The parasite is best known for changing the behavior of rodents in ways that make them easier prey—an infection seems to make mice permanently lose their fear of cats. Research in humans is nowhere near conclusive, but some studies have linked infections with the parasite to personality changes, increased aggression, and impulsivity.
“That’s an example of microbiology that we know affects the brain and could potentially affect the legal standpoint of someone who’s being tried for a crime,” says Allen-Vercoe. “They might say ‘My microbes made me do it,’ and I might believe them.”
There’s more evidence linking gut microbes to behavior in mice, which are some of the most well-studied creatures. One study involved fecal transplants—a procedure that involves inserting fecal matter from one animal into the intestines of another. Because feces contain so much gut bacteria, fecal transplants can go some way to swap out a gut microbiome. (Humans are doing this too—and it seems to be a remarkably effective way to treat persistent C. difficile infections in people.)
Back in 2013, scientists at McMaster University in Canada performed fecal transplants between two strains of mice, one that is known for being timid and another that tends to be rather gregarious. This swapping of gut microbes also seemed to swap their behavior—the timid mice became more gregarious, and vice versa.
Microbiologists have since held up this study as one of the clearest demonstrations of how changing gut microbes can change behavior—at least in mice. “But the question is: How much do they control you, and how much is the human part of you able to overcome that control?” says Allen-Vercoe. “And that’s a really tough question to answer.”
After all, our gut microbiomes, though relatively stable, can change. Your diet, exercise routine, environment, and even the people you live with can shape the communities of microbes that live on and in you. And the ways these communities shift and influence behavior might be slightly different for everyone. Pinning down precise links between certain microbes and criminal behaviors will be extremely difficult, if not impossible.
“I don’t think you’re going to be able to take someone’s microbiome and say ‘Oh, look—you’ve got bug X, and that means you’re a serial killer,” says Allen-Vercoe.
Either way, Prescott hopes that advances in microbiology and metabolomics might help us better understand the links between microbes, the chemicals they produce, and criminal behaviors—and potentially even treat those behaviors.
“We could get to a place where microbial interventions are a part of therapeutic programming,” she says.
This article first appeared in The Checkup, MIT Technology Review’s weekly biotech newsletter. To receive it in your inbox every Thursday, and read articles like this first, sign up here.
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