Genetics : Are our fates are sealed in our DNA ? Part 1
Written in the Code · Blog 1
In 1978, a woman in the Netherlands wrote to her university medical centre with a concern about the men in her family. She had noticed a pattern. Over five generations, the males on one branch of her family had been involved in an unusual number of violent incidents — not ordinary arguments, but serious, impulsive acts. One had committed arson. One had been convicted of rape. Several had attacked family members, including one who had stabbed a superior with a pitchfork after a minor disagreement at work. Another had attempted to run over his employer with a car. Yet the women in the family were entirely unaffected.
She suspected a genetic cause. She was right. Fifteen years later, in 1993, a team led by Han Brunner at Nijmegen University Medical Centre published their findings in Science. Every affected male in the family carried the same thing: a point mutation in a gene on the X chromosome that completely eliminated the function of an enzyme called monoamine oxidase A. The gene was MAOA. The syndrome was named Brunner syndrome. And the question it raised — whether a single gene could predispose a person to violence — has not been answered without controversy in the thirty years since.
What MAOA Does — and What Happens When It Doesn't
Monoamine oxidase A is an enzyme found primarily in the outer membrane of mitochondria in neurons and other cells. Its job is straightforward and critical: it breaks down monoamine neurotransmitters — primarily serotonin, dopamine, and norepinephrine — after they have been released into synapses and reabsorbed. MAOA is, in the language of neuroscience, the cleanup crew. It ensures that these neurotransmitters do not accumulate to excess after firing.
When MAOA is absent or severely impaired, these neurotransmitters build up. Serotonin, dopamine, and norepinephrine all accumulate to abnormal levels in the synaptic gap and within neurons. The behavioural consequences — as the Dutch family demonstrated across five generations — include impulsive aggression, difficulty modulating emotional responses to threat or frustration, and in the most severe cases, serious violence. The affected males in the Brunner family had borderline intellectual disability alongside their behavioural traits, suggesting broad neurological disruption.
The causal chain is almost elegantly simple: break the cleanup enzyme, the chemical messengers accumulate, the brain's ability to modulate aggression fails. MAOA was the first gene ever linked directly to human aggression — a finding so politically charged that it was initially rejected for presentation at a conference in 1992 before eventually being published in one of science's most prestigious journals.
From One Family to the Whole Population: MAOA-L
The Brunner syndrome mutation — which completely eliminates MAOA function — is vanishingly rare. But it pointed researchers toward a more common variant: a length polymorphism in the MAOA gene promoter that determines how much enzyme is produced. The shorter version of this repeat sequence (2 or 3 repeats), known as MAOA-L, produces substantially less enzyme than the longer version (MAOA-H, 4 or more repeats). MAOA-L is not a broken gene — it is a quieter one. About 35% of men carry it.
For years, MAOA-L was studied with mixed results — some studies found associations with aggression, others didn't. Then in 2002, Avshalom Caspi and colleagues at King's College London published a study in Science that would become one of the most cited papers in behavioural genetics. They followed a large New Zealand birth cohort and found that MAOA-L alone did not reliably predict violent behaviour. But the combination of MAOA-L and severe childhood maltreatment was powerfully predictive. Boys who had been maltreated and who also carried MAOA-L were significantly more likely to develop antisocial behaviour, conduct disorder, and violent convictions as adults than maltreated boys with MAOA-H, or unmaltreated boys with MAOA-L.
The finding crystallised what would become a paradigm in behavioural genetics: genes and environments interact, and neither alone may be sufficient. MAOA-L was not a sentence. It was a loaded gun. The trigger was the environment.
The Courtroom: When Genes Become a Defence
The MAOA research has had an extraordinary second life in criminal courts. The logic is seductive: if a defendant's violent behaviour was partially caused by a genetic variant they did not choose to have, should their culpability be reduced? Courts in several countries have grappled with precisely this question.
In 2009, an Italian appeals court reduced the sentence of Abdelmalek Bayout — convicted of murder — partly on the basis that he carried low-activity MAOA alleles alongside several other genetic variants that expert testimony linked to aggression. The reduction was modest (one year off a nine-year sentence), but it was the first time a European court had used genetic evidence in a sentencing decision in this way. The case was reported worldwide and ignited fierce debate.
In American courts, MAOA evidence has been raised in capital sentencing proceedings — not to argue innocence, but to argue that a defendant's genetic predisposition to violence constitutes a mitigating circumstance that should weigh against the death penalty. Courts have been inconsistent in how they receive such evidence.
The X-Linked Inequality: Why Only Men
One of the most important biological features of MAOA is its location on the X chromosome. Men have one X chromosome; women have two. A man who inherits a low-activity or non-functional MAOA allele on his single X has no backup copy — that allele determines his MAOA expression entirely. A woman who inherits the same allele on one X will almost always have a functional allele on her other X, providing sufficient enzyme regardless. This is why all five affected individuals in the Brunner family were male. It is why virtually all research on MAOA-L and aggression focuses on men. The Y chromosome confers vulnerability to this particular genetic fate by virtue of what it lacks.
The Evolutionary Question: Why Is MAOA-L Common?
If MAOA-L were simply a harmful variant, evolution should have reduced its frequency over time. It hasn't. Roughly 35% of men across diverse human populations carry it. This frequency suggests that MAOA-L carried, at some point in human evolutionary history, some advantage — or that its costs only manifest in specific environments that were rare or absent for most of human prehistory.
Several hypotheses have been proposed. Lower MAOA activity means higher circulating monoamines — which could enhance vigilance, aggressive defence of resources or family, and tolerance for physical conflict. In an ancestral environment where territorial competition and inter-group violence were common, these traits may have been adaptive. The "warrior gene" label, originally applied to Māori populations in New Zealand who were found to have high MAOA-L frequencies (a finding that generated significant controversy and subsequent debate about its interpretation), reflects this evolutionary framing — though the label has been widely critiqued for its reductiveness and potential for misuse.
What is clear is that MAOA-L is not a simple defect to be eliminated. It is an ancient variant, present across human populations at substantial frequency, that shapes neurotransmitter metabolism in ways that interact with developmental environment in complex and sometimes devastating ways. The Dutch family's tragedy was not merely genetic. It was the intersection of a specific gene with specific lives, specific histories, specific provocations. The gene shaped the probability. The world pulled the trigger.
The Dutch Family: A Portrait in Five Generations
The affected branch of the family can be traced back to an ancestor whose behaviour was described in family records as "peculiar" and who had difficulty maintaining employment due to unpredictable outbursts. The MAOA mutation is presumed to have arisen or been inherited in this generation.
Multiple affected males across three further generations. Documented incidents include: arson (setting fire to family property following a perceived slight), attempted murder of a sister, rape of a female relative, and stabbing of a workplace supervisor. Female family members across all generations remained unaffected, consistent with X-linked inheritance.
A female family member — an obligate carrier (carrying one functional and one non-functional MAOA allele) — writes to Nijmegen Medical Centre requesting investigation of the hereditary pattern she has observed. This initiates the fifteen-year investigation.
Brunner, Nelen, Breakefield, Ropers, and van Oost publish their findings identifying the MAOA point mutation (premature stop codon) as the cause of Brunner syndrome. Every affected male in the family is confirmed to carry it. Unaffected males and all females do not. The paper becomes one of the most discussed in neuroscience history — and one of the most politically fraught.
The woman who wrote that letter in 1978 had no idea she was handing science one of its most significant — and most troubling — discoveries. She wanted to understand why the men in her family kept doing terrible things. The answer was in their chromosomes. Whether that answer changes what those things meant, who was responsible for them, and what we owe people who carry such genes — those questions remain open, and the MAOA debate is unlikely to close them soon.
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