Which statement best captures temporality as a requirement for causality in epidemiology?

• A. The outcome must occur before exposure.
• B. Exposure and outcome must occur at the same time.
• C. Timing is not important for causality.
• D. Exposure must occur before the outcome.

Answer: (D)

Temporality means the cause must come before the effect. For an exposure to be considered a possible cause of an outcome, there has to be a clear time order with the exposure occurring first, then the disease or outcome developing after some lag. If the outcome is observed before the exposure, causality cannot be claimed because the sequence would imply reverse causation or that the exposure is not related to the development of the outcome at all. This is why study designs that track exposure first (like prospective cohorts) are so valuable—they establish that the exposure precedes the outcome and allow for the latent period between exposure and disease. The other statements fail because they either mix up the direction of time, imply simultaneity, or deny the importance of timing in causal reasoning.