The use of neuroscientific and genetic evidence in criminal cases has been called a double-edged sword. On one hand, it may reduce the apparent culpability of defendants and mitigate their perceived responsibility for their crimes, leading to more lenient sentences. On the other hand, it may support an assumption that these offenders are prone to committing future acts of violence, and lead to more punitive sentences. Whether genetic evidence is mitigating or aggravating is an important question for defense attorneys and prosecutors alike in determining whether and how to introduce such evidence in a criminal trial. Genetic evidence has been introduced into criminal cases in the United States, Canada, Europe, and Australia. 

Of particular interest is how this evidence is viewed by legal professionals in the United States compared to how it is viewed by those in Germany. One study (hereafter the “German law student study”) concluded that in the United States biological information has a mitigating effect on sentencing, whereas in Germany it seems to increase the rate of involuntary commitment to forensic psychiatric hospitals. The question of whether genetic evidence in criminal cases is a double-edged sword whether it will mitigate or aggravate perceived culpability depends strongly on the particular criminal justice system of a given country.

Neuroscience and Genetic Evidence

Neuroscience and genetic evidence have been increasingly introduced in criminal cases. This evidence consists of brain imaging and genetic evidence that seek to explain an offender’s behavior.

Research in these fields as applied to criminality has largely focused on psychopathy because it is considered one of the strongest predictors of aggression and violence. Psychopaths are charged with committing severe acts of violence, with committing twice as many crimes as non-psychopathic offenders, and of having a rate of recidivism that is five times higher than their non-psychopathic counterparts. Psychopathy is defined by behavioral and interpersonal characteristics that include impulsivity, lack of empathy or remorse, and manipulativeness, among others. Perspectives on psychopathy range from it being a biological mental disorder based on structural and functional dysfunctions of the brain to it being a social disorder. Not all psychopaths are violent or criminal. In fact, research shows greater rates of psychopaths succeeding in the business world than in the general population. The causes of psychopathy are controversial. Studies to determine the causes of psychopathy investigated the correlation between physiological indices (like heart rate) and aggression, and how prenatal factors (like alcohol consumption during pregnancy) might contribute to violent behavior. Current research focuses on neurotransmitters, steroid hormones, and brain structure and function.

Sociopathy, or chronic antisocial behavior, is another area of focus. It can be a developmental or an acquired disorder (like in the case of Phineas Gage). Research into the causes of sociopathy has considered how brain trauma, strokes, tumors, infections, and aneurysms can lead to alterations in social and moral behavior.

Evidence from behavioral genetics also suggests that genetic contributions cause a significant amount of the variance in antisocial personality. One review found that 56% of the variance of antisocial personality and behavior was explained by genetics. Studies have focused on the MAOA gene which is associated with abnormal aggressive behavior.

Research into psychopathy, sociopathy, and genetics have explored how they might influence behavior. How behavior is influenced, and how this influence impacts determinations of guilt and sentencing decisions is at the heart of the double-edged sword theory.

Past Experiments

Several experimental studies have been performed to explore the influence of neurobiological or genetic evidence on judging in criminal cases. For example, one study found that defendants diagnosed with psychosis were more likely to be found not guilty by reason of insanity than those diagnosed with psychopathy. Neuroimages showing brain damage as well as testimony that the disorder began after the defendant incurred brain damage increased the likelihood of a favorable verdict. Another study examined how explanations of behavior affected the participants’ fear of the defendant. Researchers found that a genetic explanation of the defendant’s behavior amassed the most fear of the defendant, whereas an impulsivity explanation garnered the least fear. This appeared consistent with the concern that social stigmatization of people with genetic predispositions might translate into more punitive findings of responsibility of defendants under certain circumstances.

United States’ Research vs. Germany’s Research

Two studies were conducted to examine how judges might be influenced by the presentation of genetic evidence when sentencing offenders.

Aspinwall et al. (2012) tested the influence of genetic evidence on sentencing decisions made by United States judges. They presented U.S. state trial judges with a hypothetical case in which the offender was convicted of aggravated battery. All participants received a psychiatric testimony about the offender’s psychopathy. One group was told the testimony was presented by the defense while another group was told it was presented by the prosecution. One group received an explanation that the offender’s psychopathy was related to his MAOA genotype (nicknamed the “warrior gene” because of its link to aggression), while the fourth group did not receive any genetic explanation. They found that while the judges considered the psychiatric testimony to be aggravating, the presentation of neurogenetic evidence for the offender’s psychopathy significantly reduced sentencing.

Fuss et al. (2015) repeated Aspinwall’s study to determine whether the same results would be found in German judges. They found that while neurogenetic evidence significantly reduced the German’s judges estimation of the offender’s legal responsibility, the average prison sentence was not reduced. In fact, when the evidence was presented by the prosecution, it significantly increased the number of German judges who ordered an involuntary commitment in a psychiatric hospital. It was suggested that the differing results between these two studies showed the judges were highly influenced by the legal system in which they operated.

Aspinwall’s study has been highly criticized for being flawed in its design and methodology. Denno and McGivney (2013) concluded that Aspinwall’s interpretation of the effects of genetic evidence as a double edged sword was not supported by actual case law nor did it account for different evidentiary standards on either side of the sword. It is easier for the defense to use genetic evidence to justify why a defendant should not receive capital punishment than it is for the prosecution to prove a defendant’s future dangerousness based on genetic factors.

German Law Student Study

The German law student study sought to investigate the influence that different types of neurobiological explanations had on the sentencing decisions made by German law students. This study was based on the previous studies of Aspinwall and Fuss, but was modified to address some of Denno and McGivney’s criticism.

The first difference was that the hypothetical case given to participants was for manslaughter instead of aggravated assault. This was done because most real criminal cases where genetic evidence is presented involve a question of capital punishment. Another modification was the removal of the groups that were told the psychiatric testimony was presented by either the defense or the prosecution. This was because in real cases it was unrealistic for the prosecution to introduce behavior genetics evidence in any capacity. Instead, the study established three groups: one received a genetic explanation (“MAOA Gene” group), one received a neurobiological explanation (“Brain Injury” group), and one did not receive any biological explanation for the offender’s behavior (“Absent Biomechanism” group). Finally, law students were interviewed instead of judges because the researchers wanted to achieve a high response rate.

The German researchers found that the strongest difference between the three groups was observed with regard to legal responsibility. Students who received information describing a major brain injury rated the legal responsibility of the offender significantly lower that those who did not receive a biological explanation. Still, the mean prison sentence assigned by law students differed only slightly with the shortest sentence actually being assigned by the group that received no biological explanation (“Absent Biomechanism” group: 9.15 years, “Brain Injury” group: 10.06 years, “MAOA Gene”: 10.54 years) (though researchers indicated that these group differences were not statistically significant).


Whether neurobiological or genetic evidence is a double-edged sword has been answered differently by different studies. The German law student study concluded that this question cannot be answered in general, but that the answer depends strongly on the given system of criminal justice. The United States, for example, has much harsher punishment than most other Western countries, but at the same time has purposefully lax standards for admitting mitigating evidence. Understanding how these factors play into sentencing decisions does not encapsulate whether the criminal justice system of a given country can adequately rehabilitate offenders. Future research should focus on how offenders that have psychopathic, sociopathic, or genetic predisposition to violence can be rehabilitated to prevent recidivism and future violence.


Author Biography: Amanda Bini is a Senior Moderator of the International Law Society’s International Law and Policy Brief (ILPB) and a J.D. candidate at The George Washington University Law School (GW). She has Bachelor of Science Degrees in Criminal Justice and Business Administration from Northeastern University. She is also a member of GW’s Federal Circuit Bar Journal.

Editor: Elizabeth Duncan