Interesting, but very preliminary observational research, and there could be various explanations for the results suggesting that the microbiome (community of microbes) of a toddler's gut may influence their behavior. The researchers did not think these differences were diet related, but they had not studied the diet in depth. From Science Daily:
The microbiome of a toddler's gut may influence their behavior, a new study suggests. Researchers from The Ohio State University studied microbes from the gastrointestinal tracts of children between the age of 18 and 27 months, and found that the abundance and diversity of certain bacterial species appear to impact behavior, particularly among boys. The correlation exists even after the scientists factored in history of breastfeeding, diet and the method of childbirth -- all of which are known to influence the type of microbes that populate a child's gut.
Christian and study co-author, microbiologist Michael Bailey, PhD, studied stool samples from 77 girls and boys, and found that children with the most genetically diverse types of gut bacteria more frequently exhibited behaviors related with positive mood, curiosity, sociability and impulsivity. In boys only, researchers reported that extroverted personality traits were associated with the abundances of microbes from the Rikenellaceae and Ruminococcaceaefamilies and Dialister and Parabacteroides genera.
"There is definitely communication between bacteria in the gut and the brain, but we don't know which one starts the conversation," said Dr. Bailey...Overall, associations of temperament with the gut microbiome in girls were fewer and less consistent than boys. However, in girls, behaviors like self-restraint, cuddliness and focused attention were associated with a lower diversity of gut bacteria, while girls with an abundance of Rikenellaceae appeared to experience more fear than girls with a more balanced diversity of microbes.
To identify correlations between gut bacteria and temperament, researchers asked mothers to assess kid's behavior using a questionnaire which measures 18 different traits that feed into three composite scales of emotional reactivity: Negative Affect, Surgency/Extraversion and Effortful Control. Scientists looked at the different genetic types and relative quantity of bacteria found in the toddler's stool samples along with their diets.
Similar to other child behavior studies, researchers separated their findings by gender to analyze temperament. Overall, the study found few differences in the abundance and types of gut microbiota between girls and boys.Both researchers say that parents shouldn't try to change their child's gut microbiome just yet. Scientists still don't know what a healthy combination looks like, or what might influence its development.