There may be no actual butterflies in your stomach, but there are approximately 100 trillion microorganisms in your gut. Collectively, these microorganisms — a mixture of bacteria, viruses, fungi, and protozoa — are known as the microbiome. It’s increasingly clear that the microbiome influences mental and physical health. Now, scientists have taken this topic a step further and linked the microbiome to the likelihood of specific personalities.

Personality is both inherited and influenced by one’s environment. And the environment inside humans may matter just as much as the external environment, argues Katerina Johnson, a research associate at the University of Oxford.

Her research, which will be published in the March edition of the Human Microbiome Journal, suggests that gut microbiome composition may be related to differences in personality — and that the diversity of the microbes in your gut could help predict these traits.

The study reveals that how social you are may be linked to the diversity of your microbiome, with higher diversity indicating more pro-social personalities. The opposite — a low level of diversity — may be indicative of neuroticism.

The findings add a new dimension to the science of personality — and suggest that a better understanding of microbiome-gut-axis could lead to new therapies for conditions that affect mental health, Johnson says. Social interactions, where we spend our time, and what we eat all affect the gut microbiome, and likely are “affecting our behavior and psychological well-being in currently unknown ways," she says.

The gut-personality link

The results add to a growing body of research which links different gut bacteria with autism, which can affect social abilities, while other recent research suggests that people with depression also have different gut bacteria than those without. The link between social stress and gut bacteria was also explored in a fascinating 2019 study, in which scientists gave rats a transplant of gut bacteria from rats that show depressive behavior. The previously not-depressed rats began to show stressed-out behavior after the transplant.

This new study doesn't focus on a particular brain condition. Instead, it looks at the link between the composition of the human microbiome and broad personality traits.

A sample of 655 adults were given at-home “gut kits” made by a company called uBiome (the company is now bankrupt). Before the startup was raided by the FBI, it made its name analyzing the DNA of the microorganisms found in poop. Fecal samples of this kind are considered sufficiently representative of the composition of the gut microbiome.

People with larger social networks tend to have a more diverse microbiome. Getty Images

The study participants also filled out an online questionnaire that surveyed their behavioral traits, sociodemographic factors, diet, health, and lifestyle choices. Their personality traits were assessed with a standard “Big Five” personality assessment. The researcher also assessed participants' general tendency to feel anxious, as well as the Autism Spectrum Quotient, to measure the degree to which an individual displayed autistic-like behavior.

The results reveal that people with larger social networks tend to have a more diverse microbiome, while lower diversity was associated with increased levels of stress and anxiety. People who reported that they typically don’t sleep well also had a less diverse microbiome.

Interestingly, the results also show that people who eat food with more naturally occurring probiotics, like yogurt and sauerkraut, had significantly lower levels of anxiety, stress, and neuroticism. They were also less likely to suffer from a mental illness.

The same did not hold true for people who consumed probiotics in supplement form instead.

Adventurous eaters, on average, had greater gut diversity, as well — further supporting the idea that microbiome health can be improved by diet.

What drives the link?

Certain bacteria species were more abundant in individuals who were more social: Those that belong to the genera Akkermansia, Lactococcus, and Osciollospira. Johnson notes that previous research found that children with autism also have a reduction of Lactococcus and Oscillopsira — that suggests the bacteria may be linked to social difficulties in autism.

Two other bacteria genera, Desulfovibrio and Sutterella, meanwhile, were less abundant in less sociable people.

Lactococcus more abundant in individuals who were more social Wikipedia

Taken together, the results indicate that some bacterial genera may be more strongly linked to behavior than others. Future investigation of their effects on social behavior in animal models could lead to potential new therapies for autism, Johnson says.

While it offers clues to the gut-brain axis, the study does not answer whether microbiome diversity drives personality traits, or vice versa. In fact, the research suggests that it may be a two-way street, with behavior shaping the composition of the gut microbiome, and certain gut microbes affecting the body’s stress response.

There are a number of possible reasons why there’s an interaction between the gut microbial community and the brain. Scientists think what’s in the gut could be communicating to the brain through neural, immune, and endocrine pathways — but they don't have the data to prove it. What we do know is that gut microorganisms can produce various neuroactive chemicals, and that certain psychiatric conditions often come hand-in-hand with gastrointestinal problems.

More human studies are needed to know for sure — until recently, most research has involved animal models. But it does appear that our life on the outside affects life on the inside, and vice versa. How we can leverage that knowledge to live better remains to be seen, but at least we will get to know our guts in the process.

Abstract: The gut microbiome has a measurable impact on the brain, influencing stress, anxiety, depressive symptoms and social behavior. This microbiome-gut-brain axis may be mediated by various mechanisms including neural, immune and endocrine signaling. To date, the majority of research has been conducted in animal models, while the limited number of human studies has focused on psychiatric conditions. Here the composition and diversity of the gut microbiome is investigated with respect to human personality. Using regression models to control for possible confounding factors, the abundances of specific bacterial genera are shown to be significantly predicted by personality traits. Diversity analyses of the gut microbiome reveal that people with larger social networks tend to have a more diverse microbiome, suggesting that social interactions may shape the microbial community of the human gut. In contrast, anxiety and stress are linked to reduced diversity and an altered microbiome composition. Together, these results add a new dimension to our understanding of personality and reveal that the microbiome-gut-brain axis may also be relevant to behavioral variation in the general population as well as to cases of psychiatric disorders.