Scientists have discovered that being an only child doesn't just lead to behavioural differences that can set kids apart from those with siblings - it actually affects a child's brain development, too.
A new study comparing brain scans of only children and others who grew up with siblings has revealed significant differences in the participants' grey matter volume, and researchers say it's the first neurological evidence in this area linking changes in brain structure to differing behaviours.
To investigate if only children demonstrated neurological differences from their peers who grew up with brothers and sisters, researchers at Southwest University in China recruited 303 college-age students.
The mix of young people in China offers a broad pool of candidates for this area of research, owing to the nation's long-lasting one-child policy, which limited many but not all families to only raising a single child in between 1979 and 2015.
The common stereotype about being an only child is that growing up without siblings influences an individual's behaviour and personality traits, making them more selfish and less likely to share with their peers.
The participants in this latest study were approximately half only children (and half children with siblings), and were given cognitive tests designed to measure their intelligence, creativity, and personality, in addition to scanning their brains with MRI machines.
Although the results didn't demonstrate any difference in terms of intelligence between the two groups, they did reveal that only children exhibited greater flexibility in their thinking - a key marker of creativity per the Torrance Tests of Creative Thinking.
While only children showed greater flexibility, they also demonstrated less agreeableness in personality tests under what's called the Revised NEO Personality Inventory. Agreeableness is one of the five chief measures tested under the system, with the other four being extraversion, conscientiousness, neuroticism, and openness to experience.
But more importantly than the behavioural data - which have been the focus of many other studies - the MRI results actually demonstrated neurological differences in the participants' grey matter volume (GMV) as a result of their upbringing.
In particular, the results showed that only children showed greater supramarginal gyrus volumes - a portion of the parietal lobe thought to be associated with language perception and processing, and which in the study correlated to the only children's greater flexibility.
By contrast, the brains of only children revealed less volume in other areas, including the medial prefrontal cortex (mPFC) - associated with emotional regulation, such as personality and social behaviours - which the team found to be correlated with their lower scores on agreeableness.
While the researchers aren't drawing firm conclusions on why only children exhibit these differences, they suggest it's possible that parents may foster greater creativity in only children by devoting more time to them - and possibly placing greater expectations on them.
Meanwhile, they hypothesise that only children's lesser agreeableness could result from excessive attention from family members, less exposure to external social groups, and more focus on solitary activities while growing up.
It's important to note that there are some limitations to the study - first off, all the participants were highly educated young people taken from a specific part of the world, and the results only reflect testing from one point in time.
That said, the researchers say it's the first evidence that differences in the anatomical structures of the brain are linked to differing behaviour in terms of flexibility and agreeableness.
"Additionally, our results contribute to the understanding of the neuroanatomical basis of the differences in cognitive function and personality between only-children and non-only-children," the authors write in their study.
While there's still a lot we don't understand about what's going on here, it's clear that there's a link between our family environments and the way our brain structure develops, and it'll be fascinating to see where this direction of research takes us in the future.
The findings are reported in Brain Imaging and Behaviour.