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DNA Tests Offer Deeper Examination Of Accused

But what of the murderers, rapists and other violent criminals who fall outside those narrow bounds? Can some, at least, blame their behavior on their genes?

Studies have shown that up to 62 percent of antisocial and criminal behavior is "heritable," a rough measure of a genetic contribution. And in a few cases, courts have allowed arguments seemingly akin to "My genes made me do it."

The Supreme Court of South Carolina reversed a murder conviction for a man who shot a shopkeeper in the head, concurring with the killer's attorney that his actions were an outgrowth of severe, genetically rooted depression -- essentially saying that what he did was the result of an inherited disease rather than an act of free will.

But in a 2001 first-degree murder case, a Tennessee court rejected the defendant's claim that an inherited predisposition to depression and mental illness made him incapable of a premeditated act.

All told, defendants hoping to convince juries of their innocence, or at least garner enough sympathy to avoid the death penalty, have had better luck invoking family histories of mental illness than specific gene tests to raise doubts about their culpability. But that is likely to change, experts say, as specific genes get more definitively linked to violence and impulsivity.

One case that may prove to be a harbinger took place in the 1990s, after Stephen A. Mobley was convicted of murdering a Domino's Pizza store manager in Georgia. Hoping to avoid the death penalty, his attorneys asked the court to pay for tests to find out whether Mobley, who came from a family with a history of violent behavior, harbored a mutant gene for a brain enzyme known as MAO-A.

Scientists had just pegged a Dutch family's multi-generational history of fistfights and run-ins with the law to that mutant gene. But the judge in Mobley's case found the association too new, and Mobley was executed in 2005.

Since then, however, a number of studies have strengthened the link between MAO-A and violent behavior, and other genes have been added to the mix. This month, scientists in Israel reported that a version of a gene called AVPR1a is associated with "ruthlessness." And although such tests can offer only the probability that a given behavior will arise, they can sway jurors, experts said, because they seem more scientific than a doctor's clinical assessment.

It is probably only a matter of a time before gene tests are admitted in a criminal trial, at least as evidence in the sentencing phase, said Deborah W. Denno, a professor at Fordham Law School.

"The word 'genetic' is such a loaded term. It's very touchy stuff," Denno said. But it is not as though the kind of testimony already being used in sentencing is particularly scientific, she added.

"The mere fact that your family loves you or that you go to church is allowed as mitigating evidence in a death penalty case," Denno said.

How Juries Will React

Whether evidence of an inborn penchant for violence can be relied upon to evoke a jury's sympathies is another question, and there is some reason to doubt it.

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