It is a question as old as Cain and Abel: Are some kids simply born bad? At long last science has an answer, in fact two answers. The short answer is yes, many criminals share a common genetic flaw. The complete answer is more complicated, and could hold the key to eradicating violent crime. The idea that crime runs in families was revived this summer when researchers at the University of Wisconsin-Madison announced they had found a direct link between genes and behavior. This story, like all others about human behavior, begins in the brain. For better or worse, every decision we make takes place in the small spaces that separate the 100 billion nerve cells in the human brain. Contrary to appearances, neurons, as brain cells are called, do not physically touch like wires. They are separated by tiny gaps known as synapses. To communicate across this void, a nerve cell releases a small amount of a class of compounds called neurotransmitters. Imagine these molecules as tiny ignition keys and the surface of the neuron on the opposite side of the synapse as a wall lined with ignition locks. When the right keys fit into the proper locks, the adjoining neurons fire. The UW-Madison research suggests that the leftover keys could open the door to understanding how genes influence criminal behavior. Extra Keys

The job of picking up the unused keys is the work of an enzyme called monoamine oxidase A (MAO A). All brains produce some MAO A. In most people, the brain produces enough MAO A to break down excess amounts of the neurotransmitters serotonin, dopamine and norepinephrine. In 1993, Harm G. Brunner, head of the human genetics department at the St. Radbout University Medical Center in the Netherlands, announced he had found a genetic connection that could explain the abnormal amounts of serotonin often found in criminals. A study of a family of Dutch criminals revealed they suffered from a defect in the gene that determines MAO A production. Olivier Cases, of France's National Center for Scientific Research, constructed an experiment to test the idea. He studied the behavior of mice in which the MAO A encoding gene had been turned off. The animals exhibited fearless, impulsive behavior. And when their brains were examined, it was discovered that the serotonin levels were nine times the normal amount. The chain of evidence linking genes, brain chemistry and criminal behavior was complete. There was, however, one weak link. Most people with abnormal MAO A levels lead perfectly peaceful lives. Clearly, something else was involved in tipping the scale of those with this genetic flaw toward criminality. Rough Start

The answer came from an observation that had been made long before anyone even began thinking about the neurochemical origins of crime. In interviewing criminals, psychologists frequently found that violent inmates had been abused as children. Yet, a history of abuse by itself is not a predictor of future criminal behavior. One obvious possibility was that the two factors associated with criminal behavior--genes and abuse--might somehow be related. "There are known genes that protect against malaria and other parasites," says Terrie Moffitt, a psychologist who worked on the UW-Madison study. "We wanted to know if a particular MAO A genotype could protect maltreated children against antisocial behavior." A genotype is the technical term for the inheritable information that is used as a set of instructions to control how cells are constructed and operate. Moffitt found the answer in New Zealand. The Bad Gene

In 1972, researchers at the Dunedin School of Medicine in New Zealand began an ambitious data-collection effort called the Longitudinal Dunedin Multidisciplinary Health and Development Study. Psychology professor Avshalom Caspi, the head of the effort at UW-Madison, delved into this database and studied data on 442 subjects. "The team looked at both the MAO A genotype in all participants and also periodically assessed the subjects' history of abuse and criminal convictions, their penchant for violence and any symptoms of antisocial personality disorder," says a project spokesman. "Antisocial behavior includes persistent fighting, bullying, lying, stealing and disobeying the rules during adolescence. As adults, the subjects show no remorse and act impulsively and aggressively." What UW-Madison researchers found was nothing short of astounding. Only 12 percent of the abused children had low MAO A levels and these accounted for almost half of their generation's convictions for violent crimes in New Zealand. "The combination of maltreatment and the genetic variation magnified the odds by nine times," says Moffitt. The opposite appeared equally true. A surplus of MAO A may protect them against the effects of abuse. "The genotype of high MAO A activity may promote trauma resistance," Moffitt concludes. Moffitt says it's important that the team's work not be misapplied. "Low levels of the enzyme [MAO A] did not predict antisocial outcomes," she cautions. "Its relation to aggression only emerged when we considered whether the children had been maltreated." There is one aspect of the research that sings out with crystal clarity. The belief that violence breeds violence now has an unmistakable ring of scientific truth.