Your Gut Bacteria May Predict Psychopathic Traits, Portuguese Study Finds

If you've ever wondered whether the trillions of microbes living in your gut shape more than just digestion, a new study adds a striking, unsettling data point. Researchers from the University of Porto in Portugal and the University of Oxford in the UK have found that specific bacteria in the gut and mouth are statistically linked to psychopathic personality traits, the cluster of characteristics that includes low empathy, manipulativeness, and impulsivity. The study, published in Translational Psychiatry, doesn't show that bacteria cause psychopathy, but it does add psychopathy to a growing list of behaviors researchers are trying to trace back to the gut-brain axis (the two-way communication network linking gut microbes to brain function).

Key Takeaways

• A 200-person study found that gut bacteria including Allisonella and Prevotella, plus the oral bacterium Treponema vincentii, tracked with self-reported psychopathy scores.
• The overall community composition of gut bacteria (not just one species) differed meaningfully between people who scored higher versus lower on psychopathy measures.
• This is a single, cross-sectional study, it shows a correlation, not proof that gut bacteria cause psychopathic traits, and the finding needs replication before it means much clinically.

Specific Gut and Oral Bacteria Track With Psychopathy Scores

The core finding is straightforward: certain microbes recur more often in people who score higher on psychopathy measures, and others recur more often in people who score lower. In gut samples, the bacteria Allisonella, Prevotella, and Cloacibacillus evryensis were positively associated with psychopathy scores, people with more of these microbes tended to score higher. In mouth swabs, the opposite pattern showed up for Treponema vincentii (an oral spirochete, a corkscrew-shaped type of bacterium), which was negatively associated with psychopathy scores, people with more of it tended to score lower.

Why a Personality-Microbiome Link Matters Beyond This One Study

Psychopathy is usually studied as a psychological or neurological trait, but this study places it inside a much broader research trend connecting gut microbes to mood, anxiety, and behavior generally. Other recent work has found that people who self-report symptoms like anxiety, depression, and sleep problems show distinct gut microbiome differences compared with people without those symptoms, and reviews of the field describe the gut-brain axis as running through the vagus nerve, the immune system, and microbial metabolites like short-chain fatty acids. If a personality trait as specific as psychopathy shows a microbiome signature, it strengthens the broader case that gut composition is worth studying alongside genetics and environment for personality traits, not just the mood conditions the field has focused on so far.

A 200-Person Study Built to Rule Out the Obvious Confounds

The study drew on 200 participants who completed the Self-Report Psychopathy Scale Short Form, along with separate measures of empathy and anxiety, so the researchers could distinguish psychopathy-linked patterns from general negative mood or low empathy that might be an artifact. The team analyzed microbiota composition from both fecal and oral samples, plus targeted and untargeted metabolomics (large-scale chemical profiling of small molecules) in blood plasma and fecal samples, then used a statistical technique called K-means clustering to sort participants into groups based on shared biological profiles and checked whether those clusters lined up with psychopathy scores. The metabolomics data echoed the microbiome pattern: fecal levels of glucose and taurine (an amino acid involved in bile and nerve signaling) differed between the clusters and rose alongside psychopathy scores.

Notably, it was beta-diversity, a measure of how different the overall bacterial community composition is between individuals, that tracked with psychopathy scores, not alpha-diversity, which measures how many species live within a single person's gut. In plain terms: people with higher psychopathy scores didn't have a more or less diverse gut ecosystem overall, but the specific mix of species they carried looked systematically different from people with lower scores.

What This Could Mean If You're Curious About Your Gut

For now, this study doesn't translate into anything actionable for an individual reader. No test exists that reads gut bacteria and outputs a psychopathy score, and carrying Prevotella or Allisonella isn't evidence of anything about someone's personality, both are common gut genera found across a wide range of healthy people. What the study does add to is a broader pattern connecting the microbiome and behavior. Separate research out of UCLA, for instance, found that women with more Prevotella in their gut microbiome showed reduced activity in the hippocampus (a brain region involved in emotion and memory) and reported more negative feelings like anxiety and irritability, a different study and outcome, but the same genus turning up in behavior-linked microbiome research. If you're already working with a doctor on a gut-related condition, this isn't a reason to change anything, but it's a reasonable thing to bring up if you're curious how gut research connects to broader questions about mood and behavior.

The Study Can't Show Bacteria Cause Psychopathic Traits

The biggest limitation is built into the design. It's cross-sectional, meaning researchers measured gut bacteria and psychopathy scores at a single point in time rather than tracking people over months or years, so the data can show two things moving together without showing that one causes the other. It's entirely possible that psychopathic traits, associated behaviors, or diet patterns common among people with those traits shape the gut microbiome, rather than the other way around. The study is also a single result that hasn't been replicated in an independent sample, which matters in microbiome research, a field known for findings that don't hold up when other labs try to reproduce them. And psychopathy itself was measured with a self-report questionnaire, not a clinical diagnosis by a trained evaluator, so the study captures psychopathic traits on a spectrum in a general population sample rather than diagnosed psychopathy in a clinical population.

What Comes Next for This Line of Research

The researchers, who include Benedita Sampaio-Maia at the University of Porto and Philip W. J. Burnet at the University of Oxford, disclosed no competing interests and were funded through Portuguese FCT/MCTES scholarships and a Dorothy Hodgkin Early Career Fellowship, funding sources with no commercial stake in the outcome. The logical next step, as with other gut-brain axis research, is replication in an independent, larger sample, ideally paired with longitudinal tracking that can start to untangle which direction the relationship runs. Until then, this remains an early, intriguing correlation rather than an established biological mechanism.

Common Questions

Does this mean gut bacteria cause psychopathic traits? No. The study only shows an association at a single point in time; it cannot determine whether the bacteria influence psychopathic traits, the traits influence gut bacteria through diet or behavior, or a third factor drives both.

Can a gut bacteria test diagnose psychopathy? No such test exists, and this study doesn't support building one. The findings are a population-level statistical pattern, not a reliable marker in an individual person.

What is the gut-brain axis? It's the two-way communication network between gut microbes and the brain, running through the vagus nerve, the immune system, and chemical signals microbes produce. It has previously been linked to depression and anxiety, and this study adds psychopathy to that list.

Has this finding been replicated? Not yet. It's a single cross-sectional study with 200 participants, and independent replication is the natural next step before the finding can be considered established.

References

1. B. Sampaio-Maia, P. W. J. Burnet, et al. (2026). Gut and oral microbiota composition is associated with psychopathic personality: a cross-sectional study. Translational Psychiatry. https://doi.org/10.1038/s41398-026-04052-z