Jennifer, I read of a study that looked for illness occurrence in identical twins and the conclusion of the study was that only about 12% get the same disease.
What keeps me on the line of thinking about body chemistry is that being a CD occurs world wide, so it lets out local influences and family influences which is unknowingly based on Lamarck's ideas: "Lamarckism is the idea that an organism can pass on characteristics that it acquired during its lifetime to its offspring."
familial influence theory has been --obviously--discredited as an evolutionary theory. Perhaps some people don' like the idea because it takes them out of the loop--they had nothing to do with being a CD--they are "just following (biological) orders". well, we do that anyway, but I think that CDs make this special in their lives and to be sort of a puppet to nature is a little disheartening. As I pointed out, there are musical prodigies, math whizzes and all sorts of children born into ordinary families, and they generally follow their "star". Some eventually become just "ordinary " people but most excel within their gift area. The value of this being chemical in nature is that it IS the answer that we can provide that seems to support "we don't know" and supports the "I yam what I yam" crowd to boot, and the frequency world wide indicates that we are indeed NOT ALONE.