Researchers in Finland have found that transplanting gut bacteria from outgoing toddlers into rats leads to more exploratory behavior in the animals. This suggests the microbiome may influence emotional development in early life. The study highlights a potential gut-brain link via dopamine.
A team led by Anna Aatsinki at the University of Turku in Finland investigated how the gut microbiome might shape temperament. They assessed the personalities of 27 toddlers aged 2.5 years using a standard temperament evaluation and a play exercise with a bubble gun. This identified 10 exuberant children and eight inhibited, introverted ones. From these, the researchers selected four exuberant and four inhibited toddlers—half boys and half girls—and collected faecal samples.
These samples, along with controls, were transplanted into 53 young rats aged 22 or 23 days, whose bowels had been cleansed. The rats then underwent behavioral tests in various situations. Those receiving microbiomes from exuberant toddlers displayed significantly more exploratory behavior compared to controls or those from inhibited toddlers.
Brain tissue analysis revealed that rats with transplants from inhibited toddlers showed reduced activity in dopamine-producing neurons, a chemical associated with reward and risk-taking. "We couldn’t really study things like anxiety disorder in 2-year-olds, but we thought there might be behaviour differences we could look at; if they are, for instance, behaviourally inhibited versus very outgoing and extroverted," says Aatsinki.
Harriët Schellekens at University College Cork in Ireland, who was not involved, praises the work: "It suggests our microbes are active participants in emotional development, not just passive passengers." She adds, "This study beautifully shows how the gut microbiome in early life may help shape behavioural tendencies," pointing to a gut-brain route influencing curiosity and motivation through the dopamine system.
Aatsinki cautions that genetics strongly correlate with adult temperament, though environmental factors like the microbiome may affect some behaviors. It remains unclear if microbes drive personality differences or result from them, as exuberant children might develop distinct microbiomes through interactions.
The findings are detailed in a bioRxiv preprint (DOI: 10.1101/2025.10.10.681629).