Who is really at risk of CTE from repeated blows to the head?
Research shows that the risk of CTE is greatest in people who have played contact sports intensively for many years, and that the risk increases with the number of years of exposure. How large that risk is exactly for the average contact sport athlete is still difficult to say, because studies have been conducted mainly in people who already had symptoms.
Elite athletes in contact sports such as American football, rugby and ice hockey face the greatest risk of CTE. Brain damage is found more often and more severely in this group: post-mortem examinations of former elite athletes show CTE features in 73% of cases, compared with 44% in amateur athletes. The more years someone has played the sport, the greater the risk and the more severe the damage. This kind of pattern is visible across several sports and makes it plausible that repeated blows are the cause, even though this cannot strictly be proven by controlled studies.
Those percentages sound alarming, but there is a distortion built in: brains are more often donated for examination when the deceased already had symptoms. The true chance of CTE for an average contact sport athlete is therefore unknown and probably lower. One summary of available research even found no clear difference between contact and non-contact sport athletes. The literature contradicts itself on this point.
The risk is not limited to top athletes. In a study among ordinary donors (not selected on the basis of sport), 15% had CTE features. Almost all of these people had extensive exposure to repeated blows, through sport, military service or domestic violence. Contact sports are the best-known source, but not the only one. Repeated mild brain injury remains the only consistently demonstrated risk factor to date; genetic predisposition (such as the ApoE gene) did not appear to play a decisive role in the available analyses.
In nearly four out of ten cases where CTE was found, the person had died by suicide. That figure is difficult to interpret because selection also plays a role here, but it underlines how serious the consequences of CTE can be. Further research also points to repeated blows raising the risk of other brain diseases, including Alzheimer's disease and ALS, although that evidence is still preliminary.
This answer draws on two summaries of previously published studies, an early in-depth analysis of 153 confirmed cases, and several more recent individual studies, including one based on a broad community sample. The evidence is moderately strong for elite athletes with long careers, and thinner for non-athletes. A major problem across all studies is that brains are more often donated for examination when symptoms were already present, which means the figures on how common CTE is are likely too high. Two large combined analyses of many studies also contradict each other on whether contact sports carry a meaningfully greater risk. The researchers themselves emphasise that the true rate of CTE in the general population remains unknown.