Scientists Link Gut and Mouth Bacteria to Psychopathic Personality Traits

If you already follow gut-health research, you've probably seen headlines tying gut bacteria to anxiety, depression, or mood swings. A new study pushes that research into far stranger territory: the personality traits that make up psychopathy. Researchers from the University of Porto and the University of Oxford analyzed gut and mouth bacteria in 200 healthy adults and found that several specific microbial species track with how strongly someone scores on a standard psychopathic-traits questionnaire. Nobody in the study was diagnosed with anything, and the researchers are careful to call the results exploratory, but it is the first time anyone has looked for a biological signature behind this specific personality construct.

Key Takeaways

• A study of 200 healthy adults found that specific gut and oral bacteria species track with scores on a standard psychopathic-personality-traits questionnaire.
• Two fecal metabolites, glucose and taurine, were also elevated in the group with the highest combined psychopathy and anxiety scores.
• The link is correlational and small: bacterial differences explained less than 1% of the variation, and causality has not been shown.
• This is the first study of its kind, unreplicated, and does not diagnose, screen, or label any individual; the researchers stress that evaluating psychopathy requires a trained professional.

Gut and Oral Bacteria Differences Track With Psychopathic Trait Scores

Researchers found that the relative abundance of specific gut and mouth bacteria species correlates with scores on a standard measure of psychopathic personality traits (psychopathy: a personality pattern marked by low empathy, manipulation, impulsivity, and antisocial behavior), according to a study published in Translational Psychiatry. The team, led by biomedical scientist Carolina Costa, is the first to test whether this connection extends to psychopathic traits specifically, rather than the broader personality dimensions like neuroticism and conscientiousness that prior gut microbiome research has already examined, and it sits within the wider microbiota-gut-brain axis field tracking how gut bacteria interact with brain function through nervous-system, immune, and metabolic pathways.

Four gut bacteria groups, Allisonella, Prevotella, Ruminococcaceae DTU089, and Cloacibacillus evryensis, differed in abundance between participant groups, along with one species in the mouth, Treponema vincentii. Of those, Allisonella, Prevotella, and C. evryensis were positively associated with psychopathy scores, meaning higher abundance tracked with higher trait scores, while T. vincentii moved in the opposite direction. The researchers also measured 203 different metabolites (small molecules produced during digestion and metabolism) in blood and stool, and found that only two, fecal glucose and fecal taurine, differed significantly between groups, both running higher in participants with the highest combined psychopathy and anxiety scores.

Why a Gut Microbiome Link to Personality Traits Matters

This matters to anyone already tracking gut-brain research for anxiety, depression, or mood, because it extends that body of science into a personality domain that has had almost no biological research behind it. Psychopathy affects an estimated 1% of the general population as a clinical diagnosis, but like most personality traits, it exists on a spectrum, and most people carry at least some of its component traits (poor impulse control, low empathy, manipulation) at a subclinical level without ever meeting criteria for a disorder. Earlier research has already linked gut microbiota composition to broader traits such as sociability and neuroticism; this study is the first to ask whether that connection extends to the affective and interpersonal traits that make up psychopathy specifically, a construct the clinical literature still cannot fully explain through brain differences alone.

How the Study Measured Bacteria and Psychopathic Traits

The researchers recruited 200 adult volunteers (ages 19 to 64, average age 42, 77% female) from the community in Portugal, screening out anyone with obesity, active neurological or gastrointestinal disease, diabetes, recent antibiotic use, or a substance abuse history, so the sample reflects a healthy, treatment-naive population rather than a clinical or forensic one. Participants completed the Self-Report Psychopathy Scale-Short Form, a 29-item questionnaire, along with separate measures of empathy and anxiety. The team then sequenced bacterial DNA (16S rRNA gene sequencing, a standard method for identifying which bacterial species are present in a sample) from fecal and saliva samples, and ran metabolomics (chemical profiling of small molecules) on blood and stool. Using an unsupervised statistical method called k-means clustering, they sorted participants into four groups based on combined psychopathy, empathy, and anxiety scores, ranging from a low-psychopathy, low-anxiety group of 93 people to a high-psychopathy, high-anxiety group of just 23, then compared bacteria and metabolite levels across those groups.

What This Means If You're Already Researching Gut-Brain Health

If you are reading this because you suspect your own gut health connects to a mood, anxiety, or behavioral issue, the honest answer is that this study does not give you anything to act on yet. It does not identify a bacteria species you could take as a probiotic, a food to avoid, or a test you could order to learn anything about psychopathic traits, your own or anyone else's. The researchers themselves state plainly that evaluating psychopathy requires a comprehensive assessment by a qualified mental health professional, not a stool sample. What the study does offer is an early hint that the same microbiome research reshaping how scientists think about anxiety and depression might eventually extend to personality and antisocial behavior, but that is a research direction, not a tool you can use today.

The Caveats: Small Effect Sizes and No Proof of Cause

Several limits keep this from being more than a first data point. The study is cross-sectional (it measured everyone once, rather than following people over time), so the authors are explicit that it cannot establish causality, and they note the relationship could run in either direction: gut bacteria could influence brain function, or psychopathy-linked behaviors like diet and stress hormones could be shaping the microbiome instead. The effect sizes involved are also small. Bacterial community differences (beta-diversity, a measure of how different two people's overall bacterial makeup are from each other) explained less than 1% of the variation tied to psychopathy scores. This is also the first study of its kind, so none of these findings have been replicated elsewhere. Other limits include reliance on self-reported psychopathy and diet data, a sample drawn entirely from one country (Portugal), and a clustering method that can be sensitive to how the data happens to split.

What Comes Next for This Research

The authors call for the findings to be tested in clinical or forensic samples with well-defined high- and low-psychopathy groups, rather than a general community sample, to see whether the pattern holds. They also point to animal studies as the likely next step for testing causality directly, since experiments in animal models can manipulate the microbiome and measure behavior in ways that are not possible in people. Further out, the authors speculate, cautiously, that measuring gut and oral bacteria could someday complement standard psychological questionnaires or inform non-drug interventions, but they frame that as a long-term possibility built on replication that has not happened yet, not a near-term application.

Common Questions

Does this study prove gut bacteria cause psychopathic traits? No. The study only found associations between bacteria levels and trait scores, and the authors say causality has not been established and could run in either direction.

Could a stool test screen someone for psychopathy? No. The researchers state that evaluating psychopathy requires a full assessment by a qualified mental health professional, not a microbiome test.

Were any of the 200 participants diagnosed psychopaths? No. All were healthy community volunteers assessed on a continuous psychopathic-traits scale, not a clinical population with any diagnosis.

Which specific bacteria were linked to higher trait scores? Allisonella, Prevotella, and Cloacibacillus evryensis in the gut were positively linked to psychopathy scores, while Treponema vincentii in the mouth was negatively linked.

References

1. Carolina F. F. A. Costa, Daniel Radford-Smith, Alexandre R. T. Figueiredo, 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
2. Han-Na Kim, Yeojun Yun, Seungho Ryu, et al. (2018). Correlation between gut microbiota and personality in adults: A cross-sectional study. Brain, Behavior, and Immunity. https://doi.org/10.1016/j.bbi.2017.12.012
3. The Microbiota-Gut-Brain Axis (Physiological Reviews, 2019). NIH. https://pubmed.ncbi.nlm.nih.gov/31460832/