After every mass murder, the question everyone asks is why it happened. How could anyone possibly be so violent, or so evil, or so out of control, or so crazy as to engage in the wholesale and indiscriminate killing of a bunch of people who are usually complete strangers?

In some cases, there are longstanding preexisting warning signs: a history of mental illness, substance use, isolation and/or estrangement; the repeated experience of being abused or bullied; and/or the influence of a political or religious or racist cult.

Sometimes there is an event that might be seen as the immediate trigger: a fight, a humiliation or failure, a rejection, some burning of all bridges, a cutting off of hope and connection.

But often enough, the mass murderer is (like the recent knife-wielding Pennsylvania schoolboy) someone with no preexisting risk factors, has no special special current stressors, and is part of a loving family.

I have spent a long professional life as a psychiatrist judging peoples’ motivations and attempting to assess the risk that the person in front of me might harm himself or others. I have also studied the available literature and made a small contribution to it.

My conclusion is that we will never really understand the motivations of mass murderers or be able to pick them out of the vast crowd of people with the same motivations and experiences who don’t kill.

Even after everything is analyzed to death, the motivation for mass murder always remains a mystery. For every mass murderer there are tens of thousands of similar people who never go berserk. We can’t ever expect to predict who will do it and when.

The excessive focus on ferreting out the psychological motivations arises from understandable human curiosity, but it also serves a dangerous political purpose. So long as we are distracted by the why, we do not attend to the much more practically important and politically charged question of how.

The knife-wielding attack in Pennsylvania had a relatively benign casualty rate, demonstrating the obvious fact that guns are much more efficient than knives as instruments of death. Guns make it possible to kill many more people in a shorter period of time and at a greater distance.

That’s why 700 years ago, guns began replacing blades, arrows, and spears as instruments of warfare, and that’s why other developed countries see it as a sacred public-safety responsibility to regulate gun possession among civilians. In contrast, we in the United States have buried our heads in the sand and are ignoring the enormous toll of gun violence. In 2010 firearms were the means of death in 20,000 suicides and 11,000 homicides in the U.S.

Some of these deaths might have occurred anyway through less-lethal means, but there is no denying that free access and wide availability has made gun death a major threat to our public health.

My medical instinct is to favor the licensing of guns in the same way we license that other most dangerous cause of instrument-related death: cars. We should do our best to ensure that the people who have guns are responsible in their use. Those with histories of mental illness, substance abuse, or violence should not be permitted to pack heat, and I also see no reason to allow the wide dispersion of military-grade weapons and ammunition that can inflict so much damage in so short a time.

I fully understand the political obstacles that heretofore have defeated even the mildest of gun-control legislation. These will weaken inexorably (but very gradually) as the death toll rises over the coming years.

But there is a better way that could save many lives. A solid majority of Americans favor sensible gun control that respects the constitutional rights of responsible gun owners but balances this with appropriate concerns about public safety and public health. Surely a fair and reasonable compromise could have been achieved had gun control not become such a fiercely contested ideological hot potato.

We need to work past the passions and misinformation to find common-sense answers before tens of thousands more innocents die needless deaths. We spend far too much effort trying to understand the impossible-to-fathom motivation of mass murderers, and far too little effort finding common-sense gun-control compromises that could greatly reduce the lethality of their means.

Allen Frances is a psychiatrist and professor emeritus, Duke University.  He blogs at the Huffington Post.