I’m finding this discussion very interesting because of my personal background. The general population would describe me as “good at maths”. I would describe myself (because of context) as “bad at maths”. I was one of the best all the way through high school and then started an undergraduate maths course known for being challenging. After a few weeks I completely hit a wall and couldn’t progress any further with it. (I changed course to music.) My sister, father and brother-in-law all completed a whole undergraduate course in maths—I couldn’t finish the first year. So I think I am bad at maths.
Following on: I think I have a much deeper aesthetic understanding of music than my father and sister. They, the “better” mathematicians, are excellent musicians, but in a functional sense. I’d say that I, the “worse” mathematician, have a much more profound insight into music than they do.
Iron overload / haemochromatosis occurs in approx 0.5% of the population of Northern European origin (and less in other ethnicities). Undiagnosed and untreated the iron will build up in the liver and other organs and cause a variety of unpleasant side effects. Venesection is the standard treatment, though I suggest that less than 0.5% of the population is not significant enough to explain the other studies.
Source: http://www.haemochromatosis.org.uk/index.html