There has been a lot of second-guessing about Andreas Lubitz, the Germanwings co-pilot who deliberately crashed his plane into the French Alps, killing himself and 149 others. If only Lufthansa had regular mental evaluations of pilots, if only people at the airline knew what obvious signs to look for, this tragedy could have been avoided.

But psychiatrists know that isn’t true. It isn’t just fellow workers who fail to pick up the supposed subtle hints that indicate that someone might be a danger to themselves or others. “We have no indication what could have led the co-pilot to commit this terrible act,” said Carsten Spohr, Lufthansa’s chief executive.

Psychiatrists themselves have a very poor record. Identifying mental illness is a long way from thinking that the person poses a danger. Look at the inability of psychiatrists to identify mass shooters. It’s very common for mass killers to be seeing psychiatrists before their attacks, including Elliot Rodger (Santa Barbara), Ivan Lopez (the most recent Fort Hood shooter), Adam Lanza (Sandy Hook elementary school), James Holmes (Aurora, Colo., movie theater), and Seung-Hui Cho (Virginia Tech).

It can be very difficult for mental-health professionals to accept that their patients may pose a serious violent threat.

Rodger had already been receiving top-quality mental-health counseling for years. Indeed, one of his psychiatrists, Dr. Charles Sophy, is nationally known and medical director for the LA County Department of Children and Family Services.

It can be very difficult for mental-health professionals to accept that their patients may pose a serious violent threat.

The Army psychiatrist who last saw Lopez found no “sign of likely violence, either to himself or to others.” While Holmes’ psychiatrist warned University of Colorado officials about his patient’s violent fantasies, she “rejected the idea” that the threat was sufficiently serious for him to be taken into custody.

Seung-Hui Cho was deemed “an imminent danger to himself as a result of mental illness.” Nonetheless, he was determined not to be “an imminent danger to others as a result of mental illness.” The judge stated that it was not necessary to have him involuntarily committed.

Again, these prominent mass killers certainly didn’t lack mental health care. The problem was that even good psychiatrists failed to identify real threats.

Psychiatrists have strong incentives to get the diagnosis right. Besides their own professional pride and desire to help, they have legal obligations to inform authorities of threats. Holmes’ psychiatrist was sued by the families of victims.

It can be very difficult for mental-health professionals to accept that their patients may pose a serious violent threat. Indeed, they tend to deny it to themselves.

The problem is severe enough that there is a whole academic literature devoted to it. It has been suggested that psychiatrists become desensitized to danger or try to prove their fearlessness. It’s possible that added training may help improve diagnoses of unusual cases.

However, it’s simply hard to predict these extremely rare outcomes.

Monday morning quarterbacking is always easy. What seem like obvious telltale signs in retrospect are often not so obvious before the attack, even to the experts.

There is also the risk of placing too much stigma on mental illness. Extremely few mentally ill people go on to become mass killers. Even among schizophrenics, the rate is much lower than one person out of every 100,000.

There are no cheap or easy answers. If someone poses a true danger to others, why not lock them up? Or provide outpatient caregivers to monitor them?

No one wants a dangerous person to have a weapon. But our mental-health system can’t be the last line of defense. There are just too many mistakes.

John R. Lott, Jr. is a columnist for FoxNews.com. He is an economist and was formerly chief economist at the United States Sentencing Commission. Lott is also a leading expert on guns and op-eds on that issue are done in conjunction with the Crime Prevention Research Center. He is the author of nine books including "More Guns, Less Crime." His latest book is "The War on Guns: Arming Yourself Against Gun Control Lies (August 1, 2016). Follow him on Twitter@johnrlottjr.