A criminal defense attorney has many arrows in his/her quiver. The latest is the "warrior gene." Having this gene saved Bradley Waldroup from a first-degree murder conviction.

The charges stemmed from a bloody rampage in which Waldroup shot his wife's friend, Leslie Bradshaw, eight times and killing her before attempting to kill his wife by chopping her up with a machete.

Waldroup had been drinking as he waited for his estranged wife and their four children who were to spend the weekend at his trailer home in the mountains of Tennessee. When his wife said that she was leaving with her friend, he removed the key from Penny Waldroup's van to ensure that they could not leave, thereby establishing criminal intent. Waldroup then launched his deadly attack on the pair.

The "warrior gene"

Waldroup's defense attorneys ordered a test and established that he had the warrior gene. Like most such biological defenses, there is a germ of scientific truth combined with a hefty dose of junk science, including clever labeling. The warrior gene might be called other things, such as the gambling gene, the gene, the irritability gene, or even the live-in-a-trailer gene. Its effects are contingent on an abusive .

The scientific rationale for diminished responsibility is that a variant of the relevant gene, known as MAO-A is linked to the underactive prefrontal cortex, this being a key area of the brain that inhibits antisocial impulses. The gene is also associated with antisocial behavior in European Americans (but not others) but only if they were abused as children (1).

The gene has recently acquired some evidence linking it to impulsive . In an experiment where subjects were provoked by having monetary winnings taken from them, people with the MAO-A variant proved slightly more vengeful but only if they lost the higher of two amounts of money (2). They asked for the provoker to drink a larger amount of hot sauce as . Whether this experiment is more relevant to homicidal aggression, or sensitivity to the taste of hot sauce is anybody's guess.

So far, a skilled defense lawyer might weave a tale that the bad gene had gotten the better of the European American defendant. The key scientific problem is that about 34 percent of Europeans have the warrior gene. Yet, homicide is extremely rare at a population level with only about one person in 100 committing a homicide during their lives. If the gene were used to predict homicide, it would be wrong more than 33 times for every occasion that it was right (3).

Just the facts

This brings us back to the Waldroup case tried in March 2009, where the warrior gene formed the kernel of a diminished responsibility defense. This defense received an enthusiastic endorsement in a recent NPR report by Barbara Bradley Haggerty ("Can Your Make You Murder?")

Waldroup's defense was not a simple genetic defense because it was combined with the normally ineffective abuse excuse. Defense expert William Bernet of Vanderbilt University argued that the combination of the warrior gene and being abused as a child was a dangerous cocktail that increased the likelihood of committing a violent offense.

Some of the jurors were persuaded by this defense. According to one, Debbie Beatty: "A diagnosis is a diagnosis, it's there. A bad gene is a bad gene." Junk science is also junk science. There is no getting away from that either, especially if it helps the defense to save a defendant's life.