Probiotics Linked To Increased Sense Of Fairness

An intriguing study by scientists at the University of Bonn, Germany suggests that altering the gut microbiome with pro- and prebiotics could make people more attuned to fairness, even when it means earning less money themselves.

The researchers recruited 101 participants, half of whom took a supplement containing Lactobacillus and Bifidobacterium [found in kombucha] for seven weeks, while the other half received placebos. Before and after the supplementation period, all participants played the “ultimatum game,” a two-player scenario where one player controls a pot of money and offers a share to the second player, who can either accept the offer or reject it, leaving both players empty-handed. Rejecting an unfair offer, known as “altruistic punishment,” means sacrificing potential gains to punish the other player for being ungenerous.

After taking the probiotics, participants were more likely to reject offers, particularly those in the 30%–40% range. The effect was most pronounced in players who started with a high ratio of Firmicutes to Bacteroidetes bacteria in their gut, and who experienced the greatest changes in gut composition. Interestingly, the supplements reduced levels of tyrosine, a dopamine precursor, in some participants’ blood, and this group showed the most significant increase in altruistic punishment.

The findings suggest that shifting the gut microbiome towards a healthier state may make people less rational and more sensitive to social considerations. As Plassmann and her team continue to explore this uncharted territory, the implications could reach far beyond the laboratory. From boardrooms to courtrooms, from negotiations to relationships, the invisible inhabitants of our gut may be subtly shaping our sense of fairness and our willingness to stand up for it, even at a personal cost.

PNAS Nexus, Volume 3, Issue 5, May 2024, page 166

In other words, the experiment demonstrated that “fairness” became more important than financial gain for those with a healthy gut. This suggests that the composition of the gut microbiome influences social behavior.

Whether this limits the ability of kombucha companies to succeed in business is up for debate 🙂

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1 Response

  1. the_editor says:

    I was curious to see what a generative AI tool would make of this research. So I asked Claude AI from Anthropic. I cut and paste the complete content and gave the prompt:

    Review the research in this paper and list strengths and weaknesses.

    The response:


    1. Novel research: The study explores a relatively new area of research, investigating the link between gut microbiome and social decision-making, specifically fairness sensitivity.
    2. Randomized, placebo-controlled design: The use of a randomized, placebo-controlled trial strengthens the study’s internal validity and helps to establish a causal relationship between the probiotic intervention and the observed changes in behavior.
    3. Objective measures: The study incorporates objective measures, such as blood levels of tyrosine, to explore potential mechanisms underlying the relationship between gut microbiome and social decision-making.
    4. Interdisciplinary approach: The research combines insights from microbiology, neuroscience, and behavioral economics, offering a comprehensive perspective on the gut-brain-behavior axis.


    1. Small sample size: The study included only 101 participants, which may limit the generalizability of the findings to the broader population.
    2. Short intervention period: The 7-week intervention period may not be sufficient to observe long-term changes in gut microbiome composition and their impact on social decision-making.
    3. Limited diversity: The study does not provide information on the participants’ demographics, such as age, gender, and ethnicity. A more diverse sample would strengthen the external validity of the findings.
    4. Lack of functional analysis: While the study suggests a link between changes in gut microbiome and social decision-making, it does not delve into the specific functional changes in the gut microbiome that may be responsible for the observed behavioral effects.
    5. Unclear clinical relevance: The study demonstrates an effect on social decision-making in a laboratory setting, but the real-world implications and potential clinical applications of these findings remain unclear.
    6. Potential confounding factors: Although the study controlled for the placebo effect, other factors, such as diet, stress, and lifestyle, may have influenced the participants’ gut microbiome and social decision-making during the intervention period.


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