In Philosophy of the mind this is much debated and not understood,, modern experimentational evidence seem to be pointing towards this as true.
http://en.wikipedia.org/wiki/Epiphenomenalism
Arguments for
A large body of neurophysiological data seems to support epiphenomenalism. Some of the oldest such data is theBereitschaftspotential or "readiness potential" in which electrical activity related to voluntary actions can be recorded up to two seconds before the subject is aware of making a decision to perform the action. More recently Benjamin Libet et al. (1979) have shown that it can take 0.5 seconds before a stimulus becomes part of conscious experience even though subjects can respond to the stimulus in reaction time tests within 200 milliseconds. Recent research on the Event Related Potential also shows that conscious experience does not occur until the late phase of the potential (P3 or later) that occurs 300 milliseconds or more after the event. In Bregman's Auditory Continuity Illusion, where a pure tone is followed by broadband noise and the noise is followed by the same pure tone it seems as if the tone occurs throughout the period of noise. This also suggests a delay for processing data before conscious experience occurs. Norretranders has called the delay "The User Illusion" implying that we only have the illusion of conscious control, most actions being controlled automatically by non-conscious parts of the brain with the conscious mind relegated to the role of spectator.
The scientific data seem to support the idea that conscious experience is created by non-conscious processes in the brain (i.e., there is subliminal processing that becomes conscious experience). These results have been interpreted to suggest that people are capable of action before conscious experience of the decision to act occurs. Some argue that this supports epiphenomenalism, since it shows that the feeling of making a decision to act is actually an epiphenomenon; the action happens before the decision, so the decision did not cause the action to occur.
[edit]
Arguments against
Benjamin Libet's results are quoted [6] in favor of epiphenomenalism, but he believes subjects still have a "conscious veto", since the readiness potential does not invariably lead to an action. In Freedom Evolves, Daniel Dennett argues that a no-free-will conclusion is based on dubious assumptions about the location of consciousness, as well as questioning the accuracy of Libet's results.
Others have argued that data such as the Bereitschaftspotential undermine epiphenomenalism for the same reason, that such experiments rely on a subject reporting the point in time at which a conscious experience occurs, thus relying on the subject to be able to consciously perform an action. That ability would seem to be at odds with early epiphenomenalism, which according to Huxley is the broad claim that consciousness is “completely without any power… as the steam-whistle which accompanies the work of a locomotive engine is without influence upon its machinery”. [7]
In favor of interactionism, Green (2003) argues that epiphenomenalism does not even provide a satisfactory ‘out’ from the problem of interaction posed by substance dualism. Although it does not entail substance dualism, according to Green, epiphenomenalism implies a one-way form of interactionism that is just as hard to conceive of as the two-way form embodied in substance dualism. Green suggests the assumption that it is less of a problem may arise from the unexamined belief that physical events have some sort of primacy over mental ones.
Donald Symons dismisses epiphenomenalism from an evolutionary perspective. He says that the view that mind is an epiphenomenon of brain activity is not consistent with evolutionary theory, because if mind were functionless, it would have disappeared long ago, as it would not have been favoured by evolution. [8]