The prevailing media narrative is that concussions or repetitive subconcussive blows "cause" chronic traumatic encephalopathy (CTE) and that there is a proven link between the two. It thus may come as a surprise that, despite widespread media coverage and speculation regarding the late-life or post-retirement risks of cognitive impairment in athletes who engaged in sports involving repetitive trauma, ... there has been very little in the way of peer-reviewed scientific literature involving data that establishes such risk. 
No consensus on causation
Rejecting the blanket conclusion that there is a definitive causal and effect connection between repetitive head trauma and CTE, the most recent international consensus statement on concussion in sport  cautions that a relationship has "not yet been demonstrated between CTE and concussions or exposure to contact sports."
It notes that "there are no published epidemiological, cohort or prospective studies relating to modern CTE. Owing to the nature of the case reports and pathological case series that have been published, it is not possible to determine the causality or risk factors with any certainty."
"As such, the speculation that repeated concussion or subconcussive impacts cause CTE remains unproven," the statement continues. "The extent to which age-related changes, psychiatric or mental health illnesss, alcohol/drug use or co-existing medical or dementing illnesses contributed to this process is largely unaccounted for in the published literature. At present, the interpretation of causation in the modern CTE case studies should proceed cautiously."
Remarkably, the consensus statement concludes with a direct shot at the media, in which it recognizes the importance of addressing the "fears of parents/athletes from media pressure related to the possibility of CTE."
Media stoking CTE fears?
Such a view was subsequently echoed by Christopher Randolph, PhD. of Loyola University in Chicago in a follow-up study of retired NFL players - who largely comprise the highly limited, self-selected universe from which the case studies of brains showing the presence of CTE have been drawn, and who the media have widely reported as being at high risk of CTE - in which he laments that "the media attention to this issue continues to far outweigh any meaningful results from sound experimental science."
Randolph found that when compared with healthy controls and with non-athlete patients with a clinical diagnosis of mild cognitive impairment (MCI), commonly presumed to reflect the earliest stage of Alzheimer's disease, the patterns of impairments of the retired NFL players involved in the study were virtually identical to those exhibited by the non-athletes with MCI, thus leading him and his colleagues to conclude that CTE might not be a distinct neurodegenerative disorder at all.
The study appears to lend support to Randolph's theory, first proposed in a 2009 paper, that a long history of repetitive head trauma in contact sports does not lead to CTE, but might eventually result in a diminished cerebral reserve leading in some unknown percentage of cases to an earlier than normal expression of other common, age-related neurodegenerative diseases such as AD and Parkinson's (PD). His theory was that the ways in which such diminished cerebral reserve would be expressed (e.g. MCI, AD, PD, ALS) would not differ from individuals with those diseases who lacked such a history of head trauma, which is precisely what his 2013 study suggests.
The Randolph study, co-authored by no less a concussion authority than Kevin Guskiewicz, Kenan Distinguished Professor, Director of the Matthew Gfeller Sport-Related Traumatic Brain Injury Research Center at The University of North Carolina at Chapel Hill, and Director of the Center for the Study of Retired Athletes at UNC, has been heavily criticized by those who have identified CTE is a distinct condition as being flawed. Chris Nowinski, co-director of the Center for the Study of Traumatic Encephalopathy at the Boston University School of Medicine, told PopSci.com  that it was "preposterous" for Randolph to conclude that CTE might not be its own disease because the retired football players had impairments similar to those of other patients with mild cognitive impairment.
Likewise, the pushback upon the release of the international consensus statement in March 2013 questioning the causal link between reptitive brain trauma and CTE, from the research group at Boston University's Center for the Study of Traumatic Encephalopathy most associated with the position that CTE is a distinct neurodegenerative disorder suffered only by athletes in contact and collision sports, and that repetitive trauma can be in some cases the sole cause of the disease, was immediate, sustained, and widely reported.
Typical was the reaction of one of the co-authors, Dr. Robert Cantu, Director of the Center. Dr. Cantu told NewJersey.com:  "When I saw that [it said] we need more data in terms of CTE, I wrote to the other authors, in essence, 'What the hell do you mean that we need more data?' The whole breadth of the document is large, and 99 percent of it it I strongly support. But that part of it, I don't support at all. Frankly, it stunned me."
Dr. Cantu's Boston University colleague, Dr. Ann McKee, likewise expressed befuddlement at the consensus statement's treatment of CTE, telling NewJersey.com, "This is a time that calls for immediate action to reduce the amount of head trauma experienced by athletes in all sports to prevent CTE."
Dr. McKee asserted that it would be "irresponsible to justify inaction by requesting a level of scientific proof that will take decades to acquire," expressing the fear that, to suggest that CTE "may not be part of the impact exposure, but rather due to other yet unidentified factors," could give tacit permission to those who play collision sports to proceed as if there is no urgent problem when concussions arise.
It wasn't intended to have such an implication, Dr. Rosemarie Scolaro Moser, Director of the Sports Concussion Center of New Jersey and MomsTEAM.com's sports concussion neuropsychologist, told NewJersey.com. Instead, she cautioned that it would be jumping to conclusions to say, 'If you play football, you're probably going to get CTE.' Which is not the case. So what differentiates those with CTE from those who don't have it? That's what we need to know."