IB Psychology SLTopic 1 — Mental Health DisordersPaper 1 & 2Biological approach~9 min read
What Animal Studies Tell Us About Depression
You cannot ethically make a person depressed to see what happens. So researchers do something uncomfortable instead: they create depression-like states in animals. It works surprisingly well for studying the brain — and it fails completely at the parts of depression that live inside a person’s head.
📚 What you need to know
An animal model means using animals to study a human condition such as MDD.
The two models on your syllabus are chronic mild stress (CMS) and learned helplessness.
Animals are used because some research would be unethical or impractical on humans.
Around 29 million animals are used in experiments each year across the USA and the EU.
Higher-order animals such as chimpanzees share about 98.8% of their DNA with humans.
Models can copy behaviour (giving up, losing interest) but not thought (guilt, worthlessness).
The concept links here are responsibility (ethics) and perspective (how far findings generalise).
Why use animals at all?
The honest answer is control. With animals a researcher can decide exactly what stress happens, when it happens, and for how long, and then look directly at the brain afterwards. You could never do that with people.
Ethical rules stop researchers from deliberately harming human participants.
Animals have shorter lives, so long-term effects appear quickly.
There are real similarities between human and animal brain structures, especially in the areas handling stress and reward.
Animals are already used widely in biological research, for example in studies of drug tolerance and addiction.
Learn the models as three-step chains: what was done, what the animal did, and which human symptom it resembles. Examiners want that third step, and it is the one students leave out.
The chronic mild stress model
The idea is that steady, low-level, unpredictable stress wears an animal down — the way constant small hassles wear a person down. In one study, mice were exposed to unpleasant but mild conditions for nine weeks, such as rat odours or dirty bedding in the cage.
The animals developed anhedonia: they stopped chasing things they used to enjoy.
Chronic, uncontrollable stress damaged the working of the brain’s reward system.
Scans showed changes in the prefrontal cortex and hippocampus, areas involved in decision-making and memory.
The conclusion is that long-running stress can physically change how the brain works, and one result is a reduced capacity for enjoyment — which is exactly what anhedonia looks like in humans.
The learned helplessness model
This model is about control rather than stress. Animals were repeatedly given electric shocks they could not avoid. Later, they were put in a situation where escape was possible — and they did not take it.
In one well-known version, dogs that had been shocked were placed in a cage alongside dogs that had not. The dogs that had not been shocked used the exit ramp. The ones that had been shocked stayed put in the unpleasant environment, even though the way out was right there.
The psychological point. The animal has learned something: “nothing I do makes any difference.” That learned expectation then carries into a new situation where it is no longer true. In people, this maps onto the passivity and hopelessness seen in MDD — the sense that trying is pointless.
Notice how neatly this links to other topics. Learned helplessness explains why someone in an abusive relationship may not leave, which is a sociocultural issue, not just a biological one. Cross-linking like that is exactly what “perspective” questions are asking for.
What animal models cannot do
Here is the part that separates a middling answer from a strong one. Depression is not only a set of behaviours. It is also a set of thoughts: I am worthless, nothing will ever change, this is my fault. Those thoughts are central to the diagnosis.
You cannot ask a rat whether it feels worthless. So an animal model can copy the outward signs of depression while telling you nothing about the inner experience that defines it.
What the models capture well
Loss of interest and reduced activity.
Giving up in the face of an uncontrollable situation.
Measurable changes in brain structure and chemistry.
How long-term stress accumulates over time.
What they miss completely
Guilt, worthlessness and self-blame.
Hopeless thinking about the future.
The role of language, culture and relationships.
Whether the animal experiences anything like sadness at all.
Anthropomorphism
Anthropomorphism means reading human feelings into animal behaviour. A mouse that stops exploring might be depressed — or tired, unwell, or simply behaving sensibly in a threatening environment. Naming the behaviour “helplessness” already assumes the human interpretation before it has been tested.
Ethics: the responsibility question
Deliberately exposing animals to harmful stimuli is hard to justify if there is another way to get the same knowledge. The usual framework asks researchers to look for:
🧩 Reducing harm in animal research
Use fewer animals where the same conclusion can be reached with a smaller sample.
Use less sensitive species where possible, rather than higher-order animals.
Replace animals entirely with alternatives such as human brain imaging, computer modelling, or natural experiments on people already exposed to stress.
There is a genuine tension here, and examiners reward you for naming it rather than picking a side. The same features that make an animal a good model — a brain and stress system like ours — are the features that make harming it hardest to defend.
EXAM ANSWER
Evaluate the use of animal models in researching one disorder. [22 marks]
“Evaluate” means weigh it up and reach a judgement. Build it in balanced pairs.
Pair 1 — control vs realism
Animals allow controlled manipulation of stress that would be unethical in humans, but a cage is nothing like a human life, so ecological validity is low.
Pair 2 — biology vs cognition
Brain scans of CMS mice showed real prefrontal and hippocampal change, but no animal model can reach the negative thoughts that define MDD.
Pair 3 — usefulness vs ethics
The findings have led to real treatments, but that benefit has to be weighed against deliberate harm to sentient creatures.
Judgement: useful for mechanisms, weak for meaninga judgement is not “it depends” — say what the models are good FOR
💡 Exam tips
Always finish an animal study by saying which human symptom it maps onto. Without that link you have described biology, not psychology.
Use the word extrapolation for generalising animal findings to humans. It is the precise term and it reads well.
Keep anhedonia in your vocabulary — it is one word that replaces a whole sentence.
For ethics, argue the trade-off rather than just saying “it is unethical”. The trade-off is the answer.
These models also work as examples for the learning approach, since learned helplessness is learned behaviour.
If a question asks about research methods, note that these are true experiments with manipulated variables, unlike the correlational human studies.
⚠ Common mix-ups
Saying the animals “had depression”. They showed depression-like symptoms. The wording matters.
Mixing the two models up. CMS is about repeated mild stress; learned helplessness is about lack of control.
Forgetting the control group. In the dog study, the unshocked dogs escaping is what makes the result meaningful.
Claiming animal brains are the same as ours. Similar in some structures, not the same, and the differences are largest in exactly the areas doing complex thought.
Treating high shared DNA as proof of similar minds. Genetic overlap does not mean shared experience.
Only writing about ethics. The question usually asks about value as well. Do both.
Up next: Beck’s Cognitive Model of Depression — we have just said that animal models cannot reach a person’s thoughts. So now we go straight at the thoughts themselves.
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