The Surprising Gene Shared By Criminals - Kathryn Paige Harden

Kathryn Paige Harden explains that for most antisocial behaviors, the heritability is not inherently different between sexes. Research using fraternal twins shows that the same genetic liabilities for aggression and risk-taking predict behavior in both boys and girls. However, the average level of these behaviors is shifted higher for men, often due to their greater capacity to enact violence. Social factors also play a role, as seen in the narrowing gap of smoking and drinking rates as social acceptance for women increases. This section establishes that while the 'raw materials' are similar, the real-world impact varies by gender.

The speaker notes that most contemporary genetic studies have focused on the 22 pairs of autosomes which are identical across sexes. This focus has largely ignored the 23rd pair, the sex chromosomes, due to technical complexities in research. Men possess an XY configuration, making them significantly more vulnerable to any negative variants found on the X chromosome. Because women have two X chromosomes, they have a 'backup' copy that can compensate for genetic errors. This biological reality explains why conditions like colorblindness are far more prevalent in the male population.

Harden distinguishes between common genetic variants and rare mutations within the population. Common variants are those found in at least 1% to 5% of people and typically exert very small individual effects on behavior. Evolution tends to weed out common variants that have large negative impacts, creating a trade-off between frequency and power. In her lab, they look at thousands of these tiny effects added together to reach a meaningful conclusion about a person's traits. This polygenic approach helps explain the broad spectrum of human personality and behavioral tendencies.

A famous 1990s study of a Dutch family revealed a rare X-linked mutation in the MAOA gene that caused extreme violence. The MAOA enzyme acts like a 'Pac-Man' for neurotransmitters like serotonin and dopamine, and its failure leads to a massive chemical buildup in the brain. In this specific family, the men committed heinous crimes including arson and assault, while the women remained typically functioning. The women were protected by their second X chromosome, which provided a functioning version of the enzyme. This case study serves as a powerful example of how a single genetic 'glitch' can manifest solely in males.

The speaker argues that morality should be viewed as a biological faculty that is susceptible to physical disruption. By changing just one letter of the genome, the capacity to regulate behavior can be severely impaired or destroyed. Interestingly, the violent men in the Dutch study were not initially seen as having a medical or genetic problem by the legal system. Their behavior was indistinguishable from other offenders, and only familial data revealed the underlying biological cause. This raises profound questions about how many other persistently violent individuals may have undiscovered neurobiological explanations for their actions.

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