There are two ways to go about studying a disease. Let’s call them the retrospective and prospective methods. In the retrospective method, scientists identify individuals with the disease and ask about the circumstances that led to the illness. In the prospective method, they start with a representative sample of people and track them over time to see who develops the disease.

Both methods have yielded important discoveries, but the retrospective method is much more prone to distortion than the prospective method. Consider the following example. Using the retrospective method, 100 percent of alcoholics drink alcohol. Yet drinking alcohol does not necessarily lead to alcoholism, as can be determined by the prospective method in which it can be seen that the proportion of those who enjoy alcoholic drinks and become alcoholics is less than 100 percent.

Boston University’s Chronic Traumatic Encephalopathy (CTE) Center recently reported that 99 percent of NFL alumni who made brain donations at the time of death have CTE (a similar finding was reported in 2013). While researchers acknowledge that those who make brain donations are not representative of retired NFL players (much less those with sports-related concussions in general) it is remarkably easy to make the same mistake as in the alcoholism example—that is, making the assumption that this finding generalizes to the broader population of athletes exposed to concussion.

CTE is a neurodegenerative disease initially termed “dementia pugilistica” in boxers by Dr. Harrison Stanford Martland in 1928. It is characterized by changes in mood and cognition but can only be diagnosed through the examination of brain tissue at death. The neuropathologist Bennet Omalu found the same neuropathology in a professional football player in 2005, giving inspiration to the movie Concussion.

The seriousness of CTE in those affected is not in doubt. Yet the broader context of CTE research, and its implications for our health and that of our loved ones, has been lacking. CTE has been identified in a few hundred individuals, nearly all of them athletes exposed to hundreds, if not thousands, of impacts to the head. The alarming case studies of suicide or premature death in people with CTE have led many to fear that disability or even death due to CTE is inevitable in those who have suffered concussions. Yet professional football players enjoy a longer lifespan than the average person with risk of suicide half that of the general population.

Mental changes in retired professional athletes are far more complicated than a binary classification of positive or negative for CTE. Many of those with CTE have complicated brain pathology reflecting multiple neurodegenerative conditions. Ex-professional athletes face numerous challenges, including psychiatric conditions and chronic pain overlaid on a unique profile of skills and personality characteristics that presumably attracted them to high level sports competition in the first place. Moreover, the number of people with CTE pathology in their brains who have no outward symptoms is unknown. In fact, it is not unusual for neuropathology of other types, such as Alzheimer’s disease, to be found in people who did not show signs of dementia.

At Baycrest’s Rotman Research Institute in Toronto, we have been fortunate to interview, test, and scan the brains of more than 50 National Hockey League (NHL) alumni in our research on the remote effects of concussion. We found no evidence of cognitive impairment, yet there was an elevated rate of psychological problems, such as depression, anxiety and substance abuse.

While CTE cannot be ruled out, these conditions, which are commonly observed in the absence of concussion history, are treatable. Given media accounts of CTE, many alumni are frightened and wonder if they are on a path towards inexorable decline. Ex-NHLer Todd Ewan, for example, (not a participant in our study), took his life after struggling with depression. His family was convinced that he had CTE, but there was no evidence of this upon neuropathological examination.*

Research on brain changes including CTE in retired professional athletes may lead to important new discoveries about brain trauma, aging and neurodegenerative disease. Although we regard CTE as a legitimate concern for professional athletes with major concussion exposure, is there similar concern warranted for recreational contact sports? Out of the millions exposed to concussions through recreational sports, the count of confirmed cases of CTE in the research literature is in the tens. Yet not even Dr. Omalu is immune to the reasoning error defined above, stating that “there is a 100 percent risk of exposure to permanent brain damage" in those participating in recreational contact sports, and that youth football is a form of child abuse.

Many sports carry risks of injuries that should be managed. On the other hand, lack of exercise and overly sedentary behavior contribute to numerous negative health outcomes. Participation in recreational sports, including contact sports, has many benefits to health and social functioning. Long before the latest wave of research on CTE, it was well known that concussions can be associated with significant functional impairment for weeks following the injury. They also increase the risk of subsequent concussions, where symptoms can be potentiated by prior concussion, especially if full recovery has not occurred.

If you or someone close to you has had a concussion, medical attention is warranted. Avoid activities that could result in another concussion; multiple concussions are associated with poorer recovery. If there are mental changes or psychological distress, seek help. None of these recommendations is altered by the present state of knowledge about CTE, which is in its infancy.

*Author's note: Since this article was published, a separate analysis of Mr. Ewan's brain tissue did find evidence of CTE.