On December 14, 2012, 20-year-old Adam Lanza killed 20 children at a Connecticut elementary school, as well as 6 school staffers, his mother, and himself. Within two weeks, the Connecticut Medical Examiner commissioned a group of geneticists to screen Lanza’s DNA.

And for what, exactly? Who knows. There are any number of genetic variants the scientists could zero in on — variants that have been linked to a propensity for violence, aggression, psychopathy, or psychiatric disorders. One thing I’d bet on: The screen will find something. Each of us carries genetic mutations somewhere along our 3-billion-letter DNA code. Some mutations are benign, some are not; some have huge effects, others tiny. But there’s no way to know how (or whether) any of them affects behavior.

Another thing I’d bet on: The media (and the public) will use the results of that genetic screen to explain what Lanza did. We all want answers, and a genetic test seemingly provides a long string of them. Answers from science, no less. But, as was pointed out by many scientists and commentators at the time, searching for answers in Lanza’s DNA is futile. “There is no one-to-one relationship between genetics and mental health or between mental health and violence,” read an editorial in Nature. “Something as simple as a DNA sequence cannot explain anything as complex as behaviour.”

The Connecticut Medical Examiner is apparently the first to ever request a genetic screen of a dead murderer. It’s an odd move, and perhaps one that can be blamed on intense public scrutiny in the wake of the tragedy. But using genetics to inform criminal cases is not new or even all that rare. As I learned in a fascinating commentary published in today’s issue of Neuron, behavioral genetics has a long history in the American justice system.

The author of the commentary, Paul Appelbaum of Columbia University, cites, for example, the Buck v. Bell Supreme Court case from 1927. The court upheld a Virginia law authorizing mandatory sterilization of people who are intellectually disabled, or “feeble minded”, because they threaten the gene pool. I’m not exaggerating. “It is better for all the world if, instead of waiting to execute degenerate offspring for crime or to let them starve for their imbecility, society can prevent those who are manifestly unfit from continuing their kind,” wrote Justice Oliver Wendell Holmes in the majority opinion. (If you want to be depressed all day, go read the Wikipedia entry about the case.)

Explicit genetic testing entered the courts in the late 1960s, but this time it was on behalf of the accused. Lawyers representing men carrying an extra Y chromosome — known today as ‘XYY syndrome’ — argued that because this genetic condition was overrepresented in prisons, it must drive violent behaviors. But most courts, according to Appelbaum, weren’t sympathetic to this logic, and refused to allow the genetic information into evidence.

Most cases calling on behavioral genetics, like the XYY example, do so in an attempt to lessen the culpability of a defendant who committed a crime. This isn’t usually relevant when deciding the verdict of the case (except for the very rare instances in which a defendant is found not guilty by reason of insanity). But mitigating factors — such as child abuse, drug use, abnormal brain activity, or genetic disposition — can matter a great deal during sentencing proceedings, particularly if the death penalty is on the table. “Judges tend to be fairly permissive at death penalty hearings,” Appelbaum writes.

In 2011 Deborah Denno, a law professor at Fordham University, reported 33 recorded* instances of neuropsychiatric genetic evidence in criminal courts between 2007 and 2011. She had previously reported 44 instances between 1994 and 2007, suggesting that it’s becoming slightly more common. In almost every instance, genetic evidence was used as a mitigating factor in a death penalty case.

The genetic evidence in Denno’s reports tended to be fairly crude: a family history of a condition. But specific genetic tests are beginning to seep into court, too. In 2007, several psychiatrists and geneticists described their experiences presenting evidence at criminal trials related to two gene variants: a variant of monoamine oxidase A, which when mixed with child maltreatment increases the risk of violent behavior, and a variant of the serotonin transporter gene, which when mixed with multiple stressful life events ups the risk of serious depression and suicide. A couple of cases used these scientific links to argue that defendants didn’t have the mental ability to plan their crime in advance. But most of the time genetic evidence was used to mitigate sentences. In 2011, for example, an Italian court reduced a female defendant’s sentence from life in prison to 20 years based on genetic evidence and brain scans that supposedly proved “partial mental illness.”

None of these examples trouble me too much. The U.S. court allows “any aspect of character or record” to be used as a mitigating factor during sentencing, including a defendant’s age, stress level, childhood experiences, criminal history, employment history, and even military service. So why not genetic predisposition, too? It also seems that, so far at least, judges and juries are showing an adequate level of skepticism about this kind of evidence. In 2010, I wrote a story about serial killer Brian Dugan, whose lawyers tried to use brain scans to show that he was a psychopath and didn’t deserve the death penalty. The jury wasn’t swayed.

Most shocking, to me, is how genetic evidence might be used in the civil court system, at least according to Appelbaum. Last year in Canada, a tenant sued her landlord for a fire that, she claimed, caused several injuries that will prevent her from ever working again. The plaintiff had a family history of Huntington’s disease, and the court ordered her to have a blood test to screen for the mutant gene to help determine whether her injuries were the result of the fire or her DNA. She didn’t want to take the test, but if she didn’t she’d have to drop the lawsuit. Appelbaum envisions other possible scenarios in future civil cases:

Employers contesting work-related mental disability claims might… want to compel claimants to undergo genetic testing to prove that an underlying disorder was not responsible for their impairment. Divorcing couples in child-custody disputes, in which court-ordered psychological evaluations are routine, may want to add genetic testing for behavioral traits or neuropsychiatric disorders to the list of procedures that their estranged spouses must undergo to assess their fitness to parent a child. Plaintiffs seeking to establish that a defendant acted recklessly (e.g., in precipitating an auto accident) might attempt to seek data regarding the defendant’s genetic predisposition to impulsive behavior. With increasing utilization of next-generation sequencing in medical settings, and arguments being made for sequencing newborns at birth, adverse parties in civil litigation may not need to compel genetic testing but merely to seek access to existing data.

In these civil cases, which are not usually matters of life and death, I would imagine that the bar for scientific scrutiny would be set lower than in criminal cases. That’s troubling, and all the more reason that we need to better educate the public about what genes can and cannot tell us. As genetic testing continues to infiltrate our medical system, and now our justice system, too, perhaps this education will happen naturally. One can hope.

The Nature editorial regarding the Lanza testing was titled “No easy answer”, and that’s really the crux of all of this. When a person does something awful, we want to know why. But it may be an impossible question.