Epiphenomenon

An epiphenomenon (plural: epiphenomena) is a secondary phenomenon that occurs alongside or in parallel to a primary phenomenon. The word has two senses, one that connotes known causation and one that connotes absence of causation or reservation of judgment about it.

In the more general use of the word, a causal relationship between the phenomena is implied; the epiphenomenon is a consequence of the primary phenomenon. This is the sense that is related to the noun epiphenomenalism.

However, in medicine, this relationship is typically not implied, and the word is usually used in its second sense: an epiphenomenon may occur independently, and is called an epiphenomenon because it is not the primary phenomenon under study or because only correlation, not causation, is known or suspected. In this sense, saying that X is associated with Y as an epiphenomenon is preserving an acknowledgment that correlation does not imply causation. Signs, symptoms, syndromes (groups of symptoms), and risk factors can all be epiphenomena in this sense. For example, having an increased risk of breast cancer concurrent with taking an antibiotic is an epiphenomenon. It is not the antibiotic that is causing the increased risk, but the increased inflammation associated with the bacterial infection that prompted the taking of an antibiotic. The metaphor of a tree is one way of helping to explain the difference to someone struggling to understand. If the infection is the root of the tree, and the inflammation is the trunk, then the cancer and the antibiotic are two branches; the antibiotic is not the trunk.

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