Criminality and genetics

SIGNIFICANCE: The pursuit of genetic causes of criminality is a controversial field of study that has produced intriguing examples of the apparent contribution of genetic predispositions to criminal behavior. However, the nature of human criminality defies simple and straightforward explanations and instead likely involves a combination of genetic, psychological, and environmental influences. Research into the causes of human criminality has also come under strong criticism by opponents who fear that such discoveries may be used to identify and implicate certain ethnic or racial groups as genetically predisposed to deviant behavior. While experts disagree on the degree of genetic influence, there is a general consensus that genetics is one important factor among many that shape behavioral outcomes.

Biochemical Abnormalities

Scientists have long sought an answer to the heritability of criminality. Early attempts to identify the roots of human criminal behavior were based on the concept of biological determinism, which explains and justifies human behavior as strictly a reflection of inborn human traits, with little or no attention paid to psychological or environmental influences. For example, Italian physician Cesare Lombroso reported in L’uomo delinquente (1876; The criminal man) that certain “inferior” groups, by virtue of their “apish” appearance, were, in actuality, evolutionary throwbacks with criminal tendencies. Since that time, Lombroso's theories have been thoroughly discredited as pseudoscientific and racially biased. More sophisticated, integrated biopsychosocial models and scientific theories have been developed to identify the multiple etiologies of human behavior, including criminality.

Among the best-known theories of human behavior to find support in the scientific community are those suggesting certain biochemical imbalances, particularly involving neurochemicals, potentially play a role in generating a wide range of abnormalities. Neurotransmitters activate behavioral tendencies and patterns in explicit brain areas, so imbalances in these chemicals might also negatively affect behavior. Research into the neurochemical foundations of behavior suggests that neurotransmitter imbalances may contribute to aggression, impulsivity, and other behavioral traits associated with criminality. Abnormalities in the brain’s levels of dopamine (another primary neurotransmitter) have also been implicated in aggressive and antisocial behavioral regulation in complex and context-dependent ways.

One of the most frequently studied neurotransmitters in this context is serotonin, which is linked to emotional regulation, aggression, and impulsivity. Low serotonin has been observed in individuals who engage in arson or animal cruelty, but these studies have been widely mixed. Another key neurotransmitter, dopamine, is primarily associated with reward processing and motivation. Dysregulation of dopamine has been observed in individuals with substance use disorders, antisocial traits, and attention deficit hyperactivity disorder (ADHD)—all conditions that may increase the risk of engaging in criminal behavior. Scientists have also explored the roles of the neurotransmitters gamma-aminobutyric acid (GABA) and glutamate. These chemicals are critical in maintaining neurological balance and regulating stress responses. Disruptions in GABA activity have been linked to increased anxiety, irritability, and behavioral dyscontrol. However, the link between these neurotransmitters and criminality is not direct or causal. Imbalances may influence behavioral tendencies, but they do not cause criminal behavior. Research also suggests that early life stress can affect the expression of genes involved in neurotransmitter regulation through epigenetic mechanisms, further complicating the relationship between biology and behavior.

Another proposed genetic explanation for criminal behavior involves an abnormality of the enzyme monoamine oxidase A (MAOA). This vital enzyme degrades certain neurotransmitters, including dopamine and epinephrine. Theoretically, criminal behavior is more liable when the normal levels of neurotransmitters in the brain are disrupted, leading to behavioral alterations. To date, no definitive causal link to criminal behavior has ever been established in individuals with a MAOA abnormality.

Perhaps the most widely researched theories of criminality have addressed potential genetic influences. Most early investigations in this area examined the role of an abnormality of the sex chromosomes—47,XYY—involving the presence of an additional Y chromosome in an otherwise normal male karyotype. Beginning in the 1960s, Dr. Patricia Jacobs proposed that those males who possess this extra Y chromosome were overrepresented in prisons and mental institutions. She studied nine males (out of over three hundred males in a maximum security prison) with an XYY karyotype. These XYY males had above-average height (generally over six feet tall) and below-average intelligence, exhibited personality disorders, and were more prone to have engaged in antisocial acts, leading to their incarceration. Several studies also supported these early findings, which generated considerable interest—and debate—into the abnormal behaviors potentially associated with an XYY condition. By the 1970s, multiple investigations into XYY males in various settings, not just prisons, yielded inconsistent findings concerning behavior. No definitive associations between XYY males and criminal behaviors have ever been demonstrated, and selection bias influenced early research.

A 2014 study in Finland discovered a link between violent crimes and two specific genes. First, the study provided further evidence that the MAOA gene and identified a variation of the cadherin 13 (CDH13) gene. In addition to violent crime, CDH13 has also been linked to attention deficit hyperactivity disorder (ADHD) and substance abuse. The study, which analyzed the genetic makeup of nearly nine hundred people convicted of violent crimes (murder, manslaughter, battery, etc.), found the majority of those studied to have one of these genes. While these researchers identified statistical correlations, causation cannot be inferred. CDH13 is more closely linked to neurodevelopmental disorders, and its role in criminality is speculative.

Researchers in the twenty-first century increasingly focused on epigenetics, which refers to changes in gene expression that do not alter the DNA sequence but are influenced by environmental factors. Epigenetic mechanisms can effectively “turn on” or “turn off” certain genes, shaping how the brain develops and responds to stress, emotions, and social cues. For example, the expression of the MAOA gene has been shown to vary based on early life experiences, including exposure to trauma, neglect, or abuse. This means that two individuals with the same genetic makeup may exhibit very different behavioral outcomes depending on their environment, providing a compelling framework to explain how genetic predispositions interact with life experiences to influence behaviors associated with criminality.

The biology of criminality is comparable to the biology of aggression, with testosterone (or similar androgens) typically being referenced to explain belligerent male behavior. Yet defining male criminal behavior in terms of excessive testosterone, or another biochemical entity, has almost become a cliché and one without solid scientific merit. A multimodal approach is instead preferable. Therefore, the roles of psychology and environment in criminal behavior must also be considered. When physiological dysfunction exists secondary to genetic dysfunction, cognitive deficits, and impulsiveness may also coexist, which sets the stage for criminal tendencies to be acted out. First, a neural defect in almost any form is frequently associated with impatience, irritability, and impulsiveness. Next, misperceptions and ideation, symptoms associated with many different kinds of antisocial behavior, increased anxiety, and the tendency to “act out” or “retaliate” for both real and imagined reasons. Finally, intellectual deficits diminish judgment and lessen the person’s ability to acknowledge feelings and describe them verbally rather than through inappropriate actions.

Genetic abnormalities influence numerous human characteristics, including mental capabilities and behavior, but ignoring psychology and environment in human characteristics is simplistic. After all, criminality refers to a violation of the law. Since there are numerous types of crimes and motivations for them (anger, revenge, financial gain), it is difficult to make claims of definitive, nonenvironmental links between biochemical disorders and criminal behavior without exploring all potential variables. In other words, the nature of human criminal behavior emerges from a complex interplay of biological, psychological, and environmental factors. The exact causes of aberrant behavior are complex and involve multiple influences, of which genetics is one critical component.

Impact and Applications

Research into the biological and genetic causes of criminality entered the public spotlight starting in the early 1990s as part of the US government’s Violence Initiative, championed by then Secretary of Health and Human Services Louis Sullivan. The uproar began when Frederick Goodwin, then director of the Alcohol, Drug Abuse, and Mental Health Administration (now the Substance Abuse and Mental Health Services Administration), made comments comparing urban youth to aggressive jungle primates. The public feared that research on genetic links to criminality would be used to justify the disproportionate numbers of Black and Hispanic Americans in the penal system. Psychiatrist Peter Breggin also warned that unproved genetic links would be used as an excuse to screen minority children and give them sedating drugs to intervene in their impending aggression and criminality. After all, forced sterilization laws had been enacted in thirty US states in the 1920s to prevent reproduction by the “feebleminded” and “moral degenerate.” In the early twenty-first century, the general public remained highly suspicious of any medical or genetic research that might be used to target and marginalize minority or disadvantaged groups as predisposed to “criminal” behavior.

In an era in which genes have been implicated in everything from bipolar disorders to the propensity to change jobs, the belief that genes are responsible for criminal behavior is very enticing. However, this belief may have severe ramifications. To the extent that society accepts the view that crime is the result of pathological and biologically deviant behavior, it is possible to ignore the necessity to change social conditions such as poverty and oppression that are also linked to criminal behavior. Moreover, this view may encourage individuals to claim that their “genes” made them do it. While biochemical diagnosis and treatment with medications may be simpler and, therefore, more appealing than social interventions, this is perhaps reminiscent of the days when frontal lobotomy was the preferred method of biological intervention for aggressive mental patients.

Criminality as a specific form of human behavior has been studied by scientists, psychiatrists, psychologists, sociologists, and others who ultimately seek to understand its causes, primarily hoping to lessen the occurrence and impact of its more deleterious manifestations. Researchers looking for solutions in genetics sometimes lose sight of the roles that psychology and the environment play in the various expressions of criminal behavior. The same can be said of social scientists, who sometimes ignore the roles of genetics and neurochemistry. It makes better sense to conclude that a combination of genetic, psychological, and environmental influences work in different ways for different individuals, leading to the development of criminal behavior in some but not all.

Key terms

• Metabolic pathway: a biochemical process that converts specific chemicals in the body to other, often more useful, chemicals with the help of proteins called enzymes.
• Neurotransmitter: a neurochemical that transmits messages between neurons.

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