As the 50th anniversary of the death of President John F. Kennedy approaches, we are reminded of the Bay of Pigs invasion, the Cuban Missile Crisis, the Vietnam War, his Pulitzer Prize-winning book “Profiles in Courage” and, of course, the circumstances of his death.

It is fascinating that, according to a recent Associated Press poll, more than 50 percent of Americans do not believe that Lee Harvey Oswald alone killed Kennedy. Some subscribe to elaborate schemes often called conspiracy theories, not an ideal term but one whose meaning is generally understood.

A conspiracy theory is an extreme example of something called confirmation bias, a normal process whereby we seek information that confirms our beliefs, and either dismiss or conform contradictory evidence in a way that keeps our beliefs intact. Confirmation bias is a way we justify our political, social, religious and other cultural views even in the face of conflicting views. Confirmation bias is anathema to science and so, as a scientist, it interests me.

The stereotyped scientist is objective, not susceptible to the confirmation bias that buoys conspiracy theories. However, science is not a thing but rather it is a process carried out by people, and so it is no less flawed than humanity itself.

Fortunately, a theory derived from the convergence of evidence holds up to scrutiny better than does the bias of a few. The truth, or something close to it, usually gets fleshed out eventually not because investigators are immune to subjectivity, but because a process is employed that can reach a consensus despite our biases. This process tells us that, to the best we know, the Holocaust did happen, cold fusion does not and the Earth is a sphere.

Conspiracy theories such as those surrounding the Kennedy assassination do not get resolved because there is no counterweight to confirmation bias. Conclusions take precedence over evidence, and therefore no denial of verifiable evidence is too absurd.

This bias is laid bare in the real-life trial of Clay Shaw, a man accused of being a conspirator in Kennedy’s assassination, which served as the basis of Oliver Stone’s Hollywood movie “JFK.” Shaw was prosecuted by New Orleans District Attorney Jim Garrison, who blamed the murder on a powerful cabal. Those who worked with Garrison described him as obsessed, and said he had no case. The jury agreed.

To Garrison, and apparently to Stone, the trial was sabotaged by the cabal simply because the conspiracy theory must be true. No member of the alleged cabal or anyone under its control has been identified in 50 years. No names, no confessions, no leaks, no memos, no one interested in selling the story for fame and fortune. Acceptance of Stone’s narrative persists nevertheless because lack of evidence can be construed as suppression of evidence, which can only be carried out by a cabal with nearly superhuman powers. This self-affirming, circular reasoning shields the theory from legitimate criticism. It is a caricature of confirmation bias.

Did Oswald act alone? Carl Sagan said that the absence of evidence is not evidence of absence, and so the failure to find an accomplice does not mean there was none. I prefer to ask more specifically whether Oswald fired both shots that killed Kennedy, to which I think evidence converges on the affirmative. Is this conclusion influenced by confirmation bias? Maybe, but let us take a look.

Many people may not be aware that the Warren Commission report is only one of numerous investigations, and that the testimony and expertise came mostly from the private sector. These are accessible online.

We now know that the so-called magic bullet that pierced both Kennedy and Gov. John Connally did not dip, dive and turn as originally described. Once the juxtaposition of the president with the governor in the limousine was recognized, the event is best explained as a single projectile with a straight trajectory that came from behind them. The bullet was not so magical after all.

There is consensus among experts that the spray of blood emitted from the front right side of Kennedy’s head, as documented in the Zapruder film, is characteristic of a bullet exit wound, and thus the second projectile also came from behind. Contrary to intuition, the backward snap of Kennedy’s head in response to the second bullet is not explained by a shot fired from the front. Physics tells us that the transfer of energy from the bullet to Kennedy’s head is not sufficient to cause the observed backward motion. It most likely resulted from a neuromuscular reaction.

A rifle belonging to Oswald and marked with his palm print was fired three times from the sixth floor of a building – located behind the presidential limousine – where Oswald worked and was present. This and other evidence converges on a theory that does not require a second shooter, and very strongly implicates Oswald in the crime. No unifying theory emerges from the alternate, often contradictory proposals, and denial of the best theory almost always invokes an unsubstantiated cabal.

Many doubters, probably most, may simply be unconvinced for any variety of reasons. This is skepticism, an essential component of science that should not be confused with denial. Skepticism renders a good theory vulnerable because it can be supplanted by new evidence. This is a strength, not a weakness.

A theory based primarily on confirmation bias is invulnerable, and that is its problem. Skeptics should be especially heartened to know that the scientific method is not the exclusive property of scientists. It is not a thing, it is a process held in the public domain. Follow the path of a skeptic and see what you discover.

Mark R. O’Brian, Ph.D., is a professor of biochemistry at the University at Buffalo.