In any event, mental health professionals are notoriously bad at predicting which of the world’s many misfits, cranks, and oddballs will become violent. “Over thirty years of commentary, judicial opinion, and scientific review argue that predictions of danger lack scientific rigor,” notes University of Georgia law professor Alexander Scherr in a 2003 Hastings Law Journal article. “The sharpest critique finds that mental health professionals perform no better than chance at predicting violence, and perhaps perform even worse.”

So even if the mental-health criteria for rejecting gun buyers (or for commitment) were expanded, there is little reason to think they could distinguish between future Lanzas and people who pose no threat. Survey data from the National Institute of Mental Health indicate that nearly half of all Americans qualify for a psychiatric diagnosis at some point in their lives. That’s a pretty wide dragnet.

Should half of us lose our Second Amendment rights, at least for the duration of whatever mental disorder (depression, anxiety, addiction, inattentiveness, etc.) afflicts us? Assuming a prescription for Prozac, Xanax, or Adderall is not enough to disqualify someone from owning a gun, what should the standard be?