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An epiphenomenon (plural - epiphenomena) is a secondary phenomenon that occurs alongside or in parallel to a primary phenomenon.
In medicine, an epiphenomenon is a secondary symptom seemingly unrelated to the original disease or disorder. 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 bacterial infection.
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; however, in medicine this relationship is typically not implied: an epiphenomenon may occur independently, and is merely called an epiphenomenon because it is not the primary phenomenon under study. (A side-effect is a specific kind of epiphenomenon that does occur as a direct consequence.)
Philosophy of mind and psychology 
An epiphenomenon can be an effect of primary phenomena, but cannot affect a primary phenomenon. In philosophy of mind, epiphenomenalism is the view that mental phenomena are epiphenomena in that they can be caused by physical phenomena, but cannot cause physical phenomena. In strong epiphenomenalism, epiphenomena that are mental phenomena can only be caused by physical phenomena, not by other mental phenomena. In weak epiphenomenalism, epiphenomena that are mental phenomena can be caused by both physical phenomena and other mental phenomena, but mental phenomena cannot be the cause of any physical phenomenon.
The physical world operates independently of the mental world in epiphenomenalism; the mental world exists as a derivative parallel world to the physical world, affected by the physical world (and by other epiphenomena in weak epiphenomenalism), but not able to have an effect on the physical world. Instrumentalist versions of epiphenomenalism allow some mental phenomena to cause physical phenomena, when those mental phenomena can be strictly analyzable as summaries of physical phenomena, preserving causality of the physical world to be strictly analyzable by other physical phenomena.
Free will 
According to epiphenomenalism, free will having an effect on the physical world is an illusion, as physical phenomena can only be caused by other physical phenomena. In weak epiphenomenalism, there is free will to cause some mental effects, allowing for mental discipline that is directed at other mental phenomena.
Weak versions of behaviorism in psychology, which admit for the existence of mental phenomena, but not to their meaningful study as causes of any observable behavior in psychology, view mental phenomena as either epiphenomena, or linguistic summaries, as instrumentalist tools for examination of objectively observable physical behavior in others.
In the field of complex systems, the term epiphenomenon tends to be used interchangeably with "emergent effect".
Evolutionary neurobiology 
In his Stanford Lectures, Robert Sapolsky points out that although it has been widely accepted that the dominant force in Evolution is Natural Selection, in which an organism adapts to pass the maximum number of copies of its genes to the next generation of individuals, there are parallel epiphenomena also in action, phenomena which may even be at odds with Natural Selection. An example is sexual selection - in which females of a species may prefer to mate with gaudy males, but their gaudiness makes them more prone to predation (and hence passing fewer copies of their genes to future generations). The phenomenon of sexual selection is an example of an epiphenomenon accompanying the underlying phenomenon of Natural Selection in Evolution.
Propositional theory 
Zenon Pylyshyn suggested a propositional model of cognition where people do not conceptualize ideas in images but rather in meaningful relationships. In this theory, epiphenomena refer to images because they are merely products people conceptualize from their actual thought processes. Pylyshyn defends his claim by explaining that we only see images when we envision the form of an object. While visualizing objects or actions is a frequent process in our mind, it does not occur when we are considering the meaning behind an action or the non-visual properties of an object. There are many concepts we simply cannot envision. Additionally, when envisioning an image, it changes based on our preconceived notions, suggesting that semantic relations precede visual images. Unfortunately, the idea of epiphenomena in propositional theory is largely subjective and not falsifiable.
||This article needs additional citations for verification. (July 2012)|
- Metaphysics, Richard Taylor
- Gallagher, Shaun ; In: Does consciousness cause behavior? Pockett, Susan (Ed.); Banks, William P. (Ed.); Gallagher, Shaun (Ed.); Cambridge, MA, US: MIT Press, 2006. pp. 109-124. [Chapter]
- Sapolsky, R., Stanford University, Evolutionary Neurobiology, https://www.youtube.com/watch?v=Y0Oa4Lp5fLE
- Sternberg, R. (2005). Cognitive psychology. (5th ed., p. 263). Pearson.
- Pylyshyn , Z. (2003). Return of the mental image: are there really pictures in the brain? Trends in Cognitive Sciences, 7(3), 113-118. doi:10.1016/S1364-6613(03)00003-2
- Pylyshyn, Z. (2003). Explaining mental imagery: Now you see it, now you don't. Trends in Cognitive Sciences, 7(3), 111-112. doi: 10.1016/S1364-6613(03)00004-4