A new study in 300 infants found a greater diversity in gut bacteria is associated with reduced negative emotions and fear reactivity in infants. Additionally, greater quantities of lactic-acid producing gut bacteria are associated with positive emotions in infants.

This was an associative study so causation cannot be established. The data were corrected for potentially confounding factors like mode of delivery, gestational age, infant age during sampling, antibiotic treatments and breastfeeding status.

There is a growing body of evidence suggesting gut microbiome influences neurodevelopment. The question is whether this will affect long-term changes in behavior. Animal studies suggest it may.