No single gene makes anyone a serial killer. “Serial killer gene” is a media nickname, not a scientific finding, and no gene has been shown to create killers.1SQ Online. The Truth About the Serial Killer Gene The gene behind the label is real, though. It helps clear mood-related chemical messengers from the brain, and it has a measurable but small link to aggression.2MedlinePlus Genetics. MAOA gene3PubMed. Candidate genes for aggression and antisocial behavior: a meta-analysis of association studies of the 5HTTLPR and MAOA-uVNTR That link shows up mainly alongside harsh childhood experiences, which makes it a story of genes and upbringing working together rather than fate.
Where the Serial Killer Gene Idea Came From
In the 1990s, researchers studied a large Dutch family in which several men showed mild intellectual disability and repeated impulsive aggression.4PubMed Central. X-linked borderline mental retardation with prominent behavioral disturbance: phenotype, genetic localization, and evidence for disturbed monoamine metabolism. Their behaviors included violent outbursts, arson, attempted rape, and exhibitionism.5PubMed. Abnormal behavior associated with a point mutation in the structural gene for monoamine oxidase A6PubMed Central. X-linked borderline mental retardation with prominent behavioral disturbance: phenotype, genetic localization, and evidence for disturbed monoamine metabolism. In each affected man tested, a single typing error in a gene called MAOA shut the gene down completely.7PubMed. Abnormal behavior associated with a point mutation in the structural gene for monoamine oxidase A Their bodies made no working MAOA enzyme at all, and their urine showed badly disturbed handling of brain chemical messengers.8PubMed. Abnormal behavior associated with a point mutation in the structural gene for monoamine oxidase A9PubMed Central. X-linked borderline mental retardation with prominent behavioral disturbance: phenotype, genetic localization, and evidence for disturbed monoamine metabolism.
A gene is a long string of chemical letters, and in this family one changed letter introduced a premature “stop” signal, so the cell could not finish building the enzyme. The enzyme was missing entirely, not just weaker.
Normally, MAOA makes an enzyme that breaks down serotonin, dopamine, norepinephrine and adrenaline after they have done their job.10MedlinePlus Genetics. MAOA gene It sits inside cells alongside a sister enzyme, MAOB, which this family had working normally.11PubMed Central. X-linked borderline mental retardation with prominent behavioral disturbance: phenotype, genetic localization, and evidence for disturbed monoamine metabolism.
A fully broken gene is extremely rare, and it is not the gene most people carry. Most people have a working copy in a higher or lower activity form. Headlines later blurred these two very different situations, and nobody in this research was a serial killer. Media later dubbed MAOA the “warrior gene” and the “serial killer gene.”
Does Carrying the Low-Activity Version Make You Violent
Unlike the Dutch mutation, a common difference near the MAOA gene sets it to make more or less enzyme. These are called the high-activity and low-activity versions. Pooling many studies, the low-activity version is linked to antisocial behavior, but the effect is modest.12PubMed. Candidate genes for aggression and antisocial behavior: a meta-analysis of association studies of the 5HTTLPR and MAOA-uVNTR
Results also swing widely from study to study without a clear reason.13PubMed. Candidate genes for aggression and antisocial behavior: a meta-analysis of association studies of the 5HTTLPR and MAOA-uVNTR Some studies found a clear link and others found little or none.
Among Finnish prisoners, the low-activity version and another gene, CDH13, were more common in men with 10 or more violent crimes.14PubMed Central. Genetic background of extreme violent behavior But that comes from an extreme group of repeat offenders, not ordinary people.
Much of this research focuses on men. MAOA sits on the X chromosome, so men have one copy and women have two. A man’s single copy decides his version, which makes effects easier to see in men.
The low-activity version is common, carried by roughly a third of men in many populations. If it caused violence, a huge share of the population would be violent.
“Linked to” is often misread as “causes.” It means the version is slightly more common among aggressive people, not that it drives aggression in each carrier. Most carriers live ordinary lives, and aggression is a long way from murder.
Why Childhood Abuse Changes the Picture
The gene matters most alongside maltreatment. In one study of abused children, those with the high-activity version were less likely to develop antisocial problems.15PubMed. Role of genotype in the cycle of violence in maltreated children Across many studies, the low-activity version raised the risk of antisocial behavior in people who were maltreated, but not in people facing other childhood hardships.16PubMed Central. MAOA, childhood maltreatment and antisocial behavior: Meta-analysis of a gene-environment interaction In one long-term study, however, the protective effect of the high-activity version appeared in white participants but not in non-white participants.17PubMed. MAOA and the “cycle of violence:” childhood abuse and neglect, MAOA genotype, and risk for violent and antisocial behavior So the pattern is not identical in every group. “Maltreatment” here typically means serious abuse or neglect, such as physical abuse or sustained harsh treatment, not ordinary strict parenting.
Boys and girls can differ. In a large teen study, maltreated boys with the low-activity version and maltreated girls with the high-activity version had higher delinquency risk.18PubMed. Maltreatment, MAOA, and delinquency: sex differences in gene-environment interaction in a large population-based cohort of adolescents In another study, women with the high-activity version scored higher on reactive aggression, with no difference in men.19PubMed Central. The effects of MAOA genotype, childhood trauma, and sex on trait and state-dependent aggression
“Gene times environment” means the gene raises sensitivity rather than acting as a switch. A harmful setting brings out the difference, while a safe one largely hides it.
Experience can mark genes too. Childhood trauma can change chemical tags on genes that control whether they switch on, without changing the genes themselves.20PubMed Central. Epigenetic Modifications in Stress Response Genes Associated With Childhood Trauma21PubMed. Epigenetics of childhood trauma: Long term sequelae and potential for treatment Trauma adds tags to a stress-hormone gene, and such tags appear in brain tissue of trauma survivors who died by suicide.22PubMed Central. Epigenetic Modifications in Stress Response Genes Associated With Childhood Trauma
What the Gene Seems to Do in the Brain
In scans, people with the low-activity version showed a more reactive alarm center in the brain during emotional tasks.23PubMed. Monoamine oxidase A (MAOA) genotype predicts greater aggression through impulsive reactivity to negative affect24PubMed Central. Neural mechanisms of genetic risk for impulsivity and violence in humans They also showed less activity in the brain area that acts as a brake on emotions.25PubMed. Monoamine oxidase A (MAOA) genotype predicts greater aggression through impulsive reactivity to negative affect26PubMed Central. Neural mechanisms of genetic risk for impulsivity and violence in humans The version was linked to smaller volume in emotion-related brain areas too.27PubMed Central. Neural mechanisms of genetic risk for impulsivity and violence in humans The pattern fits hot-tempered, impulsive reactions.28PubMed. MAOA and the neurogenetic architecture of human aggression29PubMed Central. Neural mechanisms of genetic risk for impulsivity and violence in humans
These scans show reactions in a controlled setting, a long way from real-life choices. The differences are group averages, not visible in one person.
Researchers separate reactive violence, done in anger, from instrumental violence, planned for a goal.30PubMed. Psychopathy in instrumental and reactive violent offenders Planned killings are more tied to psychopathic traits like lack of remorse and callousness.31Homicide Studies. A Link between Psychopathy Affect and Instrumentality in Homicide Psychopathy involves weaker links between the brain’s alarm center and the area for empathy and guilt.32School of Medicine and Public Health. Psychopaths’ Brains Show Differences in Structure and Function33PubMed Central. A cognitive neuroscience perspective on psychopathy: evidence for paralimbic system dysfunction So the MAOA pattern points toward impulsive violence, while many serial murders look planned.
Hormones add another layer. Testosterone and the MAOA version together predicted antisocial behavior in one study. In frustration tasks, men’s baseline testosterone mattered differently depending on their version. Giving extra testosterone did not consistently interact with the gene, so there is no simple formula.
Violence Involves Thousands of Genes and Many Life Events
The largest genetic study of antisocial behavior, of nearly 26,000 people, found it is shaped by many genes with small effects.34PubMed Central. Genome-Wide Association Studies of a Broad Spectrum of Antisocial Behavior Genetic effects may also differ between men and women.35PubMed Central. Genome-Wide Association Studies of a Broad Spectrum of Antisocial Behavior
To capture this, researchers combine many risk variants into a single genetic risk score.36PubMed Central. Using polygenic scores to assess liability to antisocial behavior To build one, they scan the genomes of many people and find thousands of places where letters differ slightly in people with a given trait. Each difference is weighted by how strongly it was linked, and the weights are added into one number per person. Higher scores were linked to conduct problems in childhood and criminal convictions.37PubMed. Uncovering the genetic architecture of broad antisocial behavior through a genome-wide association study meta-analysis
Scores describe groups and cannot predict any one person’s actions. Most were built from people of European ancestry, so they work less well for others, and a score is only as good as the data behind it.
Life events weigh heavily too. Childhood abuse and neglect, brain injury, substance use and poverty all add to risk alongside genes. Head injuries are common in prison populations and raise the risk of depression, substance abuse, and suicidal thoughts.38The Wiley Blackwell Handbook of Forensic Neuroscience. Neurobiology of Brain Injury and its Link with Violence and Extreme Single and Multiple Homicides One review reported that most of the most high-profile serial killers had significant brain injuries.39PubMed Central. Head Injuries and Serial Killers: Explore the Link Between Head Trauma and Criminal Behaviour But that is a small, famous group, not proof that injury causes killing.
How Courts Have Treated MAOA Evidence
Defendants have tried using the gene. In reviewed cases, MAOA evidence was allowed at trial in one of two cases and may have helped toward a lesser charge.40PubMed Central. The Forensic Use of Behavioral Genetics in Criminal Proceedings: Case of the MAOA-L Genotype It was allowed at sentencing in four of five cases, with one lighter sentence.41PubMed Central. The Forensic Use of Behavioral Genetics in Criminal Proceedings: Case of the MAOA-L Genotype Two of five appeals using it led to reduced sentences.42PubMed Central. The Forensic Use of Behavioral Genetics in Criminal Proceedings: Case of the MAOA-L Genotype It is hard to tell whether the gene evidence caused those outcomes, because its effect on any one person can’t be shown.43PubMed Central. The Forensic Use of Behavioral Genetics in Criminal Proceedings: Case of the MAOA-L Genotype The cases are few, and defendants usually present many arguments at once, such as abuse history and mental health, so no single one can be credited.
New Mexico’s Supreme Court ruled unanimously that a judge could exclude a murder defendant’s MAOA evidence.44PubMed. State v. Yepez: Admissibility and Relevance of Behavioral Genetic Evidence in a Criminal Trial Judges have reasons for caution: group findings cannot prove what drove one act, most carriers never commit crimes, and juries may overweight science-sounding evidence.
The argument cuts both ways. A defense may argue reduced blame because of genes plus abuse, while prosecutors could argue the same gene shows ongoing danger.
Should You Worry About Having the Gene
Being labeled genetically “at risk” for aggression can lead to unfair treatment at work, in insurance, and at school.45PubMed Central. Ethical issues in the use of genetic predictions of aggressive behavior in the criminal justice system: a systematic review Such labels can also damage self-esteem and a sense of control over one’s own life.46PubMed Central. Ethical issues in the use of genetic predictions of aggressive behavior in the criminal justice system: a systematic review
Some consumer DNA reports flag the low-activity version. It is common and not a diagnosis. Professionals assess behavior, not genotype.
A test reports which version someone carries, and that version is common among people who are not violent. It cannot say whether the person has been mistreated, how they react to stress, or what they will choose. A clean result does not guarantee gentleness.
What helps is support. In school programs for youth, some genetic versions predicted who benefited from particular formats, but programs reduced aggression broadly.47PubMed Central. Reducing Aggression and Impulsivity Through School-based Prevention Programs: A Gene by Intervention Interaction Support and safe environments help regardless of genes. If you are worried about anger or impulsive behavior, talk with a doctor or therapist rather than seeking a gene test.
With children, explain that genes are not destiny, and emphasize choices and support. Avoid joking labels like “warrior gene.”