How bacteria can influence our behavior

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Marie-Christine Simon from the Institute of Nutrition and Food Sciences © Studio
Marie-Christine Simon from the Institute of Nutrition and Food Sciences © Studioline Düsseldorf .

The gut microbiome influences our decisions in social contexts. This is the conclusion of a study led by Sorbonne Université and INSEAD with the participation of the University of Bonn and the University Hospital Bonn (UKB). The study has been published in Proceedings of the National Academy of Sciences Nexus.

Every day we make many decisions that affect not only ourselves but also the people around us: whether it’s dealing with differing opinions in a team meeting, tipping the waitress in a restaurant, choosing a weekend activity with friends or deciding between conventional and sustainable but more expensive products.

To explain how we make decisions in such social contexts, researchers have so far focused on self-interest, social norms and cognitive processes. In a new study led by Sorbonne University with the participation of the University of Bonn and the UKB, however, the focus was on the gut microbiome. With surprising results: the researchers led by Prof. Hilke Plassmann from INSEAD show that the trillions of bacteria and other microorganisms that live in our gut influence social decision-making. Increased bacterial diversity in the gut, caused by a change in diet, is associated with an increased sense of social fairness when making financial decisions.

The experiment

The researchers hypothesized that changing the composition of the gut microbiome could alter levels of dopamine and serotonin, two brain chemicals associated with reward-based recognition and behavior. Such changes could in turn affect social decision-making related to the perception of fairness.

To test the hypothesis, they used a classic task from behavioral economics, the so-called "ultimatum game". In this game, one person proposes to another how they should divide up a certain amount of money. The latter can either accept or reject the offer, and if they reject, neither of them gets the money.

Offers that are perceived as unfair tend to be rejected, reflecting people’s sense of fairness and desire to punish bad behavior, even if it comes at the cost of personal monetary gain. This is contrary to what standard economic theory suggests: A rational person would accept any amount, even if it is much less than the amount the first player wants to keep for himself.

The experiment was conducted with 101 participants in two identical sessions by Marie-Christine Simon, Institute for Nutrition and Food Sciences at the University of Bonn and member of the Transdisciplinary Research Area (TRA) "Life and Health" at the University of Bonn, and Bernd Weber, Institute for Experimental Epileptology and Cognition Research at the UKB and also a member of the TRA "Life and Health", and their working group. In the first session, the participants appeared sober and gave stool and blood samples. They then performed several behavioral tasks, including the ultimatum game. They had to decide whether to accept or reject 20 offers, each made by a different person and ranging from 0 to 5 euros.

After the game, the participants were randomly assigned to one of two groups. One group received a commercially available dietary supplement containing probiotics (beneficial bacteria) and prebiotics (food for beneficial bacteria), which they were to take daily for seven weeks. The other group received a placebo. Seven weeks later, all participants took part in a second, identical session.

The intervention group showed a greater tendency to reject unfair offers compared to the placebo group. The gut composition of the former group also became more diverse; this effect was more pronounced in participants with a higher body fat percentage and an unbalanced gut microbiome. The latter is associated with obesity and other clinical conditions such as depression and autism.

In order to link the changes in the gut microbiome to social behavior, the researchers measured the concentrations of precursors for dopamine and serotonin in the blood samples of the test subjects.

The intervention group showed changes in tyrosine levels, a dopamine precursor. Interestingly, tyrosine levels tended to fall in people with an unbalanced gut microbiome at the start of the study. They were also more prone to altruistic punishments than participants with a balanced gut microbiome.

The study shows for the first time that the microbiome can influence social behavior by influencing the content of dopamine precursors.

The original text was written by Hilke Plassmann.


Marie Falkenstein, Marie-Christine Simon, Aakash Mantri, Bernd Weber, Leonie Koban, Hilke Plassmann, Impact of the gut microbiome composition on social decision-making, PNAS Nexus, Volume 3, Issue 5, May 2024, pgae166 , https://doi.org/10.1093/pnasnex­us/pgae166