> Except the most effective thing you can do as an individual is have a colonoscopy, not be the sort of person who would have one :-)
So we have two hypotheses here:
1. Having the colonoscopy is the thing that reduces deaths from colon cancer
2. Having the colonoscopy correlates to some other factor, X; and it's actually X which reduces the deaths from colon cancer.
X, for instance, could be a high willingness / ability to see the doctor when you experience early symptoms of colon cancer. That is, the more willing you are to go to the doctor when you start to have early symptoms of colon cancer, the more likely you are to survive it; and the more willing/able you are to go to the doctor when you have early symptoms of colon cancer, the more willing/able you are to have a colonoscopy.
What evidence do you have to believe that #1 is true, rather than #2?
Because if #1 is the case, the medical system should push hard on colonoscopies. But if #2 is the case, pushing colonoscopies might be a red herring. In fact, it might be counterproductive -- I've heard that colonoscopies are unpleasant; if you pressure people who don't like doctors into having a colonoscopy, and they have a terrible experience, then when they experience early symptoms of colon cancer, they may be more likely to procrastinate to avoid having another one. Rather, if #2 is the case, the medical system should try find out what can be done to make people more willing / able to get early medical care.