Do Men Inherit More Violence Than Women? - Kathryn Paige Harden

Heritability of Antisocial Behavior by Sex00:00

Is there a difference in heritability of antisocial behavior that's sexed? Do men inherit more? Is the heritability greater effect on boys than on girls? Generally, no. But there's one exception to come back to. The genes associated with antisocial behavior in boys also affect girls. If you're female and have a fraternal twin that's a male sibling, his antisocial behavior predicts your likelihood of manifesting it. The same genetic liabilities are reflected in the same way. The same genetic liabilities make you more likely to be physically aggressive, relationally aggressive, substance using, and risk-taking.

It's just that the mean for men, the average for men, is shifted up. The same raw materials have a greater impact in real life. The same way women attempt suicide more than men, but men commit suicide more than women. Men's ability to enact violence and antisocial stuff tends to be greater, so it's magnified.

Part of that is around social opportunity. For many years, women were discouraged from drinking and smoking, leading to big sex differences. Now, it's more socially acceptable for women to smoke and drink, and that average difference has narrowed. The same genes are involved in both.

Role of the X Chromosome02:00

Most current studies have focused on the autosomes (22 pairs of chromosomes the same across sexes), not the sex chromosomes (XY or XX). We're now diving into the X chromosome to see if it has specific effects on antisocial behavior. Men have one X, women have two, so men are more vulnerable to X-linked genetic variants because they lack a compensating copy.

Example: Color blindness is much more prevalent in men because it's X-linked.

Most studies in the lab focus on common genetic variation (differences in at least 5% or 1% of the population). Common variants have small effects because if they had big effects, evolution would weed them out quickly. Lots of tiny effects add up to something meaningful. There are also studies of rare genetic variants.

MAOA Gene and the Dutch Family Study03:33

A famous 1990s study looked at a rare variant on a gene on the X chromosome called MAOA (monoamine oxidase). Monoamines (serotonin, dopamine) are how neurons communicate. MAOA is an enzyme like Pac-Man that eats neurotransmitters. If it doesn't work, there's a buildup of signals.

In one family, this X-linked variant made MAOA not work. All the men suffered extremely serious antisocial behavior problems, while their sisters were typically functioning:

• One raped his sister.
• One committed arson.
• One stabbed his boss with a pitchfork.

Their sisters and moms did not have this problem because they have two Xs; one functioning version compensated. Men have a 50-50 shot from mom's Xs whether they get the bad one.

That's a rare variant. The vast majority of deeply antisocial people do not have this MAOA problem; they can't use the MAOA excuse.

Implications for Morality and Undiscovered Genetics06:02

It's important for two reasons:

1. Our moral faculties (ability to not stab boss when mad) are vulnerable to disruption. Changing one letter in the genome changes one gene, one enzyme, and profoundly impairs that capacity. This supports morality as a biological faculty.
2. These men were in the Dutch criminal legal system, treated indistinguishably from other offenders based on behavior. No one thought genetic cause; they weren't found not guilty by insanity or lacking capacity. The genetic cause was clear only from familial transmission patterns.

This raises: How many people in persistently violent families have undiscovered genetic or neurobiological explanations? In the 1980s, suggesting one wrong gene for a persistently violent family would sound like science fiction, but it was true here.

Modern genomics hasn't focused much on the X chromosome, but this lab is now, and it's exciting.

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