The Genetic Basis of Antisocial Behavior: Sex Differences, the X Chromosome, and the MAOA Gene

This podcast episode delves into the heritability of antisocial behavior, exploring whether there are sex-specific differences in genetic predisposition. While general genetic liabilities for traits like physical aggression, relational aggression, substance use, and risk-taking are largely similar across sexes, manifesting as a shifted mean for men, a crucial distinction arises with the X chromosome. Most current genetic studies focus on autosomes, but new research is examining the X chromosome, which presents unique vulnerabilities for men who possess only one X, unlike women who have two and can compensate for a faulty gene.

The discussion highlights the difference between common genetic variation, which involves many variants each with small effects, and rare genetic variants that can have profound impacts. A prominent example is a 1990s study on a rare variant of the MAOA gene on the X chromosome. This variant caused the MAOA enzyme, responsible for breaking down neurotransmitters like serotonin and dopamine, to malfunction. In one family, all affected men exhibited extremely severe antisocial behaviors, including rape, arson, and violent assault, while their female relatives, despite potentially carrying the variant, were typically functioning due to the compensatory effect of their second X chromosome.

This case study offers two significant insights. Firstly, it underscores the biological underpinnings of what we perceive as moral faculties, demonstrating how a single genetic change can severely impair the capacity for prosocial behavior. This challenges purely moral or spiritual interpretations of morality by revealing its vulnerability to biological disruption. Secondly, it raises critical questions about the criminal legal system; the men in the MAOA study were indistinguishable from other offenders based solely on their behavior and were not deemed 'not guilty by reason of insanity,' yet their actions had a clear genetic cause.

The episode concludes by emphasizing the potential for undiscovered genetic or neurobiological explanations for persistent violence, particularly in families with a history of such behavior. The historical skepticism towards genetic causes for complex behaviors is contrasted with modern genomic capabilities, suggesting that many cases currently attributed solely to environmental or psychological factors might have underlying biological components yet to be identified. This calls for continued research into the X chromosome and other genetic factors to deepen our understanding of antisocial behavior and its implications for justice and societal well-being.

"the same genetic liabilities make you more likely to be physically aggressive. They make you more likely to be relationally aggressive. They make you more likely to be substance using. They make you more likely to be risk-taking."

"men are much more vulnerable to the effects of a genetic variant that's XL linked because they don't have another copy to compensate."

"color blindness for instance is much more prevalent in men versus women because it's a sex link. It's an X chromosome linked um genetic."

"The thing about common genetic variants is that um they're common which means that they are likely to have a relatively small effect in isolation because if they had a big effect evolution would make them not common would weed them out very very quickly."

"mono monoamine oxidase is an enzyme that basically is like a Pac-Man eating the neurotransmitter in your brain. And if it doesn't work well, then you get this incredible buildup of the signals that your brain ordinarily uses to communicate with each other."

"all the men in that family suffered from extremely serious antisocial behavior problems. Whereas their sisters were completely typically functioning."

"The vast majority of people who are deeply antisocial do not have this MAOA problem."

"the extent to which our morality is a biological faculty I think is very much supported by the fact that we can so profoundly disrupt it by this one change in our genome."

"these men were in the criminal legal system in the Netherlands and no one was like, "Oh, this must be a genetic problem.""

"how many other people who are persistently violent in families that are persistently violent? There might be some um genetic or neurobiological explanation that we just haven't discovered yet."