2.1 Biological approach

An animal model uses a non-human organism to study a biological process or disorder with the aim of learning something relevant to humans.

Models can allow controlled manipulation, repeated measurement and access to mechanisms that are difficult to study directly. Their value depends on construct validity: the model must share the relevant process, not merely show a similar outward behaviour.

A stress manipulation in rodents may test a pathway involved in depression, but species-specific behaviour, dose and context limit the inference. Human clinical evidence is still needed for treatment claims.

An animal result is not automatically generalisable or unethical. Evaluate welfare, model fit and the exact claim being transferred.

Evaluate animal ethics with replacement, reduction and refinement: replace animals with non-animal methods where the question can be answered adequately; reduce numbers while preserving a valid design; and refine housing, procedures and endpoints to minimize pain, distress and lasting harm. A harm-benefit review should consider species needs, scientific necessity, model validity, expected knowledge, trained care and whether less harmful alternatives exist. Weak transfer to humans reduces the ethical justification because animals would bear costs for evidence that cannot support the intended claim.

Biological reductionism explains behaviour by reducing it to genes, brain structures or chemical processes. Holistic explanations include interactions across biological, cognitive, social and cultural levels.

Reductionism can produce testable mechanisms and targeted treatments, but a mechanism at one level may not explain onset, meaning or response differences. A drug effect supports a pathway hypothesis, not a complete account of a disorder.

An antidepressant that changes serotonin signalling may reduce symptoms for some patients. Therapy, stress exposure and social support can still influence whether the change is sustained.

Biological does not mean deterministic, and holistic does not mean untestable. State what the chosen level explains and what it leaves out.

Brain-imaging methods measure structure or activity indirectly—for example, changes in volume, blood flow or metabolic signal—so researchers can compare patterns across tasks or groups.

Imaging can localise activity and track development, but resolution, reverse inference and correlational design limit what it proves. A region active during a task may support it without being the only cause.

If adolescents show different activation during inhibition, the result may indicate developmental change; it does not by itself show that maturation caused a later behaviour or that one region controls it alone.

A colourful scan is not a diagnosis or causal map. Identify the method, comparison, inference and uncertainty.

Neurotransmitters carry signals across synapses, while hormones travel through the bloodstream; both can influence behaviour by changing how neural systems respond.

A chemical explanation asks how release, receptor binding, reuptake or degradation changes a circuit. The same messenger can have different effects in different pathways, and measured levels may not equal activity at a specific synapse.

A drug that blocks reuptake may increase a messenger's availability, but symptom change depends on receptor adaptation, network effects and the person's context—not only the concentration in blood.

One neurotransmitter is not a single “happiness” or “aggression” chemical. Keep mechanism, measurement and behavioural claim separate.

The diathesis–stress model proposes that a predisposition (diathesis) combines with environmental stress to increase the probability of a disorder or outcome.

A vulnerability may be biological, psychological or social; stress can be acute or chronic. The model predicts variation: the same stressor need not produce the same outcome, and protective support can change the pathway.

Two people may experience the same workload, but only one develops persistent symptoms because prior vulnerability, sleep and social support differ. The observation supports an interaction hypothesis, not a single cause.

Diathesis is not destiny and stress is not always visible. Measure both components and avoid blaming the person or environment alone.

Genetic inheritance can contribute to vulnerability through many variants and biological pathways. A heritable tendency describes variation in a population, not a guaranteed outcome for one person.

Twin or family evidence can estimate similarity, but shared environments and gene–environment interaction complicate interpretation. A risk variant may matter only under particular developmental or social conditions.

If identical twins show different depression outcomes, genetic influence is compatible with the result: genes may alter sensitivity while stress, support and learning change exposure and expression.

Heritability is not the percentage of an individual's disorder caused by genes. Avoid deterministic or stigmatizing conclusions.

Localisation of function proposes that particular brain regions contribute more to some functions than others. Modern evidence often shows networks and interaction rather than one isolated “centre”.

Evidence can come from lesions, stimulation, imaging or task comparisons. Strong interpretation requires a clear contrast and awareness that damage can cause reorganisation or broader network disruption.

If a lesion is followed by impaired language production, it supports a contribution of the damaged network; it does not prove that every language process is stored in one spot.

Activation is not the same as exclusive control. Separate localisation, lateralisation and network-level explanation.

Neuroplasticity is the nervous system's capacity to change connections, representation or recruitment in response to development, learning or injury.

Change can involve synaptic strengthening, pruning or functional reorganisation. Sensitive periods may make some learning easier, but adult brains remain capable of adaptation; plasticity can also reinforce unhelpful patterns.

After repeated practice, a skill may recruit more efficient pathways. After injury, nearby regions may support a lost function, but recovery depends on damage, timing and rehabilitation.

Plasticity does not mean unlimited repair or that every change is beneficial. Identify the mechanism and the evidence for transfer to behaviour.

Neurotransmission is communication between neurons: an electrical signal triggers chemical release, receptors respond, and the signal is terminated by reuptake or breakdown.

A neurochemical explanation of a disorder proposes that altered signalling contributes to symptoms. Evidence from medication response, receptor studies or imaging can support a pathway, but treatment effects are not a direct measurement of the original cause.

If a reuptake inhibitor improves mood for some patients, it is consistent with serotonin involvement; it does not show that low serotonin alone caused every case or that the drug works through one mechanism only.

Do not turn a group-level association into an individual diagnosis. Separate synaptic mechanism, clinical outcome and alternative explanations.