There is a theory that frontal lobe trauma does factor into most instances of impulsive violent crime. (I'm not an expert so I have no idea.) Here's an excerpt from a long, but fascinating, article:
[Serial killer] Arthur Shawcross, who had a cyst pressing on one temporal lobe and scarring in both frontal lobes (probably from, among other things, being hit on the head with a sledgehammer and with a discus, and falling on his head from the top of a forty-foot ladder), used to walk in absolutely straight lines, splashing through puddles instead of walking around them, and he would tear his pants on a barbed-wire fence instead of using a gate a few feet away. That's the kind of behavior Pincus tries to correlate with abnormalities on the neurological examination. "In the Wisconsin Card Sorting Test, the psychologist shows the subject four playing cards-three red ones, one black one- and asks which doesn't fit," Pincus said. "Then he shows the subject, say, the four of diamonds, the four of clubs, the four of hearts, and the three of diamonds. Somebody with frontal-lobe damage who correctly picked out the black one the first time-say, the four of clubs- is going to pick the four of clubs the second time. But the rules have changed. It's now a three we're after. We're going by numbers now, not color. It's that kind of change that people with frontal-lobe damage can't make. They can't change the rules. They get stuck in a pattern. They keep using rules that are demonstrably wrong. Then there's the word-fluency test. I ask them to name in one minute as many different words as they can think of which begin with the letter 'f.' Normal is fourteen, plus or minus five. Anyone who names fewer than nine is abnormal."
This is not an intelligence test. People with frontal-lobe damage might do just as well as anyone else if they were asked, say, to list the products they might buy in a supermarket. "Under those rules, most people can think of at least sixteen products in a minute and rattle them off," Pincus said. But that's a structured test, involving familiar objects, and it's a test with rules. The thing that people with frontal-lobe damage can't do is cope with situations where there are no rules, where they have to improvise, where they need to make unfamiliar associations. "Very often, they get stuck on one word- they'll say 'four,' 'fourteen,' 'forty-four,' " Pincus said. "They'll use the same word again and again-'farm' and then 'farming.' Or, as one fellow in a prison once said to me, 'fuck,' 'fucker,' 'fucking.' They don't have the ability to come up with something else."
What's at stake, fundamentally, with frontal-lobe damage is the question of inhibition. A normal person is able to ignore the tapping after one or two taps, the same way he can ignore being jostled in a crowded bar. A normal person can screen out and dismiss irrelevant aspects of the environment. But if you can't ignore the tapping, if you can't screen out every environmental annoyance and stimulus, then you probably can't ignore being jostled in a bar, either. It's living life with a hair trigger.
A recent study of two hundred and seventy-nine veterans who suffered penetrating head injuries in Vietnam showed that those with frontal-lobe damage were anywhere from two to six times as violent and aggressive as veterans who had not suffered such injuries. This kind of aggression is what is known as neurological, or organic, rage. Unlike normal anger, it's not calibrated by the magnitude of the original insult. It's explosive and uncontrollable, the anger of someone who no longer has the mental equipment to moderate primal feelings of fear and aggression.
"There is a reactivity to it, in which a modest amount of stimulation results in a severe overreaction," Stuart Yudofsky told me. "Notice that reactivity implies that, for the most part, this behavior is not premeditated. The person is rarely violent and frightening all the time. There are often brief episodes of violence punctuating stretches when the person does not behave violently at all. There is also not any gain associated with organic violence. The person isn't using the violence to manipulate someone else or get something for himself. The act of violence does just the opposite. It is usually something that causes loss for the individual. He feels that it is out of his control and unlike himself. He doesn't blame other people for it. He often says, 'I hate myself for acting this way.' The first person with organic aggression I ever treated was a man who had been inflating a truck tire when the tire literally exploded and the rim was driven into his prefrontal cortex. He became extraordinarily aggressive. It was totally uncharacteristic: he had been a religious person with strong values. But now he would not only be physically violent-he would curse. When he came to our unit, a nurse offered him some orange juice. He was calm at that moment. But then he realized that the orange juice was warm, and in one quick motion he threw it back at her, knocking her glasses off and injuring her cornea. When we asked him why, he said, 'The orange juice was warm.' But he also said, 'I don't know what got into me.' It wasn't premeditated. It was something that accelerated quickly. He went from zero to a hundred in a millisecond." At that point, I asked Yudofsky an obvious question. Suppose you had a person from a difficult and disadvantaged background, who had spent much of his life on the football field, getting his head pounded by the helmets of opposing players. Suppose he was involved in a tempestuous on-again, off-again relationship with his ex-wife. Could a vicious attack on her and another man fall into the category of neurological rage? "You're not the first person to ask that question," Yudofsky replied dryly, declining to comment further.
Pincus has found that when he examines murderers neurological problems of this kind come up with a frequency far above what would be expected in the general population. For example, Lewis and Pincus published a study of fifteen death-row inmates randomly referred to them for examination; they were able to verify forty-eight separate incidents of significant head injury. Here are the injuries suffered by just the first three murderers examined:
I. three years: beaten almost to death by father (multiple facial scars) early childhood: thrown into sink onto head (palpable scar) late adolescence: one episode of loss of consciousness while boxing II. childhood: beaten in head with two-by-fours by parents childhood: fell into pit, unconscious for several hours seventeen years: car accident with injury to right eye eighteen years: fell from roof apparently because of a blackout III. six years: glass bottle deliberately dropped onto head from tree (palpable scar on top of cranium) eight years: hit by car nine years: fell from platform, received head injury fourteen years: jumped from moving car, hit head.