The surveillance footage shows New England Patriot Aaron Hernandez holding a gun minutes after his friend’s murder. But some suspect another culprit. Philadelphia magazine’s Joel Mathis asks, “Did football drive Aaron Hernandez to murder?”

Specifically, Mathis and others question whether chronic traumatic encephalopathy (CTE), a degenerative brain condition found in several dozen deceased players including Junior Seau, caused Hernandez’s allegedly violent behavior.

The not-guilty-by-reason-of-football defense has gone from pundit parlor-game to courtroom tactic. From speculation that CTE rather than O.J. killed Ron and Nicole, to the lawyer of former Detroit Lions receiver Titus Young explaining to a judge that his client was “still suffering from concussions” after twice missing court following three arrests in Southern California over a single week, CTE threatens to supplant celebrity as an athlete’s get-out-of-jail-free card.

Earlier this year, the defense for a former high school football player charged with murdering his 18-year-old girlfriend held that CTE, not their client, bore responsibility. A jury found the Massachusetts teen guilty, but how long before the hysteria that keeps kids away from football fields also keeps murderers out of prison?

Juries will ultimately weigh the evidence against Aaron Hernandez and Titus Young. Science has judged the evidence linking CTE with football unconvincing.

Five respected scientists from three countries assert in an April 2013 article in the British Journal of Sports Medicine that “the speculation that repeated concussion or subconcussive impacts cause CTE remains unproven.” Nine scholars based in Canada similarly conclude in a new study in Frontiers in Human Neuroscience that “it is difficult to establish a definitive link between a history of multiple concussions and CTE.”

Since Dr. Bennet Omalu first identified CTE in a football player nearly a decade ago, fellow scientists have conducted dozens of autopsies on the brains of deceased athletes. But these individual case studies, however troubling, don’t lend themselves to sweeping assertions. Logic indicts such conclusions as hasty generalizations.

Much of the public, prodded on by public intellectuals, seems to believe that a mountain of evidence links the degenerative brain disease to football.

“When you look at two teams playing football on a field,” Malcolm Gladwell said of CTE to a University of Pennsylvania audience earlier this year, “chances are that someone on that field is going to die a horrible death well before their time because of playing football.”

In fact, scientists have yet to conduct a randomized study on football players and CTE. The disease’s prevalence among the public, let alone among professional football players, remains unknown. And the selection bias admittedly employed by researchers — seeking brains from those suspected of suffering brain damage — further prejudices findings.

CTE certainly remains more of a mystery than the Aaron Hernandez case. What we know about CTE is that we don’t know much.

We don’t know why athletes who endure tremendous brain trauma never developed CTE and others experiencing far less trauma suffered from it. We don’t know if concussions cause CTE. We don’t know how to diagnose CTE in the living. We don’t know why a genetic marker found in a quarter of the population appears in three-fifths of CTE victims. We don’t know the condition’s prevalence among people who never step onto a football field, inside a ring, or upon the ice. To listen to the attorneys suing the NFL and defending its most troubled athletes, CTE is settled science. But the doctors know better than the lawyers.

Even a smoking gun on a CCTV camera may not be enough to convict Aaron Hernandez of murder in a court of law. In the court of public opinion, advocates regularly rely on flimsier evidence to convict football of killing its players. Surely a great sport, like its least savory players, deserves more than the presumption of guilt.

Daniel J. Flynn is the author of “The War on Football: Saving America’s Game.”