jgnat
Family doctors know who their JW patients are (they are the same ones who have a "no blood" directive on their files). Family doctors would also be able to give statistics on their caseload - incidence of illness (mental or otherwise) of their "no blood" patients versus the general population.
Such a study would get around the reluctance of the WTBTS to come under public, empirical examination.
This is, obviously, a good idea in the way to go to get at such information. However, for such information to be retrieved the researcher would have to have good credibility, such as by way of working for a university or hospital for research purposes. Occasionally, a good journalist could get doctors to reveal all (without their naming patients, of course) otherwise the information is never likely to see the light of day. How many of US here would a doctor reveal such information to? That's the problem. Somehow we've got to get someone with genuine credibility to do the research.
Best wishes,
Ian