Also of interest, Malcolm Galdwell's podcast* "Revisionist History" had an excellent episode on CTE in football. He made an analogy to smoking. Evidence had long suggested that smoking was dangerous, but it was only that -- evidence, not hard proof. Evidence continued to pile up, but so did the counter evidence (primarily from tobacco firms and those they funded). At what point do we say -- OK, maybe the evidence isn't 100%, but isn't there enough evidence to suggest something is wrong?

* http://revisionisthistory.com/episodes/22-burden-of-proof