While driving south on CT 15 last Thursday, not very far from Newtown, I listened to the news broadcast about yet another tragic mass shooting incident. The question asked by newscasters and witnesses to these events is always, "Why?"

The answer to this question in many, if not most, cases such as this contains certain common themes. If the behavior evidenced seems irrational and against all common sense, it's because it is. There is no way to explain this act based on what we accept as normal human behavior. Do not spend time trying to understand it.

The plain fact is that this kind of homicidal / suicidal thought and behavior is a manifestation of psychosis, defined as disordered (irrational) thinking. According to an article from the National Library of Medicine, the lifetime incidence of psychosis in the general population is about 3.5 percent. In a country like the U.S., with a population of 300 million, this means that more than 10 million people are, or will be, psychotic at some point during their lifetime.

Are these people dangerous? Do we need to arm ourselves for self-defense or wear bulletproof clothing? The truth is that the overwhelming majority of them are only dangerous to themselves, and while they constitute a public health problem, are of little danger to public safety. Some of them are the disheveled "homeless" people you see mumbling to themselves in public, sitting over steam grates, living under bridges, living in group homes, or (the luckier ones) cared for quietly by family members. Some are even able to live independently.

A small percentage of the psychotic, mentally ill commit violent acts. The vast majority of these are directed against themselves (suicide).

The homicide rate during the first episode of psychosis has been documented at about 1.6/ 1000 (first episodes of psychosis). This is 0.0016 of the population of psychotic patients. A seemingly small number until you multiply it by 10 million psychotic people: 0.0016 X 10,000,000 = 16,000 homicides.

This is a "lifetime" rather than an annual incidence. Nonetheless it adds up over time and can sometimes produce sensational headlines, as in several recent cases.

The second tragic ingredient in this picture is firearms. Guns are an exceedingly effective means of both suicide and homicide. Twentieth century "improvements" of automatic loading and firing have rendered them instruments of mass murder. In the hands of a trained professional or hunter they have legitimate uses for public safety, self-defense, or even sport. In the hands of the untrained they are a danger to the user and others. In the hands of a criminal or a person with mental illness they are a potential tragedy.

As an emergency physician, I have seen my share of gunshot cases, both accidental and intentional. The injuries can be devastating, and often lethal.

What lessons can / should we learn from yet another horrific public tragedy?

The first is that mental illness is a common human disease, with significant morbidity and mortality. It must be de-stigmatized, better understood through research, screened for, and treated effectively. Fully half of psychosis-related homicides occur during the first (pre-diagnosis) psychotic episode.

The second is that firearms are too available in our society. No one outside the military and police SWAT teams need fully automatic weapons. Any citizen in possession of a firearm must be responsible for its safe use, storage, and protection from theft or misuse.