Charles Ingram and Robert Webster were neighbors in Florida, but friends said the two older men had little love for each other and often quarreled. On a spring day in 2010, the two men, both gun enthusiasts who had state permits to carry concealed weapons, got into another argument across their lawns.

This time, police later said, both men pulled out their weapons. When Mr. Webster began approaching, Mr. Ingram raised his gun, as did Webster. Two shots rang out simultaneously, and both men fell. Webster died almost instantly, Ingram less than a month later.

That "Deadwood"-style neighborhood gunfight is one of 555 examples compiled by advocates of gun control detailing how the mere presence of legal guns can turn mundane moments into tragedies – sobering rebuttals against the estimated tens of thousands of times a year Americans brandish guns in self-defense to thwart crimes in progress.

In a country that witnesses bloody gun violence of all kinds on a daily basis, Ingram and Webster were part of a growing cohort, a sort of standing militia of what concealed-carry advocates say are between 8 million and 11 million citizens carrying concealed guns in public in the name of protecting themselves and those around them.

Less than two decades ago, fewer than a million Americans carried concealed weapons, and they were mostly ex-police, ex-military, or owners of cash businesses.

Now, as more states expand open and concealed-gun carry to include bars, churches, airports, and college campuses, such tragedies highlight the life-and-death stakes of living in a more heavily armed America.

Complicating this rise of the concealed gun in America, new research on the psychology of what is called "embodied cognition" suggests that simply the act of holding a gun shades one's perceptions, sometimes at odds with reality.

To opponents of concealed carry, such research suggests that a toxic mix of politics and paranoia, added to 30 ounces of chromed steel tucked legally under a belt at Wal-Mart, ultimately equals a scarier and more dangerous society.

The legal right to shoot

As of January, all 50 states, with various exemptions, allow people without serious criminal records or mental illness to obtain a permit to carry a concealed gun. That expansion of concealed carry coincides with the adoption of a new breed of self-defense laws that give armed citizens more – but not total – legal cover for shooting at fellow Americans.

In a recent paper titled "Second Amendment Penumbras," University of Tennessee law professor Glenn Reynolds points to major US Supreme Court decisions, including District of Columbia v. Heller, which in 2008 struck down the city's ban on handguns, as defining new parameters of self-defense and gun carry.

Until recently, Professor Reynolds writes, "gun ownership was treated as a suspect (or perhaps 'deviant' is a better word) act – one to be engaged in, if at all, at the actor's peril. But with gun ownership now recognized as an important constitutional right belonging to all Americans, that deviant characterization cannot be correct."

Illinois, the last holdout against concealed carry, was forced last year by the courts to allow it. A majority of states now have "shall issue" laws in which the state must award a permit if an applicant satisfies written requirements; other states have "may issue" laws that give police the authority to deny license applications.

The debate about whether the surge in public-carry laws heralds the dawn of a neo-Wild West era or simply restores the proper balance to gun rights has been punctuated by a string of incidents involving authorized gun carriers who killed fellow citizens over minor squabbles and preventable misunderstandings.

Florida, a pioneer in the liberalization of gun laws and a state where 1 in 19 people on the street is licensed to carry, has had several notable incidents involving concealed-carry permits in just the past three years: the fatal shooting in January at a Tampa movie theater, in which a well-regarded retired police captain shot a younger man, a father, in an argument involving texting and a thrown bag of popcorn; the killing of Trayvon Martin, in which a neighborhood watch captain shot the unarmed teenager after profiling him as a "punk" and scuffling with him; and an incident in which a white man with a concealed gun shot and killed a young black man in an argument over loud music (the shooter said he saw a gun; none was found).

Arguments for concealed carry

"I don't argue that there are no problems with [concealed-carry permit holders], but when you look at the data it's pretty hard to find any other group in the population that's as law-abiding as" permitted gun carriers, says John Lott Jr., an economist and gun-rights advocate and author of "More Guns, Less Crime."

"The type of person who's going to go through the process of getting a concealed-carry permit is not the kind of person you have to worry about," he says. "They're law-abiding citizens who have a lot to lose if they make a mistake."

Statistics support Mr. Lott's assertion. The number of incidents in which concealed-gun carriers kill innocent people is a fraction of 1 percent of all gun-related homicides. In North Carolina, one of only a handful of states that reveals the identities of permit holders, 200 of the 240,000 concealed carriers (.08 percent) committed felonies of all types, including eight shooting deaths, in the five-year period ending in 2011. This compares with about 2.5 percent of voting-age Americans who have a felony rap sheet, according to The Sentencing Project.

The view of gun carriers as law-abiding citizens seems to have traction and correlates with increasingly positive public attitudes toward concealed carry. In 1999, an NBC News/Wall Street Journal poll found that 73 percent of Americans disapproved of making it easier for people to legally carry concealed weapons. In a Reuters/Ipsos poll last spring, 75 percent favored concealed carry by eligible citizens.

Another possible influence on public attitudes is the notion that an armed society is largely a polite society. Statistics suggest violent crime in the United States has gone down as more citizens either carry guns openly or concealed.

Using data reported by police to the FBI, the National Crime Victimization Survey reports that Americans used guns in self-defense 338,700 times over five years ending in 2011.

Concealed carry may have a deterrent effect as well. A recent Quinnipiac University study suggests that states with stricter concealed-carry laws have higher murder rates than states that are less restrictive, though it allows there could be other explanations for this difference.

Amplifying that point, a 2004 report by the National Research Council of the National Academies warned "it is not possible to determine that there is a causal link between the passage of right-to-carry laws and crime rates."

Brian Anse Patrick, a University of Toledo professor of communications and longtime concealed-carry permit holder, offers his own anecdotal evidence that concealed carry deters crime. In an interview he said he has personally brandished (but never fired) his gun several times to stop a possible crime. One example involved him displaying his handgun to a strange man who ran up to his car in the middle of traffic and began reaching into the back seat. The man backed off when he saw the gun.

Arguments against

The "popcorn shooting" at the Tampa movie theater especially agitated pro-gun-control groups, who saw proof that even a gun carrier with decades of threat assessment experience could allow a situation to spiral out of control.

"Just because you're a law-abiding citizen today doesn't mean you're going to be one tomorrow," says Kristen Rand, legislative director at the Violence Policy Center in Washington, which keeps count of those killed by legal gun carriers in "non-self-defense" situations.

"In a lot of these cases," she says, "a shooter's life is ruined, an innocent person is dead, and there's a little girl with no father and a woman with no husband, and all because one guy believed the gun-lobby hype that 'I'm going to get this gun because someday I might need it.' "

The Violence Policy Center also marshals its own statistics on gun ownership and deaths, saying the total numbers of gun deaths, including suicides, are lower in states such as Massachusetts and Hawaii where there are fewer guns per capita than states with higher death rates, such as Louisiana and Wyoming.

For some gun-control advocates, the trend toward concealed carry also raises troubling undercurrents of race and class. They cite words often seen on pro-gun Internet forums – the "good guy" versus "the thug," a term that commentators from the blogosphere to the sidelines of the NFL (Seattle Seahawks cornerback Richard Sherman) are calling the "new N-word."

While many new concealed-carry products – clutch holster bags, slimming underwear with holsters – at January's huge SHOT gun show in Las Vegas were for women, statistics from Arizona and elsewhere suggest it is primarily white men over 30 who are arming up.

Gun-control advocates consequently see the gun-carry movement as populated at least in part by white men who feel politically unempowered and who may be inclined to indulge in displays of extra muscle and power over their fellow citizens.

"There is a certain psychology at work with some who carry openly or concealed," writes columnist Stephen Lemons, in the Phoenix New Times newspaper. "I have seen it in the nativist camp, where these grizzled old white extremists try to provoke their enemies with guns on their hips, itching to blast someone."

While that may be harsh, even some concealed-carry proponents see a strain of disturbing behavior among some carriers.

"Acting like a deadly threat is imminent, walking around stores jerking your head around ... 'on a swivel,' planning your tactical movement from the gas pump to the cash register IS paranoid behavior, unless you live in Fallujah," writes one permit holder on a concealed-carry Internet forum. "Acting like every situation involves a critical threat is goofy.... Don't confuse life with movies."

Perhaps contributing to such confusion, guns can change people's perceptions at even deeper levels, according to recent research at the University of Notre Dame psychology department. The researchers had to get special permission from campus police to use replicas of handguns in a study that found that people holding a gun are more likely than those holding a ball to perceive objects in other people's hands as guns.

"Carrying a firearm changes what you can do in the world ... and that could potentially change the way you perceive and interact with people in the world," says James Brockmole, a Notre Dame psychology professor whose research has established a link between gun handling and "embodied cognition," the theory that objects encountered by the body unconsciously influence behavior. "People pay attention to the world differently if they're armed."

Where we're going

However persuasive the recent instances of gun violence have been for gun-control proponents, they haven't changed dramatic trend lines in attitudes and gun ownership in America, which George Washington University law professor Bob Cottrol says has always been at least symbolically an armed society.

More than 300 million guns are distributed among about 40 percent of US households.

Since 9/11, the expiration of the federal assault weapons ban in 2003, and the social breakdown in New Orleans after Katrina, Americans have put aside post-Prohibition distrust of public gun carry and embraced the idea of it.

Surprisingly to some, 91 percent of 15,000 police officers polled recently by the PoliceOne organization also said they support citizens' ability to carry concealed weapons.

And while gun carry has always been popular in rural America, the current wave of concealed-carry seekers is largely urban professionals, says the University of Toledo's Professor Patrick.

Since the late 1990s no major federal anti-gun legislation has passed. Even the Sandy Hook massacre in Newtown, Conn., in 2012, in which 20 schoolchildren and six adult staff were killed, could not help President Obama and Democrats in Congress push through what they called "common-sense gun-safety reforms."

That failure last spring again drew attention to the National Rifle Association's ability to lobby lawmakers on a scale and intensity that pro-gun-control forces, even with the deep pockets of former New York Mayor Michael Bloomberg, could not match.

The NRA has long supported expanded concealed-carry laws, as well as anonymity for carriers. The nation's first recipient of a "shall issue" concealed-carry permit went to Marion Hammer, the NRA's longtime Florida lobbyist, after Florida passed its pioneering law in 1987.

While some states – notably Colorado, New York, and Connecticut – have tightened gun laws since Sandy Hook, most gun-related legislation in the states has decreased the barriers to carrying firearms in public.

That development has created some strange legislative bedfellows. Georgia Carry, a pro-gun group, lobbied last year to remove a rule banning anyone with a prior marijuana misdemeanor conviction from ever getting a permit to carry a concealed weapon in Georgia.

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Lots of otherwise law-abiding middle-aged men were being denied gun-carry rights because of a commonplace youthful indiscretion, says Jerry Henry, the group's director.

"I personally don't understand why everybody's so scared of [guns]," he says. "They don't do anything by themselves."