What exactly does it take to kill someone? Here's how 21-year-old West Texas Army Pvt. Steven Green described shooting a man who refused to stop at an Iraqi checkpoint: "It was like nothing. Over here, killing people is like squashing an ant. I mean, you kill somebody, and it's like, 'All right, let's go get some pizza,' " he told the military newspaper Stars & Stripes.

"I mean, I thought killing somebody would be this life-changing experience. And then I did it, and I was like, 'All right, whatever.' "

[Podcast: Vicki Haddock and John Koopman on Killing 101.]

In February, the soldier's comments struck embedded correspondent Andrew Tilghman as unremarkable, a reflection of the fact that he and Green were immersed in the treacherous hellhole of Mahmoudiya, at the edge of what GIs have dubbed the Triangle of Death. Green's statements didn't even make it into the Stars & Stripes article, which ran earlier this year.

It was only recently -- when the honorably discharged soldier appeared in federal court pleading not guilty to the rape of a 14-year-old Iraqi girl and the cold-blooded murder of her family -- that Tilghman recalled the quotes with a newfound chill.

The reality is that the brains of human beings -- unless they fall within the demographic sliver we call psychopaths -- are hardwired not to kill other humans. Like rattlesnakes that fatally bite other species but fight fellow rattlers by wrestling them, humans overwhelmingly recoil from homicide. That's usually a good thing, because it prevents society from disintegrating into bloodthirsty anarchy.

But it poses an occupational hazard to some -- particularly soldiers, police officers, spies and victims of savage crimes. All of them may face situations in which hesitating to kill is the surest way to get killed.

That's why military training camps, police academies and even some self-defense pros are constantly searching for more effective methods of suppressing the human revulsion to taking human life -- virtually rewiring the brain to react first in certain situations with an automatic response to kill.

Target practice on hollowed cabbages filled with ketchup to mimic the way a bullet rips open a human head. Marching to chants of "kill, kill, kill." Video game simulations that reward points for every successful "shot." These are among hundreds of techniques that experts say can recondition the human brain.

What that reconditioning requires, and the psychological toll it ultimately takes on the killers, make up the taboo scientific inquiry sometimes known as "killology." To outsiders, the subject is distasteful, even repellant. To practitioners, it is simply a fact of life -- and death.

"Once the bullets start flying, most combatants stop thinking with the forebrain (that portion of the brain that makes us human) and start thinking with the midbrain (the primitive portion of our brain, which is indistinguishable from that of an animal)," writes retired Lt. Col. Dave Grossman, a former U.S. Army ranger and West Point professor of military science who coined the term, on his Web site killology.com. "In conflict situations, this primitive, midbrain processing can be observed in the existence of a powerful resistance to killing one's own kind. ... This is an essential survival mechanism that prevents a species from destroying itself during territorial and mating rituals."

The only thing that has any hope of silencing the midbrain, he argues, is what influenced Pavlov's dogs: conditioning.

The need for new drills became apparent once researchers noted that a majority who had been trained in other ways to kill, surreptitiously refused to do it.

In World War II, when U.S. soldiers got a clear shot at the enemy, only about 1 in 5 actually fired, according to sensational and controversial research by Army historian Brig. Gen. S.L.A. Marshall. It wasn't that they were cowards: On the contrary, they performed other perilous feats, including running onto the battlefield to rescue fellow soldiers, and sometimes they even placed themselves in greater personal danger by refusing to fire. And yet at the moment of truth, they just couldn't kill.

While modern scholars have debated his methodology, other contemporary researchers have reached conclusions similar to Marshall's that "fear of killing, rather than the fear of being killed, was the most common cause of battle failure in the individual."

Go back even further in U.S. history. Grossman noted this "Civil War Collector's Encyclopedia" citation about recovered muskets after the Battle of Gettysburg: Almost 90 percent were loaded, half of those multiple times. Given that a Civil War soldier would spend 95 percent of his time laboriously loading his musket and only 5 percent aiming and firing it, that many loaded muskets seems to make sense only if the soldiers in battle were faking it -- all the while looking busy so that their comrades would never know the difference.

The FBI discovered a similar problem among law enforcement officers through the early 1960s: a startling number were refusing to fire at suspects even when other lives were endangered.

Even those who fired their weapons were not necessarily trying to kill -- it is hard for an observer to detect soldiers or cops who fire high to intentionally miss.

Psychologists who advised the military and law enforcement agencies began to push for changes that would revolutionize training to improve kill rates. Their methods -- familiar to those who operate boot camps, police academies and aggressive-response self-defense courses -- are a distasteful mystery to most in the outside world. But they work.

The Pentagon improved firing rates. Research suggests that 55 percent of U.S. soldiers fired on the enemy in the Korean War. By Vietnam that rate had climbed to more than 90 percent. Police studies document similar changes in recent decades.

One of the key changes was to get rid of the old firing ranges, where shooters took target practice in an open field aiming at a bull's-eye. This failed miserably at preparing shooters for real-world confrontations.

Today's apprentice killers train in situations designed to simulate combat as closely as possible, and they rehearse in a fashion that would be instantly recognizable to pioneers of behavior modification, from Ivan Pavlov to B.F. Skinner. The bull's-eyes have been replaced by human-shaped targets that pop up without warning, for example, with polyurethane faces on balloon bodies inside uniforms. A trainee spots the targets, fires almost on instinct and gets rewarded with points, badges and three-day passes. Over and over, these "kill drills" build muscle memory and acclimate the brain to the act of killing.

Aggressive self-defense courses filled with students who may be more hesitant to go for the jugular use similar rehearsals to form a kinetic memory of how to react if attacked. Instructors may push students to overcome their squeamishness by, for example, taping a peeled orange over an actor's eye and having students practice sticking their thumbs into it.

But most apprentice killers have had years of moral training reinforcing the commandment "thou shalt not kill." Suppressing that is the greatest challenge of killology.

Some training focuses on rationales for killing -- to overcome an enemy that threatens the "American way of life" or "wages war against freedom" or simply trying to kill innocent victims. But a key part of many programs is to make killing more palatable -- even socially acceptable and desirable.

Consider an excerpt of a lecture on mines to Marines at Parris Island, who grunted their approval.

"You want to rip (the enemy's) eyeballs out, you want to tear apart his love machine, you want to destroy him, privates. ...You want to send him home in a Glad bag to his mommy!"

Such bloodthirsty language helps "desensitize them to the suffering of an 'enemy' at the same time they are being indoctrinated in the most explicit fashion, as previous generations of soldiers were not, with the notion that their purpose is not just to be brave and to fight well; it is to kill people," observes military historian Gwynne Dyer in his book "War: The Lethal Custom."

Another technique is to create physical and emotional distance between the killer and the target by fostering a sense of us versus them. While physical distance is achieved with bombs, rocket launchers and even night-vision goggles, which reduce humans to ghostly green silhouettes, emotional distance often is achieved by categorizing targets as different because of their race, ethnicity or religion. The military does whatever it can to deny the fellow humanity of enemy soldiers and is loath to repeat the spectacle of Christmas Day in 1914, when German and British soldiers crawled out of their trenches to share cigarettes, candy and soccer.

In his autobiography, top Marine sniper Jack Coughlin writes from Iraq: "So far in this war I had fired six shots and had six kills -- exactly the right ratio. I considered the ill-trained, poorly led soldiers of Iraq to be hamburger in my scope, practically begging me to kill them, and I was more than ready to grant their wish."

Social dynamics often are critical as well. Soldiers and cops forge deep bonds with their colleagues and some studies report that their greatest fear on the battlefield or in the line of fire is not dying but letting their buddies down -- a potent motivator for killing.

And finally, organizations like armies and police forces rely on elaborate codes and strict authority figures who order killing to be done. In a most amazing and famous demonstration, Yale psychologist Stanley Milgram established that almost two-thirds of people would be willing to administer shocks to others -- even to the point of a lethal 450 volts -- simply because they were ordered to do so by a scientist in a white lab coat.

The institutions that teach killing emphasize that they do so with built-in safeguards against indiscriminate violence. Their pupils know, at least theoretically, that if they fire in an unauthorized way, they can be ostracized, washed out, charged or court-martialed. In his seminal work "On Killing," Grossman takes pains to distinguish that kind of training from violent video games, which mimic the techniques of operant conditioning and desensitization without any of the safeguards.

But as the rape and homicide charges against Pfc. Green and four of his fellow soldiers show, in rare instances the safeguards simply fail. Atrocities result.

A more pervasive risk, however, is that soldiers and cops who kill pay a steep psychological price for not only using the new skills they acquire but also for acquiring the skills in the first place. The Pentagon is waging an unprecedented campaign to deal with the mental and emotional scars of combat in Afghanistan and Iraq. Turning human beings into killers is a tricky business.