Original
Research was sparse and disjointed until 3 or 4 years ago, but there has been an avalanche since then. There is evidence for it being an immune disfunction, which effects a variety of tissues by inhibiting a crucial enzyme which plays a part in sugar metabolism, resulting in an excess of lactate (causing low energy and muscle pain). There are many other theories. The research is available on websites like www.meassociation.org and www.openmedicinefoundation.org.
It is NOT a psychological condition.