Introducing evidence at trial that a defendant has a genetic predisposition to violent behaviour is becoming a tactic more commonly used by defense attorneys. Known as the ‘genetic defense’ it was a strategy which worked in the case of Bradley Waldroup in Tennessee in 2009.

On 16 October 2006, Bradley Waldroup killed his wife’s friend Leslie Bradshaw by shooting her eight times before chasing after his wife Penny Waldroup with a machete and causing multiple wounds. He went on trial in March 2009 for the first-degree murder of Leslie Bradshaw and the attempted murder of his wife. His defence attorneys were successful in establishing a diminished responsibility defence by introducing evidence that Waldroup had the so-called ‘warrior gene’. This gene is monoamine oxidase A, known as MAOA and there is some scientific evidence, albeit controversial, suggesting that variants of this gene in some people predisposes them to violence and aggression.


Bradley Waldroup on trial for murder and attempted murder in 2009

The presence of this gene, Waldroup’s defense team said, which was discovered through genetic testing, coupled with a childhood full of abuse was a combination putting Bradley Waldroup at a much higher risk of committing a violent crime. “His genetic makeup, combined with his history of child abuse, together created a vulnerability that he would be a violent adult,” forensic psychiatrist William Bernet of Vanderbilt University who examined Waldroup told NPR.

It was an argument which convinced the jury who found Waldroup guilty of voluntary manslaughter instead of murder, and attempted second-degree murder and he was sentenced to 32 years in prison. A conviction of first-degree murder would have most likely sent Bradley Waldroup to death row.

The science of genetics and criminal behaviour are advancing, however, research has indicated this can be a dangerous strategy and juries do not necessarily view this evidence in the way a defense team plans.

Convincing a jury that a person has a vulnerability to violence may help for their current charges but may do the opposite when looking at the future as to whether or not this individual is safe to be released from custody. Research published in 2015 found that while jury members may be persuaded to believe a defendant should be found not guilty due to insanity after the presentation of genetic evidence, they could also believe this individual is a continued risk to society as a result of such evidence.


The research carried out at the University of Columbia with over 600 participants gave a fictional murder case to participants to read over three different experiments. Participants were told that a genetic disposition or abusive background would cause a higher likelihood that the defendant in question would display violent behaviour. One group was given a case version which highlighted a “genetic variation associated with aggression and violent tendencies”, while a second case version given to different group focused on a traumatic childhood characterized by beatings and abuse living in an area rife with gang violence. A further group read the murder case with none of this evidence relating to childhood trauma or any suggested genetic components to the defendant’s behaviour.

Participants in the study, after reviewing the case they were given, then gave their verdicts on the defendant. It was found that although participants were more likely to support an insanity defence and believe the defendant could not control their behaviour, they were just as likely to support a guilty verdict, regardless of their background or genetics.

Those who read the genetic evidence case study did believe this person had less control over their actions, however, they only supported a shorter sentence in one out of the three experiments carried out. This indicates that a belief of lack of control due to genetics or abusive backgrounds does not necessarily equate to a lessor punishment for the individual in question. In fact, the acceptance of such evidence could encourage the jury to ensure this person is not out on the streets where such factors could once again promote violent and aggressive behaviour that they are unable to control.

“Judges or jurors may believe the perpetrator couldn’t control his actions, but they also may think he is a danger to society who will strike again.”

Genetics, as with all our biology, is out with our control. If science discovers an abnormality or the presence of genes which could have a significant influence on our behaviour, this could, in turn, convince some that the behaviour on trial was not a simple wilful decision by the defendant.  However, as pointed out by Stephen Morse, professor of law and psychiatry at the University of Pennsylvania “Brains don’t kill people. People kill people,” he said in a Nature article discussing science in court.


In the case of the Sandy Hook Elementary School shooter Adam Lanza who shot and killed his mother and 20 children and six adults in Newtown, Connecticut, before shooting himself on 14 December 2012, the medical examiner took the unusual step of requesting the University of Connecticut carry out genetic testing on an already dead Lanza.

A move taken in an attempt to shed some light on what could have caused this 20-year-old to carry out such drastic and deadly violence, it caused a sea of criticism from DNA research projects such as The Genetic Literacy Project to news outlets and high-profile science journals. “Something as simple as a DNA sequence cannot explain anything as complex as behaviour.” wrote Nature in 2013. Quite whether this testing revealed anything within his genetics is unknown but it has highlighted the ongoing debate that surrounds genetic DNA testing and criminal behaviour.

In the search for answers behind violent crimes such as murder, the examination of genetics is one avenue that constantly evolving scientific technologies is opening up for further research. While science can often be viewed as a black and white picture, in reality, the interpretation of scientific findings especially when applied to the complexities of human behaviour is a delicate balance.

We will no doubt see more genetic evidence make an appearance inside the criminal trials of those who have committed some of the most heinous acts of violence against others but, as research indicates, how this evidence is understood and used by a jury to arrive at their verdicts, is not always predictable.