First, as the authors themselves note, the study is affected by selection bias. CTE has been in the news a lot, and it is extremely likely that family members who had reason to suspect CTE in their deceased former football player were probably likelier to donate brains for the study. The authors write, “[E]stimates of prevalence cannot be concluded or implied from this sample,” yet headline writers did precisely that.

Second, the study had no control group. (Frankly, it is amazing that JAMA could publish a study like this with no control group.) The researchers did not examine the brains of everyday people (i.e., non-football players) to determine the baseline prevalence of CTE. It is quite possible that developing CTE-like brain lesions is a normal part of aging. Consider this: If they live long enough, most men will develop prostate cancer, even though they probably won’t die from it. Perhaps something similar is occurring with CTE.

Third, the authors shed no light whatsoever on some of the large outstanding questions in regard to CTE and football. For instance, why do former NFL players live longer than the general public? And why do the brains of soccer players, who endure nothing like the sort of head banging that American football players endure, also show signs of CTE?