA would mean identical outcome, every time. It is not 100% in any study ever run.
So the same genotype (the genes you carry) can lead to different phenotypes (how you actually turn out). Something else has to be doing work, and the obvious candidate is life itself.
This single picture answers the most common exam follow-up: if the explanation is genetic, why do identical twins so often differ?
Evaluation of the genetic explanation
Strengths
Large samples and hard numbers. Twin and adoption studies collect data from hundreds of pairs, so the findings are reliable and easy to compare across countries and decades.
Concordance rates are clean data. A simple percentage lets you see a pattern straight away, such as MZ pairs agreeing more than DZ pairs.
It explains a real-world observation. Clinicians have always noticed that depression clusters in some families, and this explanation gives a testable reason for that.
Limitations
Concordance is never 100%. If the explanation were complete, identical twins would always match. They do not, so genes are at best part of the answer.
The “same environment” assumption is shaky. MZ twins are often treated more alike than DZ twins — dressed the same, given the same friends, mistaken for each other. Some of that extra MZ agreement could be extra shared environment, not extra shared genes.
It is reductionist. Boiling a complicated disorder down to inheritance ignores poverty, abuse, loss, loneliness and thinking style.
Hospital samples are not typical. McGuffin’s twins were all registered with one psychiatric hospital, so they were probably at the severe end. Milder depression, which most people experience, is under-represented.
EXAM QUESTION
Discuss one genetic explanation of one disorder. [22]
Here is how to plan it so you cover both command words — explain the idea and weigh it up.
Step 1: define the disorder in 2–3 linesMDD, its main symptoms, and that it is diagnosed from behaviour, not a lab test.Step 2: state the explanation clearlyVulnerability to MDD is partly inherited; heritability is tested using twin and adoption studies and measured with concordance rates.Step 3: use one study in detailMcGuffin (1996): 46% MZ vs 20% DZ, blind assessors, Maudsley register.Step 4: discuss — this is where the marks areTwo strengths (large sample, blind procedure) and two limitations (never 100%, equal-environment assumption). Finish by linking to diathesis–stress: genes give vulnerability, life events pull the trigger.Never end on “so depression is genetic”End on the interaction. That is the conclusion the mark scheme is looking for.
Link to concepts
Perspective
The genetic account is one level of explanation, not the only one. The same case of depression can also be described as faulty thinking (cognitive), as a loss of rewarding activity (behavioural), or as poverty and isolation (sociocultural). A strong essay says which level it is working at and admits the others exist.
Causality
Twin studies show a relationship, not a cause. Nobody manipulated anyone’s genes, so all we have is a pattern of agreement. Higher MZ concordance is consistent with a genetic cause — it does not prove one.
💡 Exam tip
Quote the two numbers together (46% and 20%). One on its own means nothing; the comparison is the whole point.
Use the words genotype and phenotype once each. They are cheap, precise marks.
Say “vulnerability to MDD is inherited”, never “MDD is inherited”. The first is defensible; the second is wrong.
The equal-environment assumption is the sharpest criticism you can make of a twin study. Learn it as one sentence.
If the question says discuss or evaluate, roughly half your writing should be critical.
Adoption studies are your backup evidence: adopted children raised away from a depressed biological parent still show raised risk.
⚠ Common mix-up
Concordance is not the same as heritability. Concordance is agreement within pairs; heritability is how much of the variation in a population is linked to genes.
MZ does not mean “male”, DZ does not mean “double”. MZ = one fertilised egg splitting; DZ = two eggs, two sperm.
Writing “46% of people with a depressed twin get MDD” as if that proves genes. Shared homes could produce agreement too. You need the DZ comparison.
Saying twin studies are experiments. They are natural, correlational designs. No variable was manipulated.
Describing the study but forgetting the disorder. Questions ask about a disorder, so define MDD before you touch the research.
Confusing 46% with “46% of the cause”. It is the proportion of pairs who match, nothing more.
Up next: Neurotransmitters and Depression — if genes only set the risk, what is actually happening in the brain of someone who is depressed? We follow serotonin across the synapse and see how SSRIs work.
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