There’s an old saying that tells us to trust our gut. It’s a nifty little metaphor for intuition—that quiet, instinctive feeling that something is right or wrong. But as it turns out, the phrase might be true in a much more literal way than we ever imagined. The trillions of bacteria that live in our intestines and mouths are not just passive hitchhikers. They help us digest food, regulate our immune systems, and even produce chemicals that our brains rely on. Scientists have known for years that these microscopic residents can influence our mental state, from anxiety to depression to stress. Now, a team of researchers has pushed the idea into darker territory. They wanted to know whether the bacteria in our bodies might be linked to psychopathic traits—the constellation of characteristics that includes lack of empathy, antisocial tendencies, impulsiveness, selfishness, and manipulativeness. In a study led by Carolina Costa from the University of Porto in Portugal, the researchers recruited 200 healthy adults and asked them to fill out a psychopathy questionnaire. Then they analyzed their blood, saliva, and fecal samples. What they found was surprising: people who scored higher on the psychopathy scale tended to have more of certain types of gut bacteria. The findings, described in a paper that at the time of reporting had not yet been peer-reviewed, but nonetheless reported by Science Alert and making waves in the science world, suggest that psychopathy might have a biological footprint not just in the brain, but in the gut. The researchers’ hypothesis was that the composition of the gut and oral microbiota may be associated with what are called “subclinical” psychopathic personality traits. In other words, you don’t have to be a criminal mastermind to have these traits. They exist on a spectrum, and all of us have a little bit of them. The question is whether the bacteria inside us are part of the reason why.
Let’s dive into the study itself, because it’s both fascinating and easy to misunderstand. The 200 participants were not prisoners, not serial killers, not people diagnosed with antisocial personality disorder. They were everyday adults. The researchers were interested in normal variation in human personality—the small, sometimes uncomfortable ways that we all can be callous, impulsive, or self-centered. Each volunteer completed the Self-Report Psychopathy Scale-Short Form, a well-known survey designed to quantify psychopathic traits in the general population. After that came the less glamorous part of the research: providing blood samples, saliva swabs, and stool samples. The team then analyzed the microbial communities in those samples, looking at both the diversity of bacteria and the presence of specific species. At first glance, the results seemed to show nothing. Individual microbial diversity—which is usually a sign of a healthy microbiome—didn’t correlate with psychopathy scores. You might expect that people with less diverse gut ecosystems would have more psychological problems, but that wasn’t the case. However, when the researchers compared participants to each other, not just looking at diversity but at the actual types of bacteria present, a different pattern emerged. Subjects with greater levels of three specific types of gut bacteria—Allisonella, Prevotella, and Cloacibacillus evryensis—also scored more highly on the psychopathy survey. Conversely, one microbe, Treponema vincentii, showed the opposite association: it was more common in people with lower psychopathic traits. The reason for that protective or negative association is unclear, but it adds a layer of complexity. This doesn’t mean that everyone walking around with Prevotella in their gut is a psychopath. Obviously not. These bacteria are common in many people, and most of us have a mix of all kinds of microbes. What it means is that, statistically speaking, there is an association between certain microbial communities and certain personality traits. And that is a very intriguing lead.
So what could explain this bizarre connection? The researchers are careful to say they don’t know the exact mechanism yet, but they have several ideas. First, Allisonella has been linked to a variety of diseases, and it may have the capacity to induce inflammation in the body. Inflammation has long been connected to mental health problems like anxiety and depression. If certain gut bacteria cause chronic low-grade inflammation, that inflammation could, in theory, affect the brain in ways that make a person more impulsive, less sensitive to others’ feelings, or more prone to aggressive reactions. It’s a pathway we’re seeing more and more in psychoneuroimmunology: the immune system and the brain are in constant conversation. Second, Prevotella is a genus that has previously been linked to psychiatric afflictions. Some research has associated it with depression, anxiety, and even autism spectrum conditions. It’s not that Prevotella is “bad” in some simple way—it’s actually a normal part of a healthy gut for many people—but in certain contexts, it may interact with the immune system or produce metabolites that influence mood and behavior. Third, Cloacibacillus evryensis is perhaps the most colorful character in this story. The researchers note that this bacterium produces a chemical in the brain that affects impulse control. That’s a big deal, because impulsivity is one of the core features of psychopathy. If a microbe in the gut can influence the production or regulation of a brain chemical that helps us pump the brakes before acting, then it might be nudging some people toward more impulsive, reckless, or even callous behavior. Again, all of this is speculative. The researchers did not prove that these bacteria cause any of these effects. They only observed associations. But the pattern is striking enough that they described it as a “measurable biological footprint for psychopathy that extends beyond traditional neurocognitive models.” In plain language: psychopathy isn’t just about the structure of the prefrontal cortex or the wiring of your brain. It might also be about the ecosystem of bacteria living in your digestive tract. That’s a radical idea, but it’s also one that fits with everything we’re learning about the gut-brain axis.
Before you start panicking and demanding a stool test, let’s take a deep breath. This research does not prove that a certain microbial formula is to blame for psychopathy. It only shows a correlation. Correlation is not causation, and the study itself has limitations. It’s possible that the bacteria influence the brain and behavior, which is the exciting and slightly terrifying hypothesis. But it’s equally possible that the relationship runs the other way. People with psychopathic traits might make different lifestyle choices: they might sleep poorly, eat badly, drink more alcohol, take risks, or experience chronic stress. All of those behaviors can shape the microbiome. Maybe the personality comes first, and the gut bacteria simply reflect the life that person is living. It could also be that a third factor—like genetics, early childhood experiences, or even environmental exposures—shapes both the brain and the gut in parallel, creating a false correlation. And it might be circular: bacteria affect mood, mood affects behavior, behavior affects diet, and diet affects bacteria. In other words, the exact recipe for evil, if it even exists, is a long way from being known. The study, by the researchers’ own admission, is preliminary and has not yet completed peer review. That doesn’t mean it’s worthless or wrong. It just means other scientists haven’t had the chance to scrutinize it fully. Also, the sample size is 200 people, which is respectable but far from enormous. And the psychopathy questionnaire relies on self-reporting, which comes with a built-in problem: people aren’t always honest about socially undesirable traits like manipulation, selfishness, and lack of remorse. Some people might even exaggerate, and some might downplay. So the data is messy, as human data always is. Still, even with all these caveats, the study is valuable. It points to something bigger: the idea that our minds are not confined to our brains. Our guts, our immune systems, and our microbial guests are all part of what makes us who we are.
If this line of research continues, it could open up entirely new ways of thinking about mental health and personality. For decades, the study of psychopathy has focused heavily on the brain, especially the prefrontal cortex, which is responsible for decision-making, impulse control, and empathy. Brain scans have shown differences in the structure and activity of this region in people with antisocial traits. But this study suggests that we might also need to look southward, toward the enteric nervous system—the complex network of neurons in the gut that communicates directly with the brain. The gut is often called the body’s “second brain,” and for good reason. It produces around 95% of the body’s serotonin, a chemical intimately involved in mood regulation. It’s also home to the microbiome, an ecosystem of thousands of microbial species that collectively weigh about as much as the brain itself. These microbes are not just passive passengers; they actively metabolize food, produce neurotransmitters, and send signals up the vagus nerve to the brain. The field of nutritional psychiatry is already exploring how diet and probiotics might help with depression and anxiety. Some researchers are even studying “psychobiotics”—probiotic strains designed to improve mental health. Could there one day be a psychobiotic for psychopathy? That sounds like science fiction, but the idea isn’t completely outlandish. If certain bacteria can influence impulse control or empathy, then changing someone’s microbiome might conceivably alter their personality traits, at least at the margins. Of course, that’s a very long way off, and we need to be careful not to dehumanize people who score high on psychopathy scales. Nobody is just their bacteria. People are complex, multifaceted beings, shaped by genes, upbringing, culture, and choice. But maybe the more we learn about the biological underpinnings of antisocial behavior, the less we’ll see it as some kind of mystical evil, and the more we’ll see it as a health issue that deserves research, compassion, and prevention.
So, what should we take away from all this? For one thing, this study is a reminder that human beings are walking ecosystems. We are not brains in jars. We are made up of our own cells plus trillions of bacterial guests, and those guests are active participants in our daily lives. They help us digest food, fight off disease, and maybe, just maybe, shape our moods and personalities. The researchers from the University of Porto have added a new and slightly unsettling piece to the puzzle. They’ve shown that there might be a measurable biological footprint of psychopathic traits that extends beyond the traditional boundaries of neuroscience. That’s strange. It’s humbling. It’s the kind of finding that makes you look at your dinner plate differently, wondering whether the probiotics in your yogurt are making you a better person. But it’s also hopeful. If personality traits are influenced by the microbiome, then perhaps they are more malleable than we thought. Maybe there are dietary, microbial, or therapeutic interventions that could nudge people toward more empathy, more self-control, and less impulsivity. Not just for the rare psychopaths of the world, but for all of us who have moments of selfishness, anger, or thoughtlessness. Before any of that can happen, though, we need much more research. We need longitudinal studies that follow people over time. We need experiments that test whether changing the microbiome can actually change behavior. We need replication, because one early study is never enough. The exact recipe for evil—if it exists—remains a mystery. Until then, maybe the best advice is also the oldest: take care of your gut. Eat your vegetables, get enough sleep, manage your stress, and remember that your microbiome is part of your story. It might be shaping you, or it might just be reflecting you, but either way, it’s with you for the ride. And if you’re worried about psychopathy, don’t rush to get your stool tested. Just try to be a little more thoughtful, a little kinder, and a little less impulsive. That, at least, is within your control. And that, in the end, is what matters most.
