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Scientists are increasingly demonstrating that the gut microbiome has roles beyond the colon. Here we explore a recent research article where scientists have shown a link between gut microbiome composition and its influence on decisions relating to fairness.
Social decision-making is a complex process involving many cognitive mechanisms to help us reach important decisions about trust, fairness, altruism, learning, and social punishment[i]. If nothing else, decision-making is tiresome. It uses mental energy that isn’t in an endless supply, leading to fatigue.
Now, researchers have found evidence that social decision-making is impacted by something else. You’ve guessed it, the composition of the gut microbiome. Scientists have found that the gut microbiota can influence decisions relating to fairness, challenging the classic cognitive science view that social decision-making is solely a function of higher cognitive processes in the brain’s cortex.
Here, we’ll explore the latest research and find out how the gut microbiome can influence decisions and what this could mean for the future.
The gut and the brain have a unique relationship, and have a direct line to each other via a bi-directional communication pathway, called the gut-brain axis. It’s a bit like an instant messaging service between the two that links the cognitive and emotional centers in the brain with the gastrointestinal tract[ii].
The gut-brain axis allows the brain to influence the activity of the gut, and the gut to influence brain activities, such as cognition, mood, and mental health[iii]. Because of this unique link, there have been exciting developments in our understanding of the gut-brain axis as scientists study the impact of the gastrointestinal tract on cognition, behaviour, and brain function. One of these emerging research areas is the influence of the gut on social behaviour.
Social behaviour explains the way people interact and influence other individuals, impacting the way we behave but also the way people respond to current situations. This includes the concept of social decision-making, which is about the decisions people make in a social context and how these decisions can have consequences for the person and others.
A new study published on 14 May 2024, found that the gut microbiome can influence social decision-making, influencing our perceptions of fairness and how we treat others[iv]:
"Falkenstein M, etc. Impact of the gut microbiome composition on social decision-making. PNAS Nexus. 2024"
Growing evidence has shown a connection between the gut-brain axis and socio-affective behaviour in animals. For example, transplanting the gut microbiota from autistic human models into germ-free mice was sufficient to induce autistic behaviour in the mice[v].
However, this current study cements the connection between the gut-brain axis and its influence on social decision-making, highlighting that manipulating the gut microbiome through diet could change people’s decisions when presented with a social dilemma. This could be important for future exploration and have implications for education and policy.
Falkenstein and colleagues built on recent evidence that diet could influence social decision-making. They tested the effects of a 7-week dietary intervention using a synbiotic supplement (a combination of prebiotics and probiotics) on altruistic social punishment in a one-shot ultimatum game.
Video:Find out what the ultimatum game is in this short video.
The ultimatum game has become popular in economic experiments. During the game, one player is given a sum of money that should be split with another player, either fairly or unfairly. The second player can refuse the offer if they deem it insufficient, resulting in neither player being given any money.
Refusing money is equivalent to ‘altruistic punishment’ or the impulse to punish someone even though it is costly and has no material gain[vi]. For the second person, restoring equality by refusing the offer so that no one receives money may be more important than a financial reward. All in all, the ultimatum game measures sensitivity to fairness.
The researchers recruited 101 participants for their study. Fifty-one participants were given a synbiotic dietary supplement for 7 weeks while the rest (50) were given a placebo. Everyone took part in 2 sessions of the ultimatum game, once at the start of the supplementation period and once at the end.
There were two main findings from the study; synbiotic supplements can influence social decision-making and the composition and diversity of the gut microbiome.
Let’s look at the study results in more depth.
Overall, the study results demonstrated that the participants who received the synbiotic supplement for 7 weeks were more likely to reject the unequal offers proposed by members of the placebo group in the second ultimatum game at the end of the intervention period. In other words, the supplement group showed increased altruistic punishment tendencies.
How social decision-making changed across the sessions for each group before and after taking a synbiotic supplement or placebo
Graph 1:The graph above shows the rejection rates for all the offers for each group (placebo and treatment) and each session (beginning and end of 7-week intervention). It shows that the supplement group were more likely to reject the offers, particularly after the 7-week supplementation period.
As well as the behavioural changes evoked in the synbiotic group, the researchers found changes in the gut microbiome composition. The individuals with the biggest imbalance between Firmicutes and Bacteroidetes experienced the most significant changes in line with the synbiotic supplement.
A balanced Firmicutesto Bacteroidetes ratio is critical for gut homeostasis. When it is high, the ratio has been associated with an increased risk or incidence of obesity[vii]. The synbiotic supplement given to participants was associated with the Firmicutesto Bacteroidetes ratio because it contained probiotics from the Firmicutes phylum, and the prebiotic, inulin, which is known to feed Bifidobacteria, and at the same time reduces the abundance of Bacteroides.
Overall, the participants with a less diverse gut microbiome at the start of the study, those with a higher Firmicutesto Bacteroidetes ratio, experienced the biggest changes to their microbiota composition over 7 weeks. So, the more imbalanced the gut microbiome, the greater the impact the supplement and study had.
The researchers also found that the participants with a higher Firmicutesto Bacteroidetes ratio at the start of the intervention and who received the synbiotic supplement also had a higher increase in rejecting unfair offers after the 7-week supplementation period.
Finally, the scientists explored the underlying mechanisms responsible for the altruistic punishment behaviour changes. They found that tyrosine levels increased after synbiotic supplementation. However, the increases were influenced by the balance of the gut microbiome at the start of the trial e.g., Firmicutesto Bacteroidetes ratio.
They then investigated the relationship between the tyrosine level changes and the altruistic punishment behaviour changes. At the start of the study, the participants who had a less balanced microbiome in the supplement group displayed increased rejection rates for unfair offers.
Therefore, suggesting that the overall composition of the gut microbiome could influence social decision-making through tyrosine levels, a precursor to the brain's reward neurotransmitter, dopamine.
The study is interesting because it shows that after a dietary intervention resulting in the manipulation of the human gut microbiome, the participants became less rational but were more sensitive to the fairness of the monetary offer they were given.
The study demonstrates that diet influences human behaviour, but it also contributes to the availability of dopamine precursors. Dopamine is an important player in the brain’s reward system.
Although it’s too early to determine if the composition of our gut can influence our rationality and make us more receptive to social factors, such as fairness, it does highlight the pathways which should be investigated in the future.
It’s an exciting prospect that the gut microbiome could be modulated by diet to have a positive impact on human decisions. One area where this could be useful is in the clinical setting, where decisions about health and disease must be made.
The study by Falkenstein et al., (2024) shows compelling evidence that the gut microbiome composition can influence human social decision-making, especially for altruistic punishment.
Over 7 weeks, a synbiotic supplement increased participants' willingness to reject unfair monetary offers in the ultimatum game, reflecting increased altruistic punishments. The behaviour change was associated with changes in the gut microbiome composition and tyrosine levels.
The study further cements the importance of the gut-brain axis in decision-making and socio-affective behaviour by influencing metabolic pathways. The study also demonstrated that those with the greatest imbalance showed the most significant behaviour changes.
In the future, these findings could lead to further investigations into the influence of gut microbiota in social cognition and suggest potential therapeutic interventions for improving social decision-making in the clinical world.
Written by: Leanne Edermaniger, M.Sc. Leanne is a professional science writer who specializes in human health and enjoys writing about all things related to the gut microbiome.
[i] Rilling JK, Sanfey AG. The neuroscience of social decision-making. Annu Rev Psychol. 2011;62:23-48. doi: 10.1146/annurev.psych.121208.131647. PMID: 20822437.
[ii] Carabotti M, Scirocco A, Maselli MA, Severi C. The gut-brain axis: interactions between enteric microbiota, central and enteric nervous systems. Ann Gastroenterol. 2015 Apr-Jun;28(2):203-209. PMID: 25830558; PMCID: PMC4367209.
[iii] Appleton J. The Gut-Brain Axis: Influence of Microbiota on Mood and Mental Health. Integr Med (Encinitas). 2018 Aug;17(4):28-32. PMID: 31043907; PMCID: PMC6469458.
[iv] Falkenstein M, Simon MC, Mantri A, Weber B, Koban L, Plassmann H. Impact of the gut microbiome composition on social decision-making. PNAS Nexus. 2024 May 14;3(5):pgae166. doi: 10.1093/pnasnexus/pgae166. PMID: 38745566; PMCID: PMC11093127.
[v] Sharon G, Cruz NJ, Kang DW, Gandal MJ, Wang B, Kim YM, Zink EM, Casey CP, Taylor BC, Lane CJ, Bramer LM, Isern NG, Hoyt DW, Noecker C, Sweredoski MJ, Moradian A, Borenstein E, Jansson JK, Knight R, Metz TO, Lois C, Geschwind DH, Krajmalnik-Brown R, Mazmanian SK. Human Gut Microbiota from Autism Spectrum Disorder Promote Behavioral Symptoms in Mice. Cell. 2019 May 30;177(6):1600-1618.e17. doi: 10.1016/j.cell.2019.05.004. PMID: 31150625; PMCID: PMC6993574.
[vi] Fehr E, Gächter S. Altruistic punishment in humans. Nature. 2002 Jan 10;415(6868):137-40. doi: 10.1038/415137a. PMID: 11805825.
[vii] Houtman, T.A., Eckermann, H.A., Smidt, H. et al. Gut microbiota and BMI throughout childhood: the role of firmicutes, bacteroidetes, and short-chain fatty acid producers. Sci Rep 12, 3140 (2022). https://doi.org/10.1038/s41598-022-07176-6
