Educerie · IB Diploma · Psychology

Topic 1 — The biological approach to understanding behaviour

The biological approach explains behaviour in terms of physiology: brain structure, neurochemistry, hormones and genes. Its power is that it deals in measurable things. Its limitation is that measurable is not the same as complete.

The brain and behaviour

Localisation is the idea that specific functions are associated with specific brain regions. The evidence comes from three sources, each with its own weakness:

• Case studies of damage. A person with damage to one region loses one capacity, suggesting the region supports it. Rich in detail and often the only available evidence — but a single case cannot be generalised, cannot be replicated, and damage is rarely confined neatly to one area.
• Neuroimaging. fMRI shows which regions are active during a task, and is non-invasive and replicable. But it shows correlation, not causation: activity during a task does not prove the region performs it, and the artificial environment of a scanner limits ecological validity.
• Animal studies. Permit controlled lesioning impossible in humans, but raise ethical objections and generalise uncertainly to human beings.

Neuroplasticity is the brain's capacity to reorganise: connections strengthen with use and are pruned without it. This matters because it qualifies the whole approach — biology is not fixed. The famous finding that experienced taxi drivers show structural differences in the hippocampus illustrates it, and also illustrates the standard caution: those differences might be a consequence of the work, or might have predisposed people to the work in the first place.

Neurotransmitters carry signals across the synapse. Serotonin is implicated in mood, dopamine in reward and motivation, acetylcholine in memory. The examinable caution: the relationship is correlational and bidirectional. Low serotonin is associated with depression; that does not establish that low serotonin causes it, since depression may alter serotonin, or a third factor may produce both.

Hormones and pheromones

Hormones are chemical messengers released into the bloodstream by glands. They act more slowly than neurotransmitters and more diffusely.

• Cortisol — the stress hormone. Adaptive in the short term; chronically elevated levels are associated with impaired memory and immune function.
• Adrenaline — the acute stress response, and implicated in the vividness of emotional memory.
• Oxytocin — associated with bonding and trust. The nuance worth knowing is that oxytocin appears to increase trust toward the in-group while sometimes increasing suspicion of out-groups, which makes "the love hormone" a misleading label.

Pheromones are chemical signals between members of a species. Their existence in humans is genuinely contested — the evidence is weak and much of it has failed to replicate. A question about pheromones is often testing whether you know this, and an answer treating human pheromones as established fact scores less than one that reports the debate.

Genetics and behaviour

Behavioural genetics asks how far individual differences are inherited. Three methods:

• Twin studies. Monozygotic twins share ~100 % of their genes, dizygotic ~50 %. A trait more concordant in MZ than DZ pairs suggests genetic influence. The confound: MZ twins are also often treated more similarly, so shared environment is not fully controlled.
• Adoption studies. Compare adoptees with biological and adoptive parents, separating genes from rearing environment more cleanly. Limited by small samples and non-random adoption placement.
• Kinship studies more broadly, tracing concordance across degrees of relatedness.

Heritability is the most misunderstood term in the subject. A heritability estimate of 0.5 does not mean half of a person's trait is genetic. It means that, in the population studied and in that environment, about half of the variation between individuals is statistically associated with genetic variation. It is a property of a population, not of a person, and it changes when the environment changes. Writing "intelligence is 50 % genetic" is a definitional error examiners notice.

Gene–environment interaction is where the field actually is. Genes influence susceptibility; environment determines whether it is expressed. Epigenetics describes how environmental factors alter gene expression without altering the sequence — providing a mechanism by which experience becomes biology, and undermining any simple nature-versus-nurture framing.

Evolutionary explanations propose that a behaviour persists because it conferred reproductive advantage. They can be illuminating, but they are difficult to falsify: a plausible adaptive story can be constructed for almost any behaviour after the fact, which is why such explanations require converging evidence rather than plausibility alone.

Research methods in the biological approach

True experiments manipulate an independent variable and permit causal claims, but often at the cost of artificiality.

Correlational studies show relationships between variables and cannot establish causation, for three reasons worth naming: the direction may be reversed, a third variable may cause both, or the association may be coincidental.

Case studies give depth on rare phenomena and are sometimes the only possible evidence, but cannot be generalised or replicated, and are vulnerable to researcher bias in interpretation.

Ethics in this approach are demanding. Brain imaging can produce incidental findings of medical significance that the participant did not consent to learn. Genetic testing raises confidentiality questions extending to relatives who never consented at all. Animal research raises the question of whether the benefit justifies the harm.

Evaluating the biological approach

Strengths. It uses objective, measurable variables; it employs well-controlled experimental methods; it has produced real applications, particularly pharmacological treatments; and it is increasingly supported by technology that did not exist a generation ago.

Limitations. It risks reductionism — explaining a complex behaviour by a single mechanism and losing what the other approaches capture. Much of its evidence is correlational yet is frequently reported causally. Many findings come from animal studies or from small, unrepresentative samples. And a purely biological account of, say, depression cannot explain why its prevalence differs across cultures and has changed over time — which is exactly where the sociocultural approach earns its place.

The strongest evaluative move is interactionist: biological factors set predispositions; cognitive and sociocultural factors determine expression. No approach alone is sufficient, and saying which one the biological account leaves out — and what it leaves out — is what a top band answer does.

What actually loses marks in this topic

1. Naming studies without using them as evidence for a claim.
2. Asserting causation from correlational or imaging evidence.
3. Defining heritability as "the proportion of a trait that is genetic".
4. Treating MZ–DZ differences as pure genetic evidence, ignoring shared environment.
5. Presenting human pheromones as established.
6. Describing a study without evaluating its sample, design or generalisability.
7. Reducing a behaviour entirely to biology when the question invites an interactionist answer.
8. Reciting ethical guidelines instead of applying them to the study in front of you.

Educerie · written from the published IB syllabus structure for Psychology Topic 1, first assessment 2027. Original text; the findings referred to are standard in the research literature. Last reviewed 5 September 2026.