During the closing arguments of the George Zimmerman trial, defense attorney Mark O'Mara asked the courtroom to be quiet for four long minutes. When he at last broke the silence, he said those four minutes were the amount of time that Trayvon Martin had had to go home.

Mr. O'Mara left no doubts: It was Trayvon's decision not to go home, but instead to "plan" an attack on Mr. Zimmerman, punching and beating him, that caused Zimmerman to fatally shoot him. Because he did not go home, O'Mara said, "Trayvon Martin caused his own death."

It is an argument, it seems, that was successful; the jury on July 13 found Zimmerman not guilty of murder or manslaughter. But it is also an argument that raises deep questions about how notions of self-defense have evolved in the era of "stand your ground" laws, and whether those changes are allowing racial fears to influence juries.

The implication in O'Mara's argument was that Zimmerman, who is white and Hispanic, who was armed, and who ignored a 911 dispatcher's instructions not to follow Trayvon, had more of a right to stand his ground than did 17-year-old Trayvon, who was black. And the verdict suggests the jurors agreed.

Data from other states with stand-your-ground laws indicate that the Zimmerman jury was not alone in being sympathetic to such a claim. Whites are significantly more successful claiming self-defense when their attacker is black than blacks are when fighting back against an attacker who is white, according to one study.

To some, such findings are a consequence of the pandemic of violence plaguing elements of the black community. But to others, they suggest that stand-your-ground laws have allowed perceptions of the black community – sometimes accurate, sometimes not – to become a legal justification for using deadly force.

Stand-your-ground laws have begun to change the calculus of self-defense in the United States. The idea behind them is to "expand the legal justification for the use of lethal force in self-defense, thereby lowering the expected cost of using lethal force and increasing the expected cost of committing violent crime," say researchers Cheng Cheng and Mark Hoekstra in a Texas A&M study.

Statistics included in the study bore that out, showing that justifiable homicides rose by 8 percent in stand-your-ground states, amounting to some 600 additional killings.

The laws have spread quickly. Since Florida passed the first stand-your-ground law in 2005, at least 30 other states have followed suit, either though legislative action or court decisions.

This comes at a time when concealed-carry gun laws are being expanded, meaning the success or failure of stand-your-ground laws will depend on "whether [people believe] guns produce a net social benefit or not," says Brannon Denning, a law professor at Samford University in Birmingham, Ala., and author of "Gun Control and Gun Rights: A Reader and Guide."

Critics say the laws upset a basic social order by, in essence, deputizing citizens. Not only does that raise the risk of minor disputes and misunderstandings becoming deadly incidents, but it also provides some legal cover for Americans to take deadly action based on their own subjective, and possibly racially tinged, views.

The Zimmerman verdict fit into a long narrative of juries refusing to convict white vigilantes on serious charges – from Bernhard Goetz in 1987 to the police in the first Rodney King trial in 1992 – for violence against black men. But a study by John Roman of the Urban Institute suggests that stand-your-ground laws could be amplifying the trend.

In states with stand-your-ground laws, the shooting of a black person by a white person is found justifiable 17 percent of the time, while the shooting of a white person by a black person is deemed justifiable just over 1 percent of the time, according to the study. In states without stand-your-ground laws, white-on-black shootings are found justified just over 9 percent of the time.

Such findings "show that it's just harder for black defendants to assert stand-your-ground defense if the victim is white, and easier for whites to raise a stand-your-ground defense if the victims are black," says Darren Hutchinson, a law professor and civil rights law expert at the University of Florida in Gainesville. "The bottom line is that it's really easy for juries to accept that whites had to defend themselves against persons of color."

The potential reasons behind this are multilayered.

On one hand, young black men are disproportionately involved in violent crime. While blacks represent 12 percent of the US population, they make up 55 percent of its homicide victims, the vast majority of those perpetrated by other blacks.

Gun control has yet to have a clear effect on the situation. Violent turf wars in Chicago have sent the murder rate soaring even though the city has some of the strictest gun controls in the entire country.

An investigative report by the Tampa Bay Times last year added more nuance to the issue of stand your ground. It analyzed 200 stand-your-ground cases in Florida and found that defendants who killed a black person were found not guilty 73 percent of the time, while those who killed a white person were found not guilty 59 percent of the time.

The paper noted that the discrepancy was due in part to the fact that black shooting victims were more likely to be armed and in the process of committing a crime when shot. In the 11 cases that involved whites killing blacks or blacks killing whites, no discrepancy in conviction rates was apparent – four of five blacks who shot and killed a white person escaped punishment while five of six whites who killed a black person escaped punishment.

"What's going on [with protests against stand your ground] is just politics," says Clayton Cramer, a gun rights historian in Horseshoe Bend, Idaho. "The real tragedy in America is not racism, but something that's gone terribly wrong within the black community."

Yet it is that perception, more than the reality, that makes stand-your-ground laws vulnerable to racial bias, say others. The fact that jurors deemed Zimmerman justified in standing his ground – but not Trayvon – points to subtle prejudices, they suggest.

"I don't think they walked past that issue. I think they walked right up to it, and said, 'Yes, we call this reasonable,' " says Jody Armour, a law professor at the University of Southern California and author of "Negrophobia and Reasonable Racism: The Hidden Costs of Being Black in America."

'We've got to understand what 'reasonable' means in the context of a great fear of crime, of great fear of violence, and of great fear of black crime and violence," he says. "And you have it again and again: juries saying it's reasonable to take drastic measures to avert drastic harm from black males."

"Encrypted attitudes" about race have a significant effect on the justice system, and stand-your-ground laws give them greater latitude, he adds. "There's a lot of unconscious biases when it comes to race," argues Mr. Armour. "And if you know that, why would you want to create laws that are going to increase the opportunities for unconscious biases to burden and harm a minority, like blacks?"

Such biases can be what Armour calls "statistically justifiable," given higher rates of violent crime and murder in the black community. The problem is that what sociologist Robert Cottrol calls "microcultures of violence" in the poor, black community falsely paint a broader swath of innocent young blacks as criminals.

These impressions have shaped many African-Americans' views about law enforcement, and so colored their views of how stand your ground was employed in the Zimmerman case.

"When you think about why, in the African-American community at least, there's a lot of pain around what happened here, I think it's important to recognize that the African-American community is looking at this issue through a set of experiences and a history that, that doesn't go away," President Obama said in an unscripted post-verdict statement.

While only 30 percent of white respondents to a Pew poll said they were dissatisfied with the verdict, 86 percent of blacks said they had problems with the jury's decision. In the wake of the verdict, both Mr. Obama and Attorney General Eric Holder called for states to roll back stand-your-ground laws, with Obama saying they "encourage ... confrontations and tragedies."

It's been widely pointed out that the Zimmerman trial was not a stand-your-ground case. But for all intents and purposes, it was. While it's true that Zimmerman chose not to request an immunity hearing under the stand-your-ground law, he still has that option if there is a civil lawsuit. If he had requested that hearing, the law would have required he take the stand and explain his actions, which the defense in this case did not want to do.

Moreover, in her instructions to the jury, Judge Debra Nelson told jurors that Zimmerman, as long as he wasn't himself committing a crime, "had the right to stand his ground and meet force with force ... if he reasonably believed that it was necessary to do so to prevent death or great bodily harm to himself...."

Two of the six women who were jurors in the trial have spoken out about the verdict, with one, "Maddy," suggesting that Zimmerman "got away with murder" because of the way the law is written in Florida.

The legal rationale for stand-your-ground laws has a lot to do with uniquely American historical dynamics, says Mr. Cottrol, a liberal African-American sociologist who has argued that "the ultimate civil right is the right to defend one's own life."

"One of the differences between English common law and what became the American majority view might be that English common law developed when confrontations were likely to occur with swords, where it's reasonable that you could run away, whereas American [self-defense] law developed in the age of the gun, where it's much more difficult to retreat if an assailant has a gun, or where there's a possibility he might have one," says Cottrol, a professor at the George Washington University Law School and author of "Gun Control and the Constitution."

Within the African-American community, the skepticism about stand your ground can run deep.

"The concern from the black community is rankled by the fact that there's a disproportionate number of African-Americans incarcerated – an entire generation – which fuels the concern by [blacks] around the country that stand-your-ground laws are aimed at them, that it's just another way that white Americans are trying to do away with generations of African-Americans, almost like a form of genocide," says Jeffrey Swartz, a former Miami-Dade judge who's now a law professor at Cooley Law School in Tampa, Fla.

Yet in Florida, some of the most ardent defenders of the law have been black defense attorneys. The reason: Their black, often young, clients are the most successful users of the law. Indeed, data show that black defendants have a high success rate in invoking stand your ground in black-on-black violence. In fact, if all cases are taken into account, black defendants have a higher success rate in claiming stand your ground than do white defendants, and they attempt to claim stand your ground at higher rates.

"There is a long history of African-American support for gun rights and the principle of armed self-defense," writes Jelani Cobb, director of the Institute for African-American Studies at the University of Connecticut, in The New Yorker. He cites the Deacons for Defense and Justice, an armed posse that protected civil rights marchers, and former NAACP head Walter White, who protected his home with a rifle during the 1906 Atlanta race riots.

History could also offer other lessons pertinent today. To that point, both gun rights and civil rights scholars find common cause in a 1921 case out of Detroit.

That year, a French-educated black doctor named Ossian Sweet moved into a white neighborhood, only to be confronted by a white mob of more than 300 people. When the menacing mob began to agitate and throw rocks, Sweet and a compatriot opened fire from a second-floor window, injuring one man and killing another.

Called out of retirement to defend Sweet, famed defense attorney Clarence Darrow made an impassioned argument to an all-white jury, suggesting they admit their biases in order to ignore them.

The jury found Sweet not guilty.