If your child is shy and introverted, new research has revealed it could be because of the bacteria in their gut.
Researchers at Ohio State University studied the guts of children between the ages of 18 and 27 months.
They found that outgoing children have fewer stress hormones impacting their gut bacteria.
Dr Lisa Christian, lead researcher in the study published in Brain, Behaviour and Immunity, said intestinal bacteria interacted with stress hormones.
"These are the same hormones that have been implicated in chronic illnesses like obesity and asthma," she said.
Researchers asked mothers of 77 toddlers to assess their children's behaviour using a simple questionnaire. Scientists then analysed the types of bacteria, and their quantities, in the children’s stool samples.
Co-author of the study, Dr Michael Bailey, said there was a strong communication between bacteria in the gut and the brain, but exactly how the interaction of hormones and bacteria affects the personality of a child is not yet known.
The researchers theorised that either the kids who are more outgoing have fewer stress hormones naturally, or perhaps the bacteria in their guts could be mitigating the production of stress hormones.
"Or it could be a combination of both," Bailey added.
Either way, your child might not be completely in control of their behaviour.
Overall, the associations between the child's temperament and the gut cell organisms were stronger in the boys than the girls, the researchers said.
"Research that suggests the type and variety of intestinal bacteria a child has could correlate with behaviour is certainly interesting.
"Establishing the direction of effect is very important though – do for instance, we feel more stressed because of the presence of certain intestinal bacteria, or could in fact our mood and stress levels influence the bacteria itself.
"Whether it is the cause or effect of children’s demeanour remains to be seen. Of course, a child’s behaviour could also be completely unrelated to this."