If drugs didn't drive Rudy Eugene to gnaw off Ronald Poppo's face, asks Yale neurologist Steven Novella, what did? Initially, he says, "I felt the most plausible hypothesis for this bizarre and violent behavior was drugs, especially given that Eugene had no history of violence." (That last part is not accurate; see below.) Although toxicological tests found no evidence that Eugene had consumed any drug other than marijuana, Novella raises the possibility that he "did use a street drug that triggered an acute psychotic episode," noting that "the toxicology can only test for known substances." He admits "it's hard to prove that Eugene did not have an unknown drug in his system." Novella even suggests that Eugene may have had "an atypical response to marijuana (which can rarely occur)." Well, yes, that's what atypical means, but does it make sense to blame Eugene's ghoulish assault on any drug, whether marijuana or "bath salts," when people who use that drug almost never behave this way? After all, Eugene's crime was an international sensation precisely because it was so unusual. It cannot possibly be explained by citing factors that Eugene had in common with millions of other people who never behave violently, let alone eat anyone's face.

There is a similar problem with Novella's favored theory, which is that Eugene had "a psychotic disorder":

Those with schizophrenia, for example, can have episodes of rapid and severe worsening where they have a "psychotic break" and can engage in extreme bizarre and violent behavior. Such psychotic episodes can be spontaneous or triggered, by going off of anti-psychotic medication, using recreational drugs, or a psychological stressor. 

As those cans suggest, Eugene's behavior was atypical even among people diagnosed with schizophrenia, most of whom are not violent, let alone cannibalistically so. In any case, Novella concedes, interviews with people who knew Eugene reveal "no discussion of any significant psychiatric history or diagnosis, and no prior episodes of violence, hallucinations, delusions, or psychotic breaks." Furthermore, Eugene was 31, "which is a little old for a first episode."

Why not examine Eugene's brain for evidence of schizophrenia? Because there is no such definitive biological test—no way to conclusively confirm or disconfirm Novella's "psychotic disorder" hypothesis, unless psychotic is just a name for the mental state of a person who strips naked, swings from a light post, scatters Bible pages, and eats a homeless man's face. Loony or crazy would do just as well, and neither counts as a scientific explanation.

"The cause of the Causeway Killer's...behavior may never be known," Novella concludes, "but I think I covered all the plausible possibilities above. Of course there are an unlimited number of implausible possibilities—alien mind control, an unknown zombie virus, demonic possession, evil fairies, vampirism, etc." (He does not mention the possibility of a voodoo curse.) Novella, of course, considers these explanations ridiculously unscientific because they are unfalsifiable. But so are Novella's proposed explanations: an unknown drug that by definition cannot be detected or an unverifiable mental disorder that cannot be traced to a brain abnormality observable in an autopsy. 

It is not quite true, by the way, that Eugene had "no history of violence." He was arrested for battery at age 16, and in 2004 police were called to his mother's house because he was wrecking the place, shoving her, and threatening to kill her. They used a Taser to subdue him after he refused to leave. This altercation surely is easier to comprehend than Eugene's grisly, unprovoked attack on Poppo, but a history of violence is, not surprisingly, a good predictor of future violence. That does not mean it is a good explanation, since we don't exactly know what makes some people more violent than others to begin with. We surely don't have a satisfying explanation for the sort of violence that generates international news coverage precisely because it is so bizarre. 

[Thanks to Mark Sletten for the tip.]