The hard facts show that the murder rate was going down for decades under the old traditional practices so disdained by the left intelligentsia, before the bright new ideas of the left went into effect in the 1960s - after which crime and violence skyrocketed.
What happened when old-fashioned ideas about sex were replaced in the 1960s by the bright new ideas of the left that were introduced into the schools as "sex education" that was supposed to reduce teenage pregnancy and sexually transmitted diseases?
Both teenage pregnancy and sexually transmitted diseases had been going down for years.
But that trend suddenly reversed in the 1960s and hit new highs.
One of the oldest and most dogmatic of the crusades of the left has been disarmament, both of individuals and of nations. Again, the focus of the left has been on the externals - the weapons in this case.
If weapons were the problem, then gun control laws at home and international disarmament agreements abroad might be the answer.
But if evil people who care no more for laws or treaties than they do for other people's lives are the problem, then disarmament means making decent, law-abiding people more vulnerable to evil people.
Since belief in disarmament has been a major feature of the left since the 18th century, in countries around the world, you might think that by now there would be lots of evidence to substantiate their beliefs.
But evidence on whether gun control laws actually reduce crime rates in general, or murder rates in particular, is seldom mentioned by gun control advocates. It is just assumed in passing that of course tighter gun control laws will reduce murders.
But the hard facts do not back up that assumption.
That is why it is the critics of gun control who rely heavily on empirical evidence, as in books like "More Guns, Less Crime" by John Lott and "Guns and Violence" by Joyce Lee Malcolm.
National disarmament has an even worse record. Both Britain and America neglected their military forces between the two World Wars, while Germany and Japan armed to the teeth.
Many British and American soldiers paid with their lives for their countries' initially inadequate military equipment in World War II.
But what are mere facts compared to the heady vision of the left?
Sowell is a senior fellow at the Hoover Institution at Stanford University in California. His website is www.tsowell.com.