The recent measles outbreaks have sparked a national debate over vaccinations and the concept of “herd immunity.” The basic idea behind herd immunity is that “the vaccination of a significant portion of a population provides a measure of protection for individuals who have not developed immunity.”

The concept of “herd immunity” applies to gun ownership and crime rates just as it does to vaccinations and infectious disease.

Crime rates in Chicago dropped dramatically in 2014 after the state of Illinois allowed legal concealed carry. From the Washington Times:

[T]he number of robberies that have led to arrests in Chicago has declined 20 percent from last year, according to police department statistics. Reports of burglary and motor vehicle theft are down 20 percent and 26 percent, respectively. In the first quarter, the city’s homicide rate was at a 56-year low.

As Richard Pearson of the Illinois State Rifle Association observed:

It isn’t any coincidence crime rates started to go down when concealed carry was permitted. Just the idea that the criminals don’t know who’s armed and who isn’t has a deterrence effect. … The police department hasn’t changed a single tactic — they haven’t announced a shift in policy or of course — and yet you have these incredible numbers.

According to AWR Hawkins, national FBI statistics also showed a significant decrease in crime in the first half 2014 even though gun sales soared in 2013:

The number of murders declined 6.0 percent, the number of rapes (revised definition) declined 10.1 percent, aggravated assaults decreased 1.6 percent, and robbery offenses decreased 10.3 percent.

This is a continuation of the longer-running trend described by Dylan Polk in Guns & Ammo: “Crime rates have dropped as gun ownership has risen, despite a population growth” over the 1993-2013 period.

As always, it can be difficult to conclusively prove a direct causal link between gun ownership and falling crime rates. But such statistics certainly undercut any claims that more guns will lead to more crime.

The recent experience of college students Nour Skargee and Sable Nehme support the thesis that guns can prevent violent crime. When home invaders burst into their apartment, Sable “ran to her bedroom, retrieved her gun and pointed it at the culprits.”

Her quick thinking and bravery paid off. The assailants fled immediately. As Skargee noted:

If we didn’t have a gun, it would have been game over. They were bigger than me, bigger than both of us … We had no way of stopping them. The gun was the one thing that made them run.

(My friend Ryan Moore tells a similar story in his interview: “Carrying a Gun Saved My Life.”)

Richard Pearson estimated that 100,000 Illinois residents will have applied for concealed weapons permits in 2014 (out of a total population of 12 million). This works out to approximately 0.8% of the population.

In other words, “the 1%” of concealed weapons permit holders in Illinois have created a measurable benefit for “the 99%.” Not a bad payoff!

The Crime Prevention Research Center claims similar results nationally:

In 10 states, more than 8 percent of adults hold concealed carry permits, and all are among the states with the lowest crime rates. Lott claims his group’s analysis shows that each one percentage point increase in the adult population holding permits brings a 1.4 percent drop in the murder rate.

The medical theory of “herd immunity” posits that enough vaccinated individuals in a population can reduce the risk of contracting a disease — even for those who aren’t vaccinated. From the experience in Illinois and around the country, a relatively small number of armed people can similarly reduce the risk of crime — even for those who aren’t armed.

So if you don’t own a gun but you are enjoying safer nights out on the town or sleeping more easily in your bed at night, give a little thanks to your neighbors who are gun owners. You’re the beneficiary of gun “herd immunity.”

Related at PJ Lifestyle: “Confessions of a Conservative Vaccine Truther“