A causal mechanism has been proven for some risk factors, but not in others.

The idea

A causal mechanism is a biological explanation of how a risk factor leads to a disease. For some factors, the mechanism is well established. For example, tobacco smoke contains carcinogens that can mutate DNA in lung cells, and this mutation can lead to uncontrolled cell division and lung cancer. For other factors, scientists observe a statistical association but cannot yet prove the mechanism. For instance, some dietary factors are linked to cardiovascular disease, but the exact biological steps may be complex or uncertain.

What earns the marks

• Define a causal mechanism as an explanation of how a risk factor produces a disease.
• Give an example where a causal mechanism is proven, such as carcinogens in tobacco smoke causing DNA mutations linked to lung cancer.
• Give an example where a causal mechanism is not proven, such as an uncertain dietary link to cardiovascular disease.
• Explain that correlation alone does not prove causation because other factors may be involved.
• Recognise that multiple interacting factors can make it difficult to isolate one cause.

Common mistakes and how to put them right

• Claiming that any statistical link proves causation.

  Put it right by stating that a mechanism must be demonstrated.

• Assuming all risk factors have a known mechanism.

  Put it right by noting that some links remain uncertain.

• Confusing a risk factor with a causal mechanism.

  Put it right by explaining that the mechanism describes the biological process, not just the association.

Exam tips

• Use the phrase 'causal mechanism' accurately and explain what it means in your own words.
• When giving an example, state the factor, the disease and the biological step that links them.
• For an unproven link, say that the evidence shows an association but the mechanism is not established.