Pregnancy and child

Is gut bacteria responsible for the 'terrible twos' in toddlers?

"Terrible twos?" asks the Mail Online, going on to say that, "the bacteria in your child's gut may be to blame for their bad behaviour". The story is based on research that showed links between the types of bacteria in stool samples from two-year-old children, and their behaviour and temperament.

Researchers have become increasingly interested in how the population of bacteria in the gut (known as gut microbiota) affects health.

Studies have already linked gut bacteria to conditions including obesity, allergies and bowel disease. Now researchers are interested in finding out if gut bacteria is also linked to mental health – for example, depression and anxiety.

So they took stool samples from 75 children in Ohio in the US, and their mothers filled in questionnaires about their temperament and behaviour. They wanted to see whether aspects of a child's temperament were linked to the bacteria in the gut.

The researchers found that both boys and girls who had greater diversity of bacteria in their gut were likely to have higher scores for "surgency" – a term used to describe a combination of impulsive behaviour and high levels of activity.

While the study found a link, it is impossible to say whether the bacteria actually caused the behaviour, or whether other factors are responsible for the link seen. This is very early exploratory research, so we can't draw too many conclusions from it.

And we certainly wouldn't advise trying to alter your toddler's gut microbiota to improve their behaviour. Just stick to a few minutes on the naughty step.  

Where did the story come from?

The study was carried out by researchers from Ohio State University in the US and was funded by grants from the university and the National Institutes for Health, and the National Center for Advancing Translational Sciences. It is published in the peer-reviewed medical journal Brain, Behavior and Immunity.

The Mail Online ignored warnings in the study that it cannot show whether bacteria causes differences in temperament or behaviour, claiming that it showed how "abundance and diversity of certain bacteria can impact a child's mood", and that parents should "blame the bacteria" in the child's gut if their toddler is "acting up".

The study did not look at "acting up" or bad behaviour, but at temperament scales, which included how extrovert and physically active a child is. 

What kind of research was this?

This was a cross-sectional study. It aimed to see whether gut microbiota (the range and amount of bacteria living in the gut) were linked to a child's temperament.

Cross-sectional studies cannot determine which factor came first – in this case, whether the differences in bacteria were present before the children developed a particular temperament. This means they cannot say which factor might potentially be influencing the other.

In addition, observational studies such as this can't show whether one thing definitely causes another, just whether the two happen to be linked in some way. Much more evidence, from a range of different studies and study designs, is needed before scientists are happy to conclude that one thing is likely to cause the other. 

What did the research involve?

Researchers sent online questionnaires to 79 mothers who volunteered for the study to assess their child's temperament, diet and feeding behaviour. Children were all aged 18 to 27 months.

The mothers then collected stool samples from the babies' nappies, which were sent to the researchers for analysis. The researchers used statistical modelling to work out whether the diversity of bacteria or the abundance of any types of bacteria were linked to particular types of temperament.

Of the 79 children tested, only 75 were included in the final analysis. In two cases the stool samples could not be analysed; the reasons for exclusion of the other two were not clear, but may relate to questionnaires showing results outside the usual expected range.

The researchers used a number of techniques to look at the variety of bacteria, how common these bacteria were in each sample, how many different types of bacteria were present in each stool sample, and what proportion they were in to each other.

The researchers used several statistical models to assess the relationship between stool sample results and questionnaire results. They looked at three main aspects of temperament.

The first, called negative affectivity, measures traits including fear, fidgeting, discomfort, shyness, sensitivity to surroundings and how easily the child can be soothed.

The second, called surgency, measures impulsive behaviour, how active a child is, how much pleasure they get from exciting situations, how sociable they are and how excited they get when anticipating pleasure.

The third, called effortful control, looks at a child's ability to stop doing something when told to, transfer their attention from one activity to another, take pleasure in normal activities and focus on a task.

Girls and boys tend to differ in their results on this questionnaire, with boys showing more surgency and girls more effortful control. Because of this, the researchers analysed the results for boys and girls separately.  

What were the basic results?

As expected, there were differences between boys and girls in the questionnaire scores for temperament. However, there was not much overall difference between boys and girls in the population of bacteria in their guts.

The researchers found both boys and girls who had greater diversity of bacteria in their gut were likely to have higher scores for "surgency". This link was stronger for boys, especially when the researchers looked at the individual scores for sociability and pleasure from exciting situations. Among girls only, they found lower levels of bacterial diversity were linked to higher scores for effortful control.

Having more of particular types of bacteria seemed to link to traits including sociability, pleasure from exciting situations and activity, but for boys and not girls. Girls who had more of one particular type of bacteria were likely to have higher scores for fear.

The researchers looked at whether the diet the children ate or how long they'd been breastfed could explain the links between gut microbiota and temperament. Although they found some links with how much vegetables or meat children ate, they say this did not explain away the links found between bacteria and temperament. 

How did the researchers interpret the results?

The researchers said they were "unable to determine" from the study whether the links they found were down to the effect of temperament on the gut bacteria, the effect of the gut bacteria on temperament, or a combination of the two.

But they went on to say that, if later studies show that gut bacteria does influence behaviour, this might give doctors the chance to treat children early to prevent later health problems, including mental health. 


This study found an intriguing link between the bacteria that live inside children's guts, and their personalities and behaviour. It's important to remember we don't know why this relationship exists, or whether it is the result of one factor directly causing the other.

For example, toddlers who are more active could have an increased exposure to bacteria, rather than bacteria leading to increased activity.

The researchers suggest there could be various explanations. For example, stress hormones can change the acidity of the gut, which could affect the bacteria that grow there. Bacteria in the gut can affect us through physical illness and may also affect how we feel or behave.

The study was small, and it only sampled bacteria living in the gut that get passed out of the body in stools. There are many other bacteria that live in the wall of the gut, which might also be important. However, it is difficult and painful to take samples of these bacteria.

The study also relied on the mother's assessment of the child's temperament. While that is important, having an assessment from fathers and impartial observers might help make the results more representative of the child's temperament as a whole, as children often behave differently in different situations.

While the researchers did try to take into account some factors that might influence both behaviour and gut bacteria (such as some aspects of diet), it is possible that these or other factors are in fact behind the association seen.

The relationship between the gut and the brain is an area of research that is attracting a lot of attention. That said, the idea that gut bacteria could impact our behaviour or mental health is not one that has gained widespread acceptance, and a lot more research is needed before it would be. 

NHS Attribution