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“The findings suggest that the gut microbiome can influence social behavior, possibly through the gut-brain axis.”

The gut microbiome, comprising trillions of microorganisms living in our gut, is critical to our overall health. Recent research, predominantly in laboratory animals has started to uncover these microorganisms’ profound impact on our brain and behavior. One intriguing area of study is the connection between the gut microbiome and social decision-making.

The study titled “Impact of the gut microbiome composition on social decision-making” was conducted by Dr. Marie Falkenstein and her colleagues from the Sorbonne Université, from the University of Bonn, and from INSEAD, and published in the prestigious journal Proceedings of the National Academy of Science (PNAS Nexus) in 2024. The researchers aimed to understand if and how the composition of the gut microbiome affects social decision-making in healthy individuals. They focused on a particular human behavior, called “altruistic punishment behavior”. This behavior refers to the act of forgoing personal gain to punish unfair behavior. The study hypothesized that altering the gut microbiome through a dietary intervention could impact this type of social decision-making.

The study involved a randomized, double-blind, placebo-controlled trial with 101 male participants. Participants were divided into two groups: one receiving a daily synbiotic supplement (a combination of probiotics and prebiotics) and the other receiving a placebo. The intervention lasted seven weeks. To assess the impact on social decision-making, participants played the ultimatum game (UG). In this game, one player proposes a division of money, and the other player accepts or rejects the offer. Rejection of unfair offers was considered a measure of altruistic punishment.

“Participants who took the synbiotic supplement rejected unfair offers more often, suggesting that altering the gut microbiome increased their willingness to punish unfair behavior.”

The study found that the synbiotic intervention significantly increased participants’ rejection of unfair offers in the game, indicating a higher tendency for altruistic punishment. This change was associated with alterations in the composition of the gut microbiome in particular in the ratio between two microbial phyla called Firmicutes and Bacteroidetes which make up the majority of gut bacterial taxa. The Firmicutes/Bacteroidetes (F/B) ratio is a measurement of the balance of the two largest phyla in the human gut microbiome. Specifically, participants with a higher baseline F/B ratio, signifying an unbalanced gut microbiome, showed greater changes in their microbiome and corresponding increases in altruistic punishment behavior after the intervention. Participants who received the synbiotic supplement demonstrated a higher rejection rate of unfair offers than those in the placebo group, implying that altering the gut microbiome increased their willingness to punish unfair behavior, even at a cost to themselves.

“This increase in tyrosine was linked to more altruistic punishment, suggesting that gut microbiome changes can affect brain chemicals involved in decision-making.”

The study measured changes in the gut microbiome before and after the intervention, finding significant alterations due to the synbiotic intake. The impact was more pronounced in participants with a higher F/B ratio at the start, a marker often associated with a less healthy gut microbiome. Additionally, the researchers measured levels of amino acids like tyrosine in the stool, which are precursors to neurotransmitters in the brain. The intervention led to an overall increase in tyrosine levels, specifically in participants with an initially high F/B ratio. This increase in tyrosine was linked to the observed rise in altruistic punishment behavior, suggesting that changes in the composition of the gut microbiome can affect brain chemicals involved in decision-making.

Despite its promising findings, the study has several limitations. Firstly, it only included male participants, which limits the generalizability of the results. Future research should include a more diverse sample to identify possible sex related differences in the results. Secondly, the study focused on a specific type of social decision-making in a controlled experimental setting. Real-world social interactions are far more complex, and further research is needed to explore how these findings may translate to everyday social behavior. In addition, the F/B ratio in healthy people varies widely and is not considered an accurate marker of gut dysbiosis. Lastly, the study’s duration was relatively short, and longer-term studies are required to understand the sustained effects of gut microbiome alterations on behavior.

The findings suggest that the gut microbiome may influence an important aspect of social behavior, possibly through modulation of brain gut interactions. The study adds to the growing evidence that our gut microbiome is intricately linked to our brain and behavior. It is intriguing to speculate that diet through its influence on the gut microbial ecosystem can impact not only physical health, but also influence social behaviors, and that maintaining a balanced gut microbiome could contribute to fair and altruistic social behaviors.