Psychopathic Traits Are Linked to the Microbes Living Inside Your Body, Study Finds
Brain scans have long pointed to structural differences in areas tied to empathy and fear, including the prefrontal cortex, as contributors to psychopathy, but those findings alone have never told the whole story. A 2026 study published in Translational Psychiatry is now pointing researchers somewhere unexpected: the microbial communities living in your gut and mouth.
The researchers, led by biomedical scientist Carolina Costa of the University of Porto in Portugal, recruited 200 healthy adult participants and asked them to complete the Self-Report Psychopathy Scale-Short Form, a validated questionnaire used to measure psychopathic traits across a general population. Each participant also donated blood, saliva, and fecal samples, which were analyzed for bacterial composition and various metabolic markers. The goal was to determine whether the microbiome and psychopathic traits overlap in any measurable way, and, according to the results, they do.
The link didn’t emerge from overall microbial diversity. When researchers looked at each person’s gut and oral microbes in isolation, no clear patterns appeared. The signal only became visible when participants’ profiles were compared with one another, and at that level, specific bacterial genera began to stand out in ways that tracked closely with psychopathy scores.
What the microbiome psychopathic traits connection actually looks like
Psychopathy is a personality construct characterized by emotional detachment, impulsivity, and antisocial tendencies, and despite a clinical prevalence of 1%, its biological underpinnings remain poorly understood. Researchers have known for years that brain structure plays a role, with abnormalities in the prefrontal cortex linked to the phenomenon. The Costa team argues that brain alterations alone are insufficient to fully explain psychopathic behavior, and that the microbiota-brain axis may account for part of what’s missing.
Three bacterial types stood out in the analysis. Higher abundance of bacteria from the genera Allisonella or Prevotella, or from the species Cloacibacillus evryensis, showed a positive association with stronger psychopathic traits. One species moved in the opposite direction: Treponema vincentii, found in the oral microbiome, showed a negative association with psychopathy markers, meaning people with more of it tended to score lower.
These are correlational findings, not causal ones. The study does not show that any of these bacteria produce psychopathic traits, nor does it suggest microbiome testing could diagnose anyone. What it does do is document a statistically measurable biological pattern that no previous research had looked for.
The bacteria involved and why they matter
Allisonella has the potential to promote inflammation, which is associated with anxiety and depression, and inflammation is increasingly recognized as a pathway through which gut activity can influence brain function. The connection to emotional blunting and reduced fear responses, both hallmarks of psychopathy, may run through that inflammatory channel, though that remains speculative at this stage.
Prevotella has previously been linked with neuropsychiatric disorders and is thought to affect emotional processing. Higher Prevotella levels correlating with psychopathy scores here adds a new dimension to that existing body of work.
The third bacterium, Cloacibacillus evryensis, may act through a different pathway altogether. Its association with psychopathy scores might be due to its suspected role as a producer of gamma-aminobutyric acid (GABA), the primary inhibitory neurotransmitter in the brain. GABA is involved in systems that regulate impulse control, one of the core traits that breaks down in psychopathy. If C. evryensis influences GABA production in ways that dysregulate impulse control, it would represent a plausible biological mechanism — though one that still requires direct testing.
The gut-brain link through which all of this could theoretically operate has been documented in broader research. The microbiota-brain axis involves bidirectional communication that runs partly through the vagus nerve, a long nerve connecting the brainstem directly to the gut, as well as through immune signaling and hormone-like molecules. The gut microbiome influences neurotransmitter production, inflammatory markers, and gene expression in ways that ripple out to affect mood, cognition, and behavior.
Limits of the research
This is a cross-sectional study, meaning it captured a single snapshot of each participant’s microbial profile and psychopathy score at one point in time. That design makes it impossible to determine whether the bacteria came first and influenced behavior, or whether behavioral patterns and lifestyle choices shaped the microbiome in the other direction, or whether both are driven by some third factor not captured in the analysis.
The study also draws from a healthy adult population. The psychopathy scores measured here reflect subclinical traits, the ordinary range of psychopathic tendencies that exist across the general population, not a clinical diagnosis. How these findings map onto individuals with severe or clinical psychopathy remains unknown.
The findings do not reduce complex traits to bacteria, and they do not justify diagnosing individuals from microbiome profiles. Instead, they point toward a future in which psychology, psychiatry, neuroscience, immunology, and microbial science work together to examine how bodily systems interact.
The study is currently available as an unedited early-release version in Translational Psychiatry and will undergo further editing before final publication, meaning additional review may refine the conclusions.
The relationship between the microbiome and psychopathic traits is new scientific territory, and at this stage it raises more questions than it answers. The research doesn’t suggest that eating certain foods or taking probiotics will prevent psychopathic behavior, and it does not provide a diagnostic tool. What it does suggest is that personality, including its most extreme expressions, may be shaped by biological systems that science is only beginning to map.
A diverse gut microbiome matters for far more than digestion. The same microbial community that affects your immune system, your mood, and your inflammation levels may also contribute, in ways still being studied, to the personality traits that define how you relate to other people. If you have questions about gut health or mental health, a licensed clinician is the appropriate starting point.
Disclaimer: The information provided here is for educational and informational purposes only and is not a substitute for professional psychological, psychiatric, or mental health advice, diagnosis, or treatment. Always seek the guidance of a licensed mental health professional, therapist, psychologist, or psychiatrist with any questions or concerns about your emotional well-being or mental health conditions. Never ignore professional advice or delay seeking support because of something you have read here.
AI Disclaimer: This article was created with the assistance of AI tools and reviewed by a human editor.
