These notes examine how the biological approach explains behavior through underlying mechanisms and why many psychologists prefer interactionist explanations that combine vulnerability with environmental stress.

This diagram shows the biopsychosocial model, where biological, psychological, and social factors overlap to shape health and behavior. In evaluation terms, it visually captures why purely biological (reductionist) accounts may miss important psychological processes and social context, supporting a more interactionist explanation. Source

Why this subtopic matters

In IB Psychology, you need to evaluate, not just describe, biological explanations. That means judging how useful they are, what evidence supports them, and whether they explain behavior fully.

Biological reductionism

What it means

Biological reductionism explains complex behavior by breaking it down into simpler biological parts, such as genes, neural activity, hormones, or neurotransmitters.

This approach is attractive because biological processes can often be measured directly, making explanations more scientific and easier to test.

Strengths

Reductionism has important strengths.

• It can produce precise, testable explanations.

• It supports scientific methods, including family studies, drug trials, and brain-based measures.

• It can lead to effective treatments when a specific biological mechanism is identified.

For example, if symptoms are linked to neurochemical activity, biological treatments may reduce them. This gives reductionism strong practical value.

Limitations

A major limitation is oversimplification. Human behavior is influenced by cognition, relationships, social context, and experience as well as biology. Explaining depression only in terms of serotonin, for example, ignores loss, beliefs, or coping.

Reductionism may also encourage biological determinism, the idea that behavior is fixed by biology. This can underestimate personal agency and change. People with similar biological characteristics do not always behave in the same way.

Another criticism is that biological findings do not always provide a complete causal explanation. A biological difference linked to a disorder may be one factor in a larger process rather than the sole cause.

For IB evaluation, the strongest judgment is usually balanced: reductionism is useful for isolating mechanisms and developing treatments, but it is often incomplete as a full explanation of behavior.

Diathesis-stress model

Core idea

The diathesis-stress model is less reductionist because it combines biological vulnerability with environmental experience.

A diathesis may be genetic, neurochemical, or physiological, while stress refers to external pressures or life events that trigger the vulnerability. The key idea is interaction: neither factor always produces the outcome by itself.

This graph illustrates the diathesis–stress (vulnerability–stress) model as a threshold process: as environmental stress increases, individuals with greater underlying vulnerability show a steeper rise in symptoms and cross the disorder threshold earlier. It helps distinguish “risk” (diathesis) from “triggering conditions” (stress) while showing why the same stressor can produce different outcomes across people. Source

This model helps explain why some people develop disorders after adversity while others do not, and why some people with a vulnerability remain healthy.

How it explains behavior

In this model:

• Diathesis creates increased risk.

• Stress activates or intensifies that risk.

• A disorder or pattern of behavior emerges when the combination becomes strong enough.

The model is often applied to depression and schizophrenia because both show evidence of inherited risk alongside important environmental influences.

A well-known example is Caspi et al. (2003). The researchers found that people with a short version of the 5-HTT gene were more likely to show depressive symptoms, especially after stressful life events. This suggests that genes alone were not enough; the environment changed the outcome.

Research by Tienari et al. on schizophrenia also supports the model. Genetic risk was more strongly associated with schizophrenia when the adoptive family environment was disturbed, showing that vulnerability and stress can interact.

Evaluation

A major strength is that the model is interactionist. It avoids the extreme reductionism of explanations based only on genes or brain chemistry, so it better reflects the complexity of behavior.

It is also supported by studies showing that biological predispositions often predict outcomes more strongly under stressful conditions. This helps explain individual differences.

The model has practical value because it suggests several intervention points:

• reduce stress exposure

• strengthen coping skills

• identify high-risk individuals early

• build protective social support

There are limits, however. Stress can be difficult to define and measure accurately. Researchers may rely on self-report or broad categories of life events, which can reduce validity.

The model can also still be too simple. Many behaviors involve multiple vulnerabilities, multiple stressors, and protective factors operating over time. Because of this, the model often explains probability rather than giving precise predictions about individuals.

Using these ideas in evaluation

In essays, direct comparison is useful.

• Biological reductionism focuses mainly on internal biological mechanisms.

• The diathesis-stress model includes biology but also recognizes environmental influence.

• Reductionism is often better for isolating causes.

• Diathesis-stress is often better for explaining variation between people.

A strong IB response should argue that biological explanations are most convincing when they are placed in context rather than treated as the only cause of behavior.