Specific bacteria in the gut and mouth, along with the chemical byproducts of digestion, are associated with traits like emotional detachment and impulsivity. These findings suggest that the microscopic organisms living inside the human body might play a role in shaping complex personality characteristics. The research was published in Translational Psychiatry.
The human body is home to trillions of microorganisms, collectively known as the microbiome. These bacteria, viruses, and fungi do a lot more than digest food and fight off infections. They communicate directly with the central nervous system through a network of chemical signals, hormones, and immune responses called the gut-brain axis.
Previous research links this biological pathway to mood disorders like depression and anxiety. More recently, researchers have started looking at how the microbiome relates to everyday personality traits and social behavior.
Psychopathy is a personality construct defined by emotional detachment, a lack of empathy, manipulative behavior, and a tendency toward impulsivity. In a clinical setting, it affects roughly one percent of the population. The exact biological mechanisms behind these traits remain poorly understood.
Carolina F.F.A. Costa, a researcher at the University of Porto and the University of Oxford, led a team to investigate this knowledge gap. The researchers wanted to see if the bacteria in the gut and mouth could be tied to subclinical psychopathic traits in the general public.
The researchers recruited 200 adult volunteers from the community in Portugal. The participants were screened to remove individuals who had taken antibiotics in the prior three months, as those medications drastically alter intestinal bacteria. The volunteers completed extensive self-report questionnaires designed to measure their levels of psychopathy, empathy, and anxiety.
These psychological assessments measured several distinct components of personality. The psychopathy survey looked at interpersonal manipulation, affective callousness, an erratic lifestyle, and antisocial behavior. Empathy was measured by testing how well participants could understand the perspectives of others and share their emotional states.
Alongside the psychological assessments, participants provided stool, saliva, and blood samples. The research team used a laboratory technique called DNA sequencing to identify the specific types of bacteria present in the gut and oral microbiomes. They also analyzed the samples for metabolites, which are the small molecules left behind when the body or bacteria break down food.
The researchers first grouped the participants based on their psychological survey scores. A computer algorithm separated the volunteers into four distinct groups characterized by varying levels of psychopathy and anxiety. As expected, participants who scored high in psychopathic traits tended to score lower in overall empathy.
Next, the team examined the bacterial makeup of the participants. They found that the overall richness of the microbiome, meaning the total number of different species present, did not vary among the four psychological groups. Any initial correlations between total diversity and psychopathy were not statistically significant after adjusting for the ages of the participants.
While overall diversity did not differ, the abundance of specific bacterial families did show a relationship with personality traits. In the gut, bacteria from the groups Allisonella and Prevotella, as well as a species called Cloacibacillus evryensis, were positively associated with higher psychopathy scores.
Participants with elevated levels of traits like an erratic lifestyle and emotional callousness had higher amounts of these specific intestinal bacteria. In the mouth, a bacterial species called Treponema vincentii showed a negative association, meaning it was less abundant in individuals with higher psychopathy scores.
Many of these specific bacteria have known biological roles that could influence the brain. Allisonella is known to produce histamine, a chemical that triggers immune responses and inflammation. Chronic inflammation in the body can send stress signals directly to the brain, which often influences mood and behavior.
Cloacibacillus evryensis has the genetic potential to produce gamma-aminobutyric acid in the gut. This specific chemical acts as an inhibitory signal in the central nervous system, helping to regulate impulsivity and self-control. Prevotella has been previously associated with differences in how the brain processes emotional and sensory stimuli.
After identifying the bacteria, the researchers looked at the metabolites in the stool and blood samples. Out of over 200 molecules analyzed, two stood out in the stool samples: glucose and taurine. Both molecules were found at elevated levels in the group of participants who scored high in both psychopathy and anxiety.
When looking at the scores on a sliding scale, higher levels of fecal glucose and taurine correlated positively with traits like antisociality and interpersonal manipulation. Glucose serves as the primary energy source for the body. The digestive tract contains specialized sensors that detect glucose and send metabolic updates to the brain.
Finding elevated glucose in feces suggests a disruption in how energy is extracted from food and absorbed into the bloodstream. Previous studies have noted abnormal glucose metabolism in the brains of individuals with high psychopathic traits, suggesting a potential link between energy use and behavior.
Taurine is an amino acid primarily produced by the liver, and it is involved in brain development and anxiety regulation. It normally binds to bile acids to help digest fats, and gut bacteria eventually separate the taurine from the bile. Finding excess free taurine in stool can indicate that the gut is struggling to absorb nutrients properly, or that specific bacterial populations are overgrown.
Finally, the team cross-referenced the bacterial data with the metabolite data. They found that higher levels of Allisonella in the gut predicted higher amounts of both glucose and taurine. This suggests that specific gut bacteria might be altering energy metabolism in ways that ultimately relate to personality expression.
The researchers note several caveats to their findings. The study relies on observations at a single point in time, meaning it cannot prove that gut bacteria cause psychopathic traits. It is equally possible that the behaviors associated with psychopathy, such as dietary choices or physiological stress responses, alter the microbiome.
The participants were volunteers from the general community rather than individuals with clinical diagnoses. The findings relate to subclinical personality traits, which may not fully represent the severe psychopathy seen in forensic settings.
The psychological data came from self-reported surveys. While these are standard tools in psychology, they are subject to personal bias, as individuals may lack self-awareness or alter their answers to appear more socially acceptable. Future research using animal models could help determine if altering gut bacteria directly changes behavior.
The study, “Gut and oral microbiota composition is associated with psychopathic personality: a cross-sectional study,” was authored by Carolina F.F.A. Costa, Daniel Radford-Smith, Alexandre R.T. Figueiredo, Rita C.M. Ribeiro, Sofia Pinto, Fay Probert, Renata Barros, Paula Serrão, Luisa Barreiros, Marcela A. Segundo, Daniel C. Anthony, Fernando Barbosa, Philip W.J. Burnet, and Benedita Sampaio-Maia.