JOHN YANG: This month in the space of a single week four young unarmed Americans were shot one of them fatally over simple everyday mistakes, pulling into the wrong driveway, ringing the wrong doorbell, getting into the wrong car in a parking lot.

These high profile cases have reignited the debate over self-defense and what justifies the use of deadly force.

According to the National Conference of State Legislatures, about 30 states have some form of so called Stand Your Ground laws.

They expand on a person's right to use force if they feel threatened.

Robert Spitzer is a Professor Emeritus of Political Science at the State University of New York at Cortland.

His latest book on U.S. gun policy is "The Gun Dilemma: How History is Against Expanded Gun Rights."

Mr. Spitzer, there are differences from jurisdiction to jurisdiction in these laws.

But what's the general principle?

What's the general idea of Stand Your Ground laws?

ROBERT SPITZER, State University of New York, Cortland: Well, there are two parts to the Stand Your Ground laws that are relevant.

The first is the general idea that if you're in a public place, and you are attacked, or somebody is about to do you grievous bodily harm, or even to kill you, instead of leaving this public place, if you could do so safely, which is a standard, some states have, Stand Your Ground laws so you can stay where you are, and resist force with force even to the extent of killing the other person.

So that's the Stand Your Ground idea, which itself I think, is somewhat controversial in legal circles, but there's an added dimension to this, which began when the state of Florida enacted a heightened Stand Your Ground law in 2005, that not only adopted the standard ground standard, but made it as they said in the law and absolute and irrevocable presumption that an individual who kills or harms another has acted in self-defense and cannot be prosecuted.

This began the spread of a heightened Stand Your Ground law in many states that change the legal presumption.

So, if you make a stand your ground claim, all you need to do is say I felt that I my life was in danger, or that I was going to be grievously harmed.

And that standard then places the burden on investigators, police prosecutors to try and make the case that No, you really didn't face the threat to claim.

And this shift in the legal standard has really opened the door wide for people to make Stand Your Ground type claims.

And in many instances, not all, obviously, to avoid any kind of prosecution.

And that widening of Stand Your Ground stand your ground on steroids, or some even call it a license to kill or shoot first laws.

That is what has really come to the country's attention in recent years.

JOHN YANG: Where does this idea come from?

Is it rooted anywhere in the sort of the history of gun laws in America?

ROBERT SPITZER: Well, Stand Your Ground laws take an older idea, something called the castle doctrine, an old principle that people may have heard of the castle doctrine goes back to the Middle Ages, a person's home is one's castle, that is to say you have a right to retreat into your home and feel safe there.

And if somebody intrudes within your home, you're entitled to defend yourself and not leave your home.

That idea goes back to the Middle Ages in Britain, it came to the United States.

But in the 19th century, another idea evolved.

And that was the standard ground idea.

That is to say, if you're in a public place, the principles of the castle doctrine in your home could apply to you in the public realm.

That is to say, instead of safe retreat, which is the legal standard, even today, in many states, you could meet force with force.

JOHN YANG: The idea behind these the sponsors of these bills and backers said it was intended to curb crime.

Is there any way of telling the result of these laws?

ROBERT SPITZER: There is zero evidence that Stand Your Ground laws of recent years have deterred crime, or improved public safety.

And a host of studies.

Numerous studies have demonstrated that what we see when these Stand Your Ground laws are enacted, is an increase in homicide and gun homicide.

For several studies have found an increase between eight and 11 percent in homicides in states after they adopt these laws, the state of Missouri which has come under focus, just in recent days, and not adopted to Stand Your Ground law in 2016, and also eliminated pistol permitting for pistol carrying, and indeed, Missouri witnessed an even larger increase in its homicide rate thereafter.

In addition, there's a race problem, which is that these laws tend to be administered in an inappropriate way between the races.

So if a white person kills another kills a black person, let's say it makes us stand your ground claim, that claim is much more likely to be accepted than the reverse.

Then there are studies that support that conclusion.

So it is deeply problematic at UK and indeed law enforcement and prosecutors have generally opposed the enactment of these laws.

JOHN YANG: You mentioned permitless concealed carry.

These two sort of trends.

We've now had the 26 state of Florida have a law that you don't need a permit for concealed carry, combined with the Stand Your Ground laws, what is the effect?

Or what do you think could be the effect?

ROBERT SPITZER: Well, it means more civilians owning guns and more civilians carrying guns with them around in society in now 26 states, and civilians lack the training, skill, judgment and the likelihood of mistakes or the likelihood of road rage or the likelihood of spontaneous anger resulting in the deployment and firing of a gun by a person carrying it are much greater and the murder statistics bear that out.

JOHN YANG: Robert Spitzer, thank you very much.

ROBERT SPITZER: You're most welcome.