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Crime Genes

Dr Mairi Levitt looks at how lawyers are increasingly looking to use defendants’ genes to defend criminal behaviour, despite the evidence for its use being largely unconvincing

Reducing a convicted murderer’s sentence because they have certain genes sounds a little far-fetched, but it has already happened. In Italy and the US, a common genetic variant, present in around a third of Caucasian men, has been successfully used in the defence  of violent criminals. Yet both the science and the ethics of the issue  are still very much under debate.

The variant in question is an allele conveying low activity of monoamine oxidase A (MAOA)[1], an enzyme involved in regulating the metabolism of several neurotransmitters, including dopamine and serotonin, which influence brain function  (Shih et al.,1999).

The first attempt to use evidence of MAOA levels as a mitigating factor for a convicted murderer was in the US in 1994. Research published the previous year had found no MAOA function in five male members of a Dutch family who exhibited low IQ and abnormal violent behaviour (Brunner et al., 1993). The MAOA gene is X-linked, and these men and others in successive generations all exhibited problem behaviour including impulsive aggression, arson and rape.

The defence team representing convicted murderer Stephen Mobley sought advice from researchers involved in the Dutch family study. Mobley had an above average IQ, but a family history of males who either exhibited violent and antisocial behaviour or were successful businessmen, such as Mobley’s father. The defence lawyers asked for a gene test for MAOA function in an attempt to commute the death penalty to a life sentence. This was refused on the grounds that the genetic research referred to did not meet the required standards for permissible scientific evidence. Mobley was executed in 2005.

An absence of MAOA function is thought to be extremely rare, but the low activity MAOA variant has been dubbed the ‘warrior gene’. Controversial research has found a high incidence among Maori men (56% in a small study) and this figure was used to explain problems of risk taking behaviour, aggression and violent crime (Lea and Chambers, 2007). Extensive critical coverage followed the reports, focusing on alternative socioeconomic explanations under media headlines such as: “White justice blamed for Maori conviction rate”. Other research papers have linked the MAOA variant to various forms of risky behaviour and to gang membership (Beaver, 2009).

Yet there are a number of huge problems when making assumptions about the link between certain gene variants and criminal behaviour. First, very different types of behaviour may be defined as antisocial, aggressive or violent depending on the particular context and historical period. And a gene variant that is present in a substantial proportion of the male population doesn’t have strong explanatory power on its own. As with other complex behaviours, researchers are looking at the interplay of genetic and environmental factors.

In an influential study that has been replicated, Caspi et al. (2002a) took the environmental factor of childhood maltreatment and the genetic MAOA factor, both of which are associated with antisocial behaviour in adulthood, and using data collected in the Dunedin birth cohort study, antisocial behaviour was assessed when the white male subjects were age 26 [1].

Twelve per cent of men with low activity MAOA genotype had experienced maltreatment in childhood, but were responsible for 44% of the convictions for violence. Of those with the genetic and environmental indicator, 85% developed some form of antisocial behaviour. Those who were not maltreated were unlikely to display adult antisocial behaviour. For those maltreated but with high levels of MAOA functioning, the genetic factor seemed to have a protective effect against later antisocial behaviour.

Since the Mobley case, evidence on MAOA levels has been brought into criminal courts. In the first Italian case, Abdelmalek Bayout stabbed a man to death who he claimed had insulted him. On appeal, his sentence was cut by one year, when evidence was produced that said he had the MAOA trait associated with violent behaviour. The research referred to at the trial did not include Bayout’s ethnic group (he was Algerian) and no evidence about his childhood environment was presented (Times newspaper report, 2009). And in the Caspi et al. study, those with low levels of MAOA and no childhood maltreatment were actually found to be less aggressive.

Genes and responsibility for behaviour

It is likely that MAOA research will be seen in UK courts in the future and judges and juries will be asked to consider its relevance. So far, such evidence has been introduced in the offender’s interests, but it is not obvious why such a genetic factor would reduce individual responsibility. Instead, it could make individuals more responsible (and blameworthy) once they have the information about their genetic risk. If they fail to act on the information, they might be held to be responsible for the consequences, in the same way as someone who continues to drive while suffering from a condition that causes unpredictable blackouts. The genetic trait could be seen as making an offender more dangerous and potentially liable to more severe penalties (that is, longer sentences) to protect the public (Denno, 2009; Levitt, 2013).

To be held morally responsible, there has to be a possibility that the person could have chosen to act differently. If there is really no choice, then the person may not be morally (or legally) responsible and is not usually thought to be worthy of blame or praise. Free will can be defined as the capacity to choose rationally from alternative actions. When human beings make choices, they choose from the practical alternatives available to them in their present situation.

It may be harder for some men, like Bayout, to refrain from criminal violence than it is for others; everyone is subject to genetic, biological and environmental influences and could be said to have genetic and environmental good luck or bad luck. Despite these differences, the legal system holds most people responsible for their actions if they are over the age of criminal responsibility and are sane[2]. Legal responsibility is a status assigned to an individual not an empirical description, and is key to the operation of the modern legal system (Barnes, 2003).

It is now acknowledged that gene and environmental interactions affect behaviour, and no doubt research will continue to find correlations between specific forms of behaviour, environmental factors and genetic traits that may or may not be replicated. Epigenetics complicates the idea of a gene environment or nature-nurture divide still further with the findings that nurture can influence not only behaviour and health in later life, but also in subsequent generations (Buchen, 2010).

Evidence of genetic traits in the normal range, such as MAOA variants, have most commonly been presented as mitigating evidence in an attempt to avoid a death sentence in the US. The combination of a genetic trait and childhood maltreatment has been correlated with an increased incidence of violent and antisocial behaviour.  It does not provide a causal link between the gene environment interaction and the specific act for which a suspect is put on trial.

Since childhood maltreatment and other environmental risk factors are already presented in mitigation in UK courts, particularly where the offenders are children, it seems likely that a defence team will also bring forward evidence of low-activity MAOA in an attempt to help their client. The jury is out on what the future holds.


1 As females have two copies of the X chromosome, they don’t neatly divide into low and high groups. Nearly half have a low-high MAOA combination and it is not known which allele is active in an individual. Only 12% of women had the low-low variant (see Caspi et al, 2002b).
2 Criminal liability requires that the individual committed the wrongful act or omission (actus rea) voluntarily and knew what  they were doing, had a wrongful state of  mind (mens rea). ‘Voluntarily’ does not imply that the person necessarily wanted to do  it, but that the act was done under the person’s control.

Barnes, B. Genes, agents and the institution of responsible action. New Genetics and Society 21(3), 291-302 (2003).

Beaver, K. M. et al. Monoamine oxidase A genotype is associated with gang membership and weapon use. Compr. Psychiatry 51(2), 130-134 (2009).

Brunner, H. G. et al. Abnormal behavior associated with a point mutation in the structural gene for monoamine oxidase A. Science 262, 578-580. (1993).

Buchen, L. In their nature. Nature 467, 146-148 (2010).

Caspi, A. et al. Role of genotype in the cycle of violence in maltreated children. Science 297(5582), 851-854 (2002a).

Caspi, A. et al. Supplementary material. Description of methods and measurements used in the Dunedin multidisciplinary health and development study. Science 297 (2002b).

Denno, D. W. “Behavioral Genetics Evidence in Criminal Cases: 1994–2007” in Farahany N. A. (ed) The impact of behavioral sciences on criminal law (Oxford University Press, chapter 10, 2009).

Lea, R. & Chambers, G. Monamine oxidase, addiction and the ‘warrior’ gene hypothesis. New Zealand Medical Journal 120, 1250 (2007).

Levitt, M. Genes, environment and responsibility for violent behaviour: “Whatever genes one has it is preferable that you are prevented from going around stabbing people”. New Genetics and Society 32(1), 4-17 (2013).

Shih, J. C et al. Monoamine oxidase: from genes to behaviour. Annu. Rev. Neurosci. 22,197-217 (1999).

Dr Mairi Levitt is a senior lecturer at Lancaster University. Her research is in the field of bioethics, focusing on the implications of genetics and medical technologies.

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